SAFETY STUDY OF WIRE STRIKE DEVICES INSTALLED ON CIVIL AND MILITARY HELICOPTERS

7 U.S. Civil Helicopter Accidents: Number of Accidents and Number of Fatalities per Year 17 8 U.S. Civil Helicopter Accidents: Trend of Number of Accidents, Number of Fatalities, and Number of Fatal Accidents per 100,000 Flight Hours 17 9 Accident Rates, 1994 Through 2003, for U.S. Air Carriers Operating Under 14 CFR 121 18 10 U.S. Civil Helicopter Accident Rates by Engine Type 19 11 U.S. Civil Helicopter Accidents: Fatal Accidents/100,000 Flight Hours by Engine Type 19 12 U.S. Civil Helicopter Accidents: Fatalities/100,000 Flight Hours by Engine Type 20 13 U.S. Civil Helicopter Accidents by Type of Operation 20 14 Comparison of Yearly U.S. Civil Helicopter Wire Strike Accidents Obtained in Three Different Studies 22 15 Civil Helicopter Wire Strike Accidents by Type of Operation for 1994-2003 24 16 U.S. Civil Helicopter Wire Strike Accidents by Type of Operation 24 17 Helicopter Wire Strike Accidents by Model 26 18 Wire Strike Accidents Involving Bell 206 26 19 Wire Strike Accidents Involving Bell 47 27 v 20 Wire Strike Accidents Involving Robinson R22 27 21 U.S. Civil Helicopter Fatal Accidents by Operation (1994-2003) 29 22 Comparison of Accidents and Fatalities by Type of Operation (1994-2003) 29 23 The WSPS Installed on a Bell 206 32 24 Maximum Wire Strike Angle 33 25 Powerline Detector Electronics Control 34 26 Zones of Scan, OASys 35 27 Zone of Scan at 100 Knots, OASys 35 28 The OASys Scan Head 36 29 The LOAS Scan Head and Electronics 37 30 HELLAS Installed on UH-60 Helicopter 38 31 SpanGUARD Installation 41 vi vii LIST OF TABLES Table Page 1 U.S. Army Accidents (Class A-D) 8 2 U.S. Civil Helicopter Accident Statistics All Accidents 16 3 U.S. Civil Helicopter Wire Strike Accidents 21 4 Comparison of Wire Strike Statistics for Two Decades 21 5 U.S. Civil Helicopter Wire Strike Accidents by Type of Operation 22 6 U.S. Civil Helicopter Wire Strike Accidents as Percentage of Wire Strikes 23 7 Comparison of Wire Strike Accidents by Type of Operation (1970-1979 and 1994-2003) 23 8 Helicopters Involved in Wire Strikes 25 9 Age Profile of Pilots Involved in Fatal Wire Strike Accidents During 1994-2003 28 10 Helicopter Flight Experience of Pilots Involved in Fatal Wire Strike Accidents During 1994 28 11 Helicopters Involved in Fatal Wire Strike Accidents 30 12 Price of WSPS for Helicopter Models 33 13 SpanGUARD Helimark Specifications 41 14 Summary of Wire Protection and Detection Devices 42 LIST OF ACRONYMS AGL Above ground level CFR Code of Federal Regulations EADS European Aeronautics and Space Company EGPWS Enhanced Ground Proximity Warning System FAA Federal Aviation Administration GPS Global positioning system HAI Helicopter Association International HELLAS Dornier’s Helicopter Laser Radar IFR Instrument flight rules IKAR International Commission for Alpine Rescue LOAS Laser Obstacle Awareness System MD McDonnell Douglas MOWAT Marking of Wires and Towers NTSB National Transportation Safety Board OASys Obstacle Awareness System OCAS Obstacle Collision Avoidance System PTS Practical Test Standards UHF Ultra high frequency U.S. United States Vac Volts alternating current Vdc Volts direct current VFR Visual flight rules VHF Very high frequency WSPS Wire strike protection system viii EXECUTIVE SUMMARY Wire strike accidents involving United States military and civil helicopters for the period 1994- 2004 were analyzed using military and the National Transportation Safety Board safety databases. The objective of the research was to conduct a study on wire strike accidents of civil and military helicopters between 1994 and 2004 to establish trends, assess the potential of existing technology for reducing wire strike accidents, and to recommend solutions that could substantially reduce the number of wire strike accidents. Trends in accidents were established for both military and civil wire strike accidents. The age group and experience profiles of the pilots involved in civil helicopter wire strike accidents were found to be similar to those found in an earlier study. Devices available for warning pilots about the proximity of wires are described and their relative merits assessed. Recommendations were then made for reducing the number of helicopter wire strike accidents. ix/x 1. INTRODUCTION. 1.1 PURPOSE. The wire strike accident rate to United States (U.S.) civil helicopters accounted for approximately 5% of all accidents from about 1963 to the present. In spite of the excellent reports on the effectiveness of wire cutters in U.S. Army helicopters, no detailed study has been carried out on these accidents and the potential of the currently available devices to reduce the accidents. The objective of the research was to conduct a study on wire strike accidents of civil and military helicopters between 1994 and 2004 to • establish trends similar to the ones presented in references 1 and 2. • assess the potential of existing and evolving technology for reducing wire strike accidents. • recommend solutions that could substantially reduce the number of wire strike accidents. 1.2 BACKGROUND. Helicopter wire strikes have been a matter of concern for both civil and military helicopters. Devices to protect the occupants in case of wire strikes have been available for some years. Systems that warn the pilots on the proximity of wires have also been developed. In spite of these developments, wire strikes continue to account for about 5% of all civil and military helicopter accidents. Tuomela and Brennan [1 and 2] analyzed the National Transportation Safety Board (NTSB) and Federal Aviation Administration (FAA) reports of 208 civil helicopter wire strike accidents for a 10-year period (1970-1979). In these accidents, 37 people lost their lives, 52 people suffered serious injury, 88 aircraft (42%) were destroyed, and 120 aircraft (58%) were damaged substantially. They concluded that some form of pilot warning device would have been beneficial in 76% of the accidents, and that wire cutters would have been effective in 49% of the accidents examined. In addition, pilot training would have been effective in 56% of the accidents. They recommended that pilot training, installation of wire cutters, use of a device to warn the pilot of wires in the flight path, and provisions to protect the main and tail rotor blades from damage due to wire strikes would be beneficial. Hart [3] reported that wire strikes constituted about 5% of the 1852 civil helicopter accidents between 1986 and 1996. Harris [4] summarized the results of the analysis of accidents from 1996 through 2000 of the U.S.-registered helicopters. Of the 934 accidents, 50 accidents (5.45%) were classified as wire strike accidents. The data showed that in every wire strike accident, the helicopter was either damaged substantially (66%, 33 accidents) or destroyed (34%, 17 accidents). Fifteen (30%) of the accidents resulted in at least one fatality, and nine accidents (18%) resulted in serious injuries. 1 At the meeting of the International Commission for Alpine Rescue (IKAR) [5], the members noted the seriousness of wire strike accidents and proposed the following recommendation: REC A 0010 - Cable Detection - Recommendation 3/2000: “In order to improve helicopter flight safety, IKAR strongly recommends that the helicopter industry adopts as a standard the implementation of active cable detection systems combined with heads-up warning devices on all helicopters.” An analysis of military helicopter accidents conducted as part of the study shows that, similar to accidents of civil helicopters, wire strikes constitute about 5% of the total accidents. Studies show that wire strikes continue to be a matter of concern because helicopters need to operate at low altitudes, wires are difficult to observe, and the background combined with sunlight can obscure wires. Also, not all helicopters can be equipped with wire cutters, and some of the recently developed devices that warn a pilot of potential wire strikes are very expensive. In this study, an analysis has been made of both civil and military accidents involving helicopter wire strikes for the period 1994 through 2004. The analysis brings out the factors that are common to the majority of wire strike accidents and also some common features between civil and military wire strike accidents. Devices that warn the pilots about the proximity of wires as well as wire cutters are described, and their potential for reducing the number of wire strike accidents is assessed. 1.3 RELATED ACTIVITIES AND DOCUMENTS. The following documents relate directly to the issues addressed herein and document the summary of wire strike accidents, as well as recommendations made to prevent future accidents: • Veillette, Patrick R., “Most Fatal U.S. Commercial Accidents Occur in Instrument Meteorological Conditions,” Flight Safety Digest of the Flight Safety Foundation, January 2003, pp. 1-56. • Australian Transport Safety Bureau, “Light Utility Helicopter Safety in Australia,” Research paper BE04/73, June 2004, (Web page: www.atsb.gov.au). • Anon., “REC A 0010 – Cable Detection, Rec 3/2000,” http://www.ikar-cisa.org. Harris, et al. [4] studied civil helicopter accidents that occurred between mid-1963 and 1997 and found that wire strikes were most frequent in single-piston engine helicopters (9.4% of all accidents of this type). Wire strikes contributed to 5.9% of all accidents of single-turbine engine helicopters and 5.4% of the total of homebuilt and amateur helicopters. Wire strikes contributed to 4.3% of all accidents in twin-turbine helicopters. They recommend discouraging flights below 750 feet above ground level (AGL), marking all man-made objects higher than 500 feet and developing a low-cast proximity spherical sensor to provide the pilot with sufficient warning to avoid obstacles. 2 The study of Tuomela and Brennan was devoted to civil helicopter wire strikes between 1970 and 1979. They found that helicopters flying agricultural missions were involved in 48% of all wire strike accidents. They noted that 83% of the accidents occurred with clear skies and unlimited visibility. They recommended installation of mechanical wire cutters, development of wire detection/pilot warning devices, and adoption of wire-avoiding flight procedures by all helicopter pilots. For the period between 1986 and 1996, Hart [3] reported that wire strikes constituted 5% of all civil helicopter accidents. Harris [5] found that wire strikes constituted 5.45% of all accidents in the period 1996 through 2000. He also noted the clear skies and unlimited visibility conditions during most of the wire strikes. In addition to wire cutters and wire markers, a number of products that give warning of wire proximity are either available or being developed. These are described in references 5-7. The Australian Transport Safety Bureau found that wire strikes constituted about 15% of all accidents to light utility helicopters operating in Australia. The legal viewpoint on wire marking has been given by Wimsatt [8] who recommends installation of markers on wires. The IKAR has made a recommendation for the installation of active cable detection systems combined with heads-up warning devices on helicopters. A joint work group consisting of experts from the HAI, FAA, IEEE, and the aviation and utilities industries has made specific recommendations for modification of Title 14 Code of Federal Regulations (CFR) Part 77 and to Practical Test Standards (PTS). Implementation of these changes would enhance the marking of obstructions and contribute to pilot awareness of wire hazards. The effectiveness of wire cutters in U.S. Army helicopters was noted by Walker and White [7]. However, there does not appear to be a detailed study on wire strike accidents to military helicopters. 2. DISCUSSION. Evaluation tasks accomplished within the framework of this effort included the summary of accidents involving U.S. military and civil helicopters, findings of the Wire Strike Warning and Protection Systems, findings based on human factors/pilot training, and recommendations for preventing wire strikes. The findings for each task are described below. Summary of accidents involving U.S. military helicopters: • During the period from 1994-2003, U.S. Army helicopters have been involved in 1160 accidents (Class A to C) of which 34 have been wire strikes. Over these 10 years, Class A-C wire strike accidents are 2.9% of the total. • There have been 147 fatalities in helicopter accidents of which 7 have been in wire strike accidents. The cost of Class A-C accidents during this period was $1483 million and the cost of wire strike accidents (Class A-C) was $87.5 million. 3 • From 1998 to 2004, the number of wire strike accidents was between 3 and 5 per year. • There has been a noticeable decrease in the number of wire strike accidents after 1990 when all military helicopters were equipped with Wire Strike Protection Systems (WSPS). • There have been no fatalities in wire strike accidents between 1996 and 2002. Summary of accidents involving U.S. civil helicopters: • The total number of accidents to civil helicopters shows an increasing trend from 1996 onwards. There were 212 accidents involving 67 fatalities in the year 2003. • The number of accidents per 100,000 flight hours also shows an increasing trend from 1996 onwards. There were 10 accidents per 100,000 flight hours in the year 2003. • In the year 2003, the number of accidents for fixed-wing aircraft was slightly above 0.3 accidents per 100,000 hours. Thus, the rate of accidents per 100,000 hours for fixed- wing aircraft is about 3% of the accident rate for helicopters. • Helicopters conducting general aviation operations were involved in 70% to 80% of all the civil helicopter accidents. Helicopters involved in aerial application operations accounted for 10% of all the accidents. • Between the years 1994 and 2004, there were 124 wire strike accidents in which 41 were fatal accidents. These accidents resulted in 65 fatalities, 45 serious injuries, and 42 minor injuries. The average age of the pilots involved in the wire strike accidents was 43.5 years, and the average rotorcraft flying experience of the pilots was about 4000 hours. • There was a 40% decrease in the number of wire strike accidents during the period 1994- 2003 compared to the period from 1970-1979. However, there was an increase of 76% in the number of fatalities, whereas the number of serious injuries decreased by 13% between 1994 and 2003. • There were 1.24 fatalities and injuries per wire strike during 1994-2003 compared to 0.84 fatalities and injuries during 1970-1979. • For the period 1994-2004, general aviation operations accounted for approximately 60% of all the wire strike accidents, while agricultural operations accounted for approximately 27% of the accidents. Other operations accounted for 13% of the accidents. (During the period 1970-1979, general aviation operations accounted for 42% of the wire strike accidents, while agricultural operations accounted for about 48% of all wire strike accidents.) • The wire strike percentage of accidents in general aviation operations has been increasing since about 1998. 4 • The Bell 47 helicopter and its variants account for about 20% of all the wire strike accidents, while wire strike accidents of the Bell 206 and its variants account for 18.6% of the wire strikes. The Robinson R22 helicopter (including three R-44 models) accounts for another 15.3% of the total wire strike accidents. • Bell 206 models were involved in nearly 26% of the fatal wire strikes and Robinson R22 models were involved in about 21% of the fatal accidents. The MD 369 and Bell 47 models accounted for 14% and 12% of the fatal accidents, respectively. • Of the 124 helicopters involved in wire strike accidents, Wire Strike Protection kits could potentially have been fitted on 32 helicopters (25.8%). Thus, nearly 74% of the helicopters involved in the wire strikes could not have been fitted with wire cutters. • The NTSB determined that the probable causes for most of the accidents are one or more of the following reasons: inadequate visual lookout (38 accidents), failure to maintain sufficient clearance with the obstacle (59 accidents), failure to maintain proper altitude (9 accidents). Other reasons (for 19 accidents) include improper judgment (e.g., decision to continue flying visual flight rules (VFR) under instrument flight rule (IFR) conditions), inadequate preflight planning, failure to see and avoid wires, intentional buzzing (low altitude flight), and selection of unsuitable area for landing. • Thirty-seven (86%) of the fatal wire strike accidents occurred in day visual meteorological conditions. Two accidents occurred in day instrument flying conditions and two occurred in night instrument flying conditions. • The majority of fatal wire strike accidents involve pilots who were between 40 and 59 years old and had more than 2000 hours of flight experience. • Seventy-eight percent of the pilots involved in the fatal wire strike accidents were over 40 years. • Fifty-six percent of the pilots had over 2000 hours of rotorcraft flight experience. • The average age of the pilots involved in fatal wire strike accidents was 47.3 years, and the average flight experience was about 3575 hours. • Helicopters operating under 14 CFR Part 91 (general aviation) account for 65% of all fatal accidents. Agricultural operations (14 CFR Part 137) and rotorcraft external load (14 CFR Part 133) account for 14% and 12% of the fatalities, respectively. Summary of the findings of the wire strike warning and protection systems: • The WSPS is most effective when the helicopter impacts the wires nearly perpendicular to the wires in a level attitude and at flight speeds of more than 30 knots. • The WSPS is available for about 25% of the helicopters. 5 • The Powerline Detector system (Safe Flight Instrumentation Corporation) senses the electromagnetic fields surrounding power lines. This detector only senses active power lines and the range of detection depends on the electrical power in the lines. The system will not detect other types of wires such as guy wires, telephone lines, and nonactive power lines. Further, the pilot is not alerted to the direction of the power lines with reference to the aircraft. • Devices that use lasers or radar to scan the surroundings for the presence of obstructions are Obstacle Awareness System (OASys) (Amphitech System), Laser Obstacle Awareness System (LOAS) (Goodrich Sensor System), and Dornier’s Helicopter Laser Radar (HELLAS) (European Aeronautics and Space Company (EADS) Dornier). They are comparatively heavy (between 35 and 60 lb) and expensive (more than $100,000). Thus, they are too heavy and quite expensive for most civil helicopters. • Honeywell Enhanced Ground Proximity Warning System (EGPWS) can warn pilots about obstacles that are over 30 ft AGL using a global positioning system (GPS) and the database maintained by Honeywell Aerospace. The EGPWS weighs about 4 lb but costs about $45,000, which makes the system too expensive for most civil helicopters. • Developing less expensive devices that can be fitted to light helicopters, such as the Robinson R22, can be helpful to about 75% of the helicopters. • Spherical markers mounted on wires will help to make the wires more visible and reduce wire strikes. The Marking of Wires and Towers (MOWAT) Work Group [9] recognized the importance of wire markers in preventing wire strikes. • The Obstacle Collision Avoidance System (OCAS) consists of units located on utility and power line towers and detects all air traffic entering a predefined warning zone and activates warning lights that illuminate the tower. The fact that the OCAS does not require any installations in the helicopters can make it attractive to helicopter operators. It is also attractive to utilities in spite of its cost ($50,000 per installation). However, the lights on the utility towers can normally be turned off. The OCAS is presently being evaluated by the FAA and has potential to prevent wire strikes. • The MOWAT Work Group [9] recognized the importance of training in preventing wire strikes and has made a recommendation to the FAA. This is a modification to the PTS that an applicant should be aware of low-level hazards and be knowledgeable in the recognition of wires, towers, and other low-level hazards. Summary of findings based on human factors/pilot training: Most of the helicopter wire strikes occur in daytime in conditions of good visibility and involve experienced pilots. These factors have been considered by specialists, who recommend reviewing aeronautical charts and conducting a reconnaissance flight at a higher altitude, before conducting low-altitude operations. Pilots need to be exposed to techniques of recognizing different types of wires, including powered and guy wires, and to anticipate their location. They 6 need to identify the power grid system and determine wire direction from the orientation of the insulating connectors on various towers. Summary of recommendations for preventing wire strikes: • All helicopter pilots flying 14 CFR Part 91 (general aviation) should avoid cruising below 750 ft when the mission does not require it. • Encourage pilots to review aeronautical charts and to conduct reconnaissance flights at a higher altitude, before conducting low-altitude operations. • Develop less expensive wire proximity warning devices that can be fitted to light helicopters such as the Robinson R22. These can be of help to about 75% of the helicopters involved in wire strikes. • Wherever feasible, install mechanical wire cutters on helicopters. • Implement the MOWAT Work Group recommendations on wire marking and pilot training. (Under consideration by the FAA in 2004.) • Conduct human factors studies to develop an understanding of the factors involved in wire identification and avoidance by pilots working in the cockpit environment. 3. EVALUATION APPROACH. 3.1 MILITARY HELICOPTER WIRE STRIKE ACCIDENTS. 3.1.1 Analysis Methodology. Data on military helicopter accidents are available at the U.S. Army Safety Center website [10] (http://safety.army.mil). In addition, Flightfax, the monthly publication of the U.S. Army Safety Center provides briefs on army helicopter accidents. Flightfax also contains papers on safety- related issues. The U.S. Army classifies accidents into the following classes: • Class A: Damage costs of $1,000,000 or more and/or destruction of an Army aircraft, missile or spacecraft, and/or fatality or permanent total disability. • Class B: Damage costs of $200,000 or more, but less than $1,000,000 and/or permanent partial disability and/or three or more people are hospitalized as inpatients. • Class C: Damage costs of $20,000 or more, but less than $200,000 and/or nonfatal injury resulting in loss of time from work beyond day/shift when injury occurred and/or nonfatal illness/disability causes loss of time from work. 7 • Class D: Damage costs are less than $20,000. • Class E: No damage costs. 3.1.2 Analysis of Accidents. Data on all the helicopter accidents in the U.S. Army (Class A-E) for the period 1994 through 2004 (September) are presented in appendix A. This appendix also contains data on wire strike accidents related to U.S. Army helicopters. The number of helicopter accidents in Class A-D and their cost for each year from 1994-2003 are given in table 1, which also gives similar data for the accidents due to wire strike. The total cost due to all accidents in this period was $1483 million, and the cost of wire strike accidents was about $94 million. During the period from 1994-2003, U.S. Army helicopters were involved in 1160 accidents (Class A-C), in which 34 were wire strikes. Over these 10 years, Class A-C wire strike accidents constituted 2.9% of the total number. Table 1. U.S. Army Accidents (Class A-D) Year Total Number Cost of All Accidents Wire Strikes Cost of Wire Strikes 1994 225 $110,023,167 9 $16,964,728 1995 199 $78,492,980 7 $10,623,388 1996 191 $130,372,509 2 $14,488 1997 153 $48,429,545 4 $2,064,623 1998 152 $111,381,217 5 $13,987,143 1999 175 $126,143,508 5 $92,036 2000 143 $44,041,440 5 $1,047,801 2001 141 $64,492,515 5 $564,333 2002 190 $328,139,151 4 $37,019,345 2003 176 $296,297,344 6 $4,316,490 There have been 147 fatalities in helicopter accidents, in which 7 were wire strike accidents. The cost of Class A-C accidents during this period was $1483 million, and the cost of wire strike accidents (Class A-C) was $87.5 million. Figure 1 shows the total number of helicopter accidents in each year from 1994 through 2004. The number for 2003 is high due to the accidents that occurred in Kuwait and Iraq since the beginning of Operation Iraqi Freedom. The accidents and the number of fatalities show an increasing trend after the year 2000. 8 0 25 50 75 100 125 1994 1996 1998 2000 2002 2004 Year Class A, B,C Fatalities 2003 Figure 1. Total Number of U.S. Army Helicopter Accidents and Fatalities (1994-2004) Figure 2 shows the yearly number of wire strike accidents. Between 1997 and 2002, the number of wire strike accidents was between two and five per year. Figure 2 also shows the number of fatalities in wire strike accidents. After 1995, there were no fatalities in accidents involving wire strikes, even though the number of wire strikes did not decreased. There were two fatalities in 2003 that involved an AH-64A model that was conducting a night reconnaissance and surveillance mission using night vision systems. One reason for this impressive safety record can be the installation of wire cutters (WSPS) on Army helicopters. 0 2 4 6 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Year Number of Accidents Number of fatalities Figure 2. U.S. Army Helicopter Wire Strike Accidents and Fatalities Figure 3 shows wire strike accidents as a percentage of the total number of accidents. There has been a continuous decrease in the wire strike accidents from about 1998. Wire strikes only constituted about 2% and 3% of all the helicopter-related accidents during the last 3 years (2002- 2004). This rate is better than the corresponding rate for civil helicopters (between 4% and 5%). 9 0 1 2 3 4 5 1994 1996 1998 2000 2002 2004 Year 2003 Figure 3. U.S. Army Helicopter Wire Strike Accidents as Percentage of Total Accidents Figure 4 gives the cost incurred by the Army due to wire strike accidents. These include the cost of the damage and the costs due to injury to military personnel. There is a decrease in the costs after 1998 and is generally less than $2 million per year. This reduction in costs is a direct result of the reduction in the number of Class A accidents. 0 2 4 6 8 10 12 14 16 1994 1996 1998 2000 2002 2004 Year Cost, $ Million Figure 4. Yearly Costs of U.S. Army Helicopter Wire Strike Accidents 3.1.3 Narrative Description of Wire Strike Accidents From 2002-2003. 3.1.3.1 June 2002. This Class A accident involved an OH-58 D-R model. “Aircraft reportedly contacted wires during training flight and landed hard on a major thoroughfare, coming to rest on its side (rolled 90 degrees). Crew members were able to egress unassisted and notified the local Chain of Command. Damage initially assessed as Class B. Pending further Estimated Cost of Damage, potential exists for Class A damage to the airframe.” 10 3.1.3.2 July 2002. This Class D accident involved a UH-60 L model. “While in flight heading 180 degrees, 100 KIAS, 125 ft AGL, the UH-60L aircraft cut through three sets of electrical wires. The aircraft still had controllability and no visual damage (while in flight); therefore, flight was continued to an approved helicopter landing zone to the west of the wire strike location.” 3.1.3.3 August 2002. This Class A accident involved an OH-58 D-R model. “While conducting a counter-drug mission, aircraft developed a vibration and made a landing. Crew inspected the damage and noted no damage. Crew spotted fire in the adjacent valley and decided to depart the area. Following take-off, the crew observed a downed power line. Aircraft was flown four miles to sheriff station without incident and shut down. Post flight inspection revealed damage to main rotor system from a wire strike. Property damage to be determined. Estimated cost of damage: $50,000 to aircraft; property: To be determined.” 3.1.3.4 November 2002. This Class C accident involved an OH-58 D-R model. “Flight of two was conducting NVG operations vicinity enter/exit point of terrain flight training area when chalk #2 noticed that they were in the wrong ravine. Chalk #2 began to scan the ridgelines when he detected power line poles. As he began to transmit this info to the lead aircraft, Chalk #1 struck three power lines. Aircraft landed without further incident. Damage includes a scratched windscreen and a voided MRB.” 3.1.3.5 January 2003. This Class B accident involved an AH-64 A model. “Crew was on an approved low level multi- ship screening mission. The aircraft struck a small set of wires that were not marked on the map causing damage to one main rotor blade, two broken antennas, as well as damage to the ALQ144 and possible arcing on one wing.” 3.1.3.6 March 2003. This Class C accident involved a UH-60 L model. “Aircraft contacted wires during an ATM training flight. The WSPS functioned as designed and severed all three wires. The aircraft sustained damage to the ALQ144 and the tail wheel strut area. (The wires were reportedly not depicted on the published hazard map).” 3.1.3.7 August 2003. This Class A accident involved an AH-64 A model. “While conducting a night recon and surveillance mission using night vision systems, a flight of two aircraft departed a named area of interest (NAI) en route to another NAI in a loose, staggered right formation. As the flight maneuvered between two hilltops, Chalk 1 struck a series of four mining cables. Both pilots were killed by the impacts and the aircraft was completely destroyed by a post-crash fire.” 11 3.1.3.8 October 2003. This Class A accident involved a UH-060 L model. “While conducting night vision goggle terrain flight, the aircraft descended to a sandbar to conduct hoist training when the aircraft struck two cables suspended across the river. After contacting the cables, the aircraft descended aft and impacted the stabilator in the riverbed. The aircraft subsequently rolled left and settled on its left side in the river. No personnel were injured in the accident.” 3.1.4 Analysis of Wire Strike Protection System Use in U.S. Army Helicopters. The U.S. Army has installed WSPS on all of its helicopters. Data collected from the distributor of WSPS, Aeroproducts Inc., Tempe, Arizona, show that most of the U.S. Army fleet of OH-58, UH-1, OH-6, AH-1, and UH-60 Black Hawk helicopters were retrofitted before 1988. The last major shipment of retrofit kits was made in 1988 for AH-64 Apache helicopters. All AH-64 Apache WSPS installations were to be completed by October 12, 1992. Therefore, all military helicopters would have had a WSPS system installed by the end of 1992. By examining the number of wire strike accidents and resulting fatalities occurring before and after the years in which WSPS units were installed, a picture of the effectiveness of the system emerges. Figure 5 shows the annual number of Army wire strike accidents (Class A-D) and the fatalities in these accidents from 1980 to 2003. Figure 5 shows that the annual number of wire strike accidents decreased after 1990. Between 1980 and 1990, the number of wire strikes varied between 8 and 20 accidents per year. After 1990, the number of accidents was reduced to below 10 per year, stabilizing at 5 per year. The reduction in accidents can be attributed to the improved awareness of the pilots to wire strike possibilities after WSPS was installed. The number of fatalities in these accidents shows a dramatic decrease after 1994. Between 1996 and 2002, there were no fatalities. However, because the database does not contain narrative details of all the accidents, it is not possible to ascribe these reductions solely to the installation of WSPS. 12 0 5 10 15 20 25 1980 1985 1990 1995 2000 Accident Count Fatalities 2003 Figure 5. U.S. Army Wire Strike Accidents 1980-2003 To determine whether the WSPS prevents the more serious accidents in a similar manner, the accidents in Classes A-C were examined over the same time span. The results are shown in figure 6. 0 2 4 6 8 10 12 14 16 1980 1985 1990 1995 2000 2005 Fatalities Accident Count 2003 Figure 6. U.S. Army Wire Strike Accidents, Class A-C 13 Figure 6 shows that there is a significant drop in the annual number of accidents after 1990. This reinforces the conclusion that the reduction in number of wire strikes and the elimination of fatalities in these accidents occurred after the installation of WSPS. 3.1.5 Summary of Military Helicopter Wire Strike Accidents. From the data and analysis presented in this section, the following points can be highlighted: • During the period 1994-2003, U.S. Army helicopters were involved in 1160 accidents (Class A-C), in which 34 were attributed to wire strikes. Over these 10 years, Class A-C wire strike accidents constituted a 2.9% of the total accidents. • There were 147 fatalities in helicopter accidents, in which 7 were attributed to wire strike accidents. During this period, the cost of Class A-C accidents was $1483 million, whereas the cost of wire strike accidents (Class A-C) was $87.5 million. • Since 1998, the number of wire strike accidents were between three and five per year. • There was a noticeable decrease in the number of wire strike accidents after 1990 when all military helicopters were equipped with WSPS. • There were no fatalities in wire strike accidents between 1996 and 2002. • There appears to be convincing evidence supporting the effectiveness of the WSPS. 3.2 CIVIL HELICOPTER WIRE STRIKE ACCIDENTS. 3.2.1 Data Sources and Methodology. A primary source for the data on civil helicopter accidents was the NTSB website. Accident data for the period 1994 through 2004 were collected and analyzed. The HAI accident database also compiles accident statistics using the NTSB database. The NTSB database can be used to extract accident data on specific types of accidents (e.g., wire strikes) during a specified period (e.g., 1994 to 2004) using keywords to query the database. However, it is necessary to use keywords like wire, wire strike, cable, and guy wire to extract all the accidents from the database. This method was used to correlate the results for the wire strike accidents generated in the present research with the results of HAI. The data included in this report cover those accidents in which wire strike was the primary cause; some cases have been included in which impacting the wires caused airframe damage or injuries to the occupants. For each accident, the NTSB database also provided a factual report that showed details about the helicopter and the pilot. The data obtained from the NTSB database is in the form of narratives. The narratives for the accidents involving helicopter wire strikes for the years 1994-2004 are compiled and presented in appendix B. Each accident was studied, and the results were tabulated for further analysis, as shown in appendix B. These tables give the date, place, NTSB identification number, number and type of injuries, and type of operation for each accident. Data on wire strike accidents for 14 the period covering 1994-2003 was analyzed in detail to identify the change in trends from those identified by Tuomela and Brennan [1 and 2] for the period covering 1970-1979. To understand the data from the NTSB, it is essential that one understands how the NTSB defines an accident, incident, fatal injury, and serious injury, as well as exceptions that apply to each. The NTSB defines an accident as “an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight until such time as all such persons have disembarked,” in which: 1. a person is fatally or seriously injured as a result of: (a) being in the aircraft. (b) being in direct contact with any part of the aircraft, including parts which have become detached from the aircraft. (c) direct exposure to jet blast. The exceptions are when the injuries are from natural causes, self-inflicted or inflicted by other persons, or when the injuries are to stowaways hiding outside the areas normally available to the passengers and crew or: 2. the aircraft sustains damage or structural failure which: (a) adversely affects the structural strength, performance, or flight characteristics of the aircraft. (b) would normally require major repair or replacement of the affected component. The exceptions for engine failure or damage is when the damage is limited to the engine, its cowlings, or accessories; or for damage limited to propellers, wing tips, antennas, tires, brakes, fairings, small dents, or puncture holes in the aircraft skin or: (c) the aircraft is missing or is completely inaccessible. The NTSB defines an Incident as “an occurrence, other than an accident, associated with the operation of an aircraft which affects or could affect the safety of the operation.” The NTSB defines a Fatal Injury as “an injury which results in death within 30 days of the incident.” The NTSB defines a Serious Injury as “an injury which: (1) requires hospitalization for more than 48 hours, commencing within 7 days from the date the injury was received; (2) results in a 15 fracture of any bone; (3) causes severe hemorrhages, nerve, muscle or tendon damage; (4) involves an internal organ; or (5) involves second- or third-degree burns.” 3.2.2 Civil Helicopter Accident Rates (All Accidents). Table 2 shows data on the accidents in which U.S. civil helicopters were involved from 1994- 2003. The total number of hours flown per year shows an increase trend up to 1999 and then a decrease from a peak of 2.74 million hours in 1999 to slightly above 2.1 million hours per year between 2001 and 2003. Figure 7 shows the number of accidents and the number of fatalities per year from 1994 to 2003. The total number of accidents shows an increasing trend from 1997 onwards. There were 212 accidents involving 67 fatalities in the year 2003. Table 2. U.S. Civil Helicopter Accident Statistics All Accidents Year 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Total hours (Millions) 1.777 1.961 2.120 2.084 2.342 2.744 2.308 2.141 2.110 2.125 Total number of accidents 218 161 176 163 191 197 206 182 205 212 Total fatal accidents 44 25 32 27 34 31 35 29 26 37 Total number of fatalities 79 45 54 43 66 57 63 51 41 67 Total number of serious injuries 53 34 34 62 26 44 42 34 51 51 Total number of minor injuries 92 54 56 79 55 81 81 71 58 82 Total number of no injuries 212 174 184 157 197 205 200 223 267 236 Fatalities/fatal accident 1.80 1.80 1.69 1.59 1.94 1.84 1.80 1.76 1.58 1.81 Total accidents/ 100,000 hr 12.26 8.21 8.29 7.82 8.15 7.18 8.93 8.50 9.72 9.98 Fatal accidents/ 100,000 hr 2.48 1.27 1.51 1.30 1.45 1.13 1.52 1.35 1.23 1.74 Fatalities/ 100,000 hr 4.45 2.37 2.70 2.06 2.82 2.08 2.73 2.38 1.94 3.15 Serious injuries/ 100,000 hr 2.98 1.73 1.60 2.98 1.11 1.60 1.82 1.59 2.42 2.40 Minor injuries/ 100,000 hr 5.18 2.75 2.64 3.79 2.35 2.95 3.51 3.32 2.74 3.86 16 0 50 100 150 200 250 1994 1996 1998 2000 2002 2004 Year Number of all accidents Number of fatalities 2003 Figure 7. U.S. Civil Helicopter Accidents: Number of Accidents and Number of Fatalities per Year A commonly used safety metric is the number of accidents per 100,000 flight hours (total number of accidents/total flight hours/100,000). Figure 8 shows the rate of the total accidents, total number of fatal accidents, and the number of fatalities per 100,000 flight hours. All these metrics show an increasing trend from about 1999. 0 2 4 6 8 10 12 14 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Year Accidetns/100,000 hours Total accidents/100,000 hours Fatal accidents/ 100,000 hours Number of fatalities/100,000 hours Figure 8. U.S. Civil Helicopter Accidents: Trend of Number of Accidents, Number of Fatalities, and Number of Fatal Accidents per 100,000 Flight Hours The rate of accidents in 2003 was close to ten accidents per 100,000 flight hours. There were 1.74 fatal accidents per 100,000 hours in the year 2003. The corresponding rates for fixed-wing aircraft operated by air carriers under 14 CFR 121 are shown in figure 9. In the year 2003, the number of accidents for fixed-wing of aircraft was slightly above 0.3 accidents per 100,000 flight 17 hours. Thus, the rate of accidents per 100,000 hours for fixed-wing aircraft is 3% of the accident rate for helicopters. 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Number of accidents/100,000 hours Fatal Accidents/100,000 hours Figure 9. Accident Rates, 1994 Through 2003, for U.S. Air Carriers Operating Under 14 CFR 121 The U.S.-registered civil helicopter fleet consists of helicopters powered by single reciprocating engines, single turbine engines, and multiple turbine engines. Figure 10 shows the accident rate (per 100,000 hours) for each of these types of helicopters. There has been a sharp increase in the number of accidents involving single reciprocating engine helicopters. The multiengine helicopters have been involved in the lowest number of accidents per 100,000 hours; this rate is also showing an increasing trend, about five accidents/100,000 hours in 2003. Figures 11 and 12 show the number of fatal accidents and the number of fatalities per 100,000 hours for the three types of helicopters. Helicopters with reciprocating engines appear more susceptible to accidents involving fatalities per 100,000 hours. Figure 13 shows the accidents in which helicopters conducting 14 CFR Part 91 (general aviation) and 14 CFR Part 137 (agricultural application) operations were involved. Helicopters conducting general aviation operations were involved in 70% to 80% of all civil helicopter accidents. Helicopters involved in agricultural application operations accounted for about 10% of all the accidents. 18 0 5 10 15 20 25 1999 2000 2001 2002 2003 US Civil Helicopter Accident Rate per 100,000 hours flown Reciprocating engine Single Turbine Multi-engine Turbine Figure 10. U.S. Civil Helicopter Accident Rates by Engine Type 0 1 2 3 1999 2000 2001 2002 2003 Year Fatal accidents/100,000hours US Civil Helicopter accidents: Fatal accidents/100,000 hours Reciprocating engine Single Turbine Multi-engine Turbine Figure 11. U.S. Civil Helicopter Accidents: Fatal Accidents/100,000 Flight Hours by Engine Type 19 0 1 2 3 4 5 1999 2000 2001 2002 2003 Reciprocating engine Single Turbine Engine Multi-eingine turbine Figure 12. U.S. Civil Helicopter Accidents: Fatalities/100,000 Flight Hours by Engine Type 0 10 20 30 40 50 60 70 80 90 1996 1998 2000 2002 2004 Year Accidents as % of Total FAR 91: General Aviation FAR 137: Aerial Application 14 CFR 91: General Aviation 14 CFR 137: Agricultural Application 2003 Figure 13. U.S. Civil Helicopter Accidents by Type of Operation 3.3 WIRE STRIKE ACCIDENTS INVOLVING U.S. CIVIL HELICOPTERS. 3.3.1 Overview of Wire Strike Accident Data. Table 3 provides some details of wire strike accidents for the years 1994 through 2004. The primary cause of these accidents was the helicopter impacting a power line, static wire, telephone wire, cable, or with a supporting structure such as a tower. There were a total of 124 wire strike accidents in which 41 were fatal accidents. The accidents resulted in 65 fatalities, 45 serious injuries, and 42 minor injuries. 20 Table 3. U.S. Civil Helicopter Wire Strike Accidents Year 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Number of wire strike accidents 14 10 8 9 13 12 14 12 16 11 5 Number of fatal accidents 7 6 2 1 5 2 4 5 6 2 1 Number of serious injuries 8 8 0 3 4 5 4 3 6 1 3 Number of minor injuries 8 6 1 4 4 1 5 4 4 5 0 Uninjured 6 1 4 5 8 12 4 7 9 7 8 Number of fatalities 11 9 2 4 9 3 5 8 9 3 2 A comparison of the wire strike accident data for the periods 1970-1979 and 1994-2003 is shown in table 4. The data for the years 1970-1979 was extracted from reference 1. Table 4 shows that there is a 40% decrease in the number of wire strike accidents in 1994-2003 compared to those in 1970-1979. However, there is an increase of 76% in the number of fatalities, although the number of serious injuries has decreased by 13% between 1994 and 2003. Table 4. Comparison of Wire Strike Statistics for Two Decades Period 1970-1979 1994-2003 Total number of wire strikes 208 124 Number of fatalities 37 65 Number of serious injuries 52 45 Number of minor injuries 85 42 Total number of injuries and fatalities 174 152 Number of injuries and fatalities per wire strike 0.84 1.24 As mentioned in section 3.2.1, a number of key words need to be used when querying the NTSB database to compile a report on a particular type of accident. Because researchers do not use the same keywords to query the database, the result is different statistics being quoted by different researchers. For example, figure 14 shows a comparison of accident data generated in the present study with similar data from Freest [11] and HAI [12]. Even though there is a difference in the number of accidents reported per year, there is fair agreement between the three sets of data. 21 0 2 4 6 8 10 12 14 16 18 Number of wire strike s Present Feerst HAI 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Year Figure 14. Comparison of Yearly U.S. Civil Helicopter Wire Strike Accidents Obtained in Three Different Studies 3.3.2 Wire Strike Statistics by Type of Operation. Table 5 presents data on helicopter wire strikes in each year for the period 1994 through 2004 by the type of operation. Table 6 presents the same data in the form of percentage per year. For the period 1994-2004, general aviation operations accounted for approximately 60% of all the wire strike accidents, while agricultural operations accounted for approximately 27% of the accidents. Table 5. U.S. Civil Helicopter Wire Strike Accidents by Type of Operation Year 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 14 CFR Part 91: General Aviation 8 6 4 5 8 6 7 8 12 6 4 14 CFR Part 133: Rotorcraft External Load 0 1 1 0 1 2 0 0 0 0 0 14 CFR Part 135: Air Taxi and Commuter 1 1 0 1 2 1 0 0 0 0 0 14 CFR Part 137: Agricultural 5 2 2 3 2 2 7 2 4 5 0 Other (Public Use) 0 0 1 0 0 1 0 2 0 0 1 Total 14 10 8 9 13 12 14 12 16 11 5 22 Table 6. U.S. Civil Helicopter Wire Strike Accidents as Percentage of Wire Strikes Year 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 14 CFR Part 91: General Aviation 57 60 50 56 62 50 50 67 75 55 80 14 CFR Part 133: Rotorcraft External Load 0 10 13 0 8 17 0 0 0 0 0 14 CFR Part 135: Air Taxi and Commuter 7 10 0 11 15 8 0 0 0 0 0 14 CFR Part 137: Agricultural 36 20 25 33 15 17 50 17 25 45 0 Other (Public Use) 0 0 13 0 0 8.3 0 17 0 0 20 As a comparison, the percentage breakdown of civil helicopter accidents was general aviation: between 70% and 80% and agricultural: 10% (figure 13). Table 7 provides a comparison of wire strike accidents by the type of operation for the two periods: 1970-1979 and 1994-2003. In 1970-1979, general aviation operations accounted for 42% of the wire strikes, while agricultural operations accounted for about 48% of all wire strike accidents. Figure 15 shows the wire strike accidents by type of operation for 1994-2003. Figure 16 shows the yearly distribution of wire strike accidents as a percentage of total in general aviation and agricultural application operations. The wire strike percentage in general aviation operations has increased since about 1999. Table 7. Comparison of Wire Strike Accidents by Type of Operation (1970-1979 and 1994-2003) Period 1970-1979 1994-2003 14 CFR Part 91: General Aviation 41.8 59.7 14 CFR Part 133: Rotorcraft External Load 1.9 4.0 14 CFR Part 135: Air Taxi and Commuter 0.6 4.8 14 CFR Part 137: Agricultural 48.0 27.4 Other (Public Use) 7.7 4.0 It may appear that helicopters involved in general aviation operations account for about 60% of wire strikes while helicopters engaged in agricultural operations account for about 27% of the wire strikes is a reflection of the overall accident spectrum. However, it was that all wire strike accidents occur at heights of less than about 150 feet AGL, an altitude band normally used for takeoff and landing operations for general aviation operations. Taking this factor into consideration, the proportion of wire strike accidents in general aviation operations is very high. 23 General Aviation (60%) Agriculural (27%) Air Taxi and Commuter (4.8%) Rotorcraft External Load (4%) Public Use (4%) Figure 15. Civil Helicopter Wire Strike Accidents by Type of Operation for 1994-2003 0 20 40 60 80 1994 1996 1998 2000 2002 2004 Year Part 91: General Aviation Part 137: Aerial Application Part 137: Agricultural Application Figure 16. U.S. Civil Helicopter Wire Strike Accidents by Type of Operation 3.3.3 Wire Strike Statistics by Type of Helicopter. Table 8 gives the list of helicopters involved in wire strike accidents and the number of accidents each year for the period 1994-2004. In this table, generic names have been used for some helicopters and all the variants have not been differentiated. For example, Bell 206 is used to designate the 206B, 206B-3, and 206L-1 models. The Bell 47 is used to designate the 47G, 47G-2, 47G-2A, 47G-5, and 47D1 models. Robinson R22 is used to designate the R22, R22A, R22B, R22-BETA II models. Three R44 helicopters were also involved in wire strike accidents. These have been included in the list of R22 accidents. 24 Table 8. Helicopters Involved in Wire Strikes Type 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Bell 206 4 3 2 1 2 1 1 1 3 3 2 MBB BO-105S 2 Eurocopter AS-350 1 1 1 1 1 Robinson R22 3 4 4 3 3 3 2 Hiller HU12V 1 2 2 3 1 Bell 47 2 3 2 2 3 4 2 5 2 MD369 3 3 1 1 1 Hughes 269 1 1 1 3 Enstrom F-280 1 1 1 2 Bell 407 1 MD-900 1 BK-117 1 Bell 212 1 Bell OH-58 1 1 Sikorsky S 58 1 1 Others 1 1 1 1 1 2 1 2 Figure 17 shows the percentage of wire strike accidents for each of the helicopter types that have been involved. The Bell 47 and its variants account for about 20% of all the wire strike accidents while the Bell 206 and its variants come close with 18.6%. The Robinson R22 (including three R44 models) accounts for another 15.3% of the accidents. Other helicopter models such as the Eurocopter AS-350 and Enstrom F280 accounted for about 31% of the wire strike accidents. Figures 18, 19, and 20 show the number of wire strike accidents for the years 1994 to 2004 for the Bell 206, Bell 47, and Robinson R22 models. Of the helicopters involved in wire strike accidents (table 8), FAA-approved wire strike protection kits (wire cutters) are available for the following models: Bell 205, 212, and 407, Eurocopter AS-350 and BK-117, and McDonnell Douglas (MD) helicopters MD-900. As shown in table 8, these helicopter models account for 32 of the 124 helicopters (25.8%) involved in wire strike accidents. FAA-approved wire cutters were not available for the other helicopters mentioned above. Thus, nearly 74% of the helicopters involved in the wire strikes could not have been fitted with wire cutters. 25 Bell 206 (18.6%) Robinson R22 (15.3%) Bell 47 (20.2%) MD 369 (7.3%) Hiller UH12 (7.3%) Others (31.4%) Figure 17. Helicopter Wire Strike Accidents by Model 0 1 2 3 4 94 95 96 97 98 99 00 01 02 03 04 Figure 18. Wire Strike Accidents Involving Bell 206 26 0 1 2 3 4 5 94 95 96 97 98 99 00 01 02 03 Figure 19. Wire Strike Accidents Involving Bell 47 0 1 2 3 4 94 95 96 97 98 99 00 01 02 03 04 Figure 20. Wire Strike Accidents Involving Robinson R22 3.3.4 Wire Strike Accidents by Probable Cause. In addition to a narrative description of the accident, the NTSB accident reports also include a list of probable causes in the case of completed investigations. A study of the NTSB wire strike accident reports (given in appendix B) shows the probable causes for most of the accidents are one or more of the following reasons: • Inadequate visual lookout (38 accidents) • Failure to maintain sufficient clearance with the obstacle (59 accidents) • Failure to maintain proper altitude (9 accidents) 27 • Other reasons for 19 accidents include improper judgment (e.g., decision to continue flying VFR under IFR conditions), inadequate preflight planning, and failure to see and avoid wires, intentional buzzing (low-altitude flight), and selection of unsuitable area for landing. Thirty-seven of the fatal wire strike accidents occurred in day visual meteorological conditions. Two accidents occurred in day instrument flying conditions and two occurred in night instrument flying conditions. Thus, 86% of the fatal accidents occurred in clear weather with good visibility. The probable causes have not been identified for all the accidents included in appendix B. 3.3.5 Analysis of Fatal Wire Strike Accidents. During the period 1994-2004 (September), there were 43 fatal accidents. Tables 9 and 10 give the profile of the age and flight experience of the pilots involved in the accidents. These data are available for 41 of the 43 accidents. Table 9. Age Profile of Pilots Involved in Fatal Wire Strike Accidents During 1994-2003 Age (years) 29 and under 30 to 39 40 to 49 50 to 59 60 and above Number 4 5 13 13 6 Table 10. Helicopter Flight Experience of Pilots Involved in Fatal Wire Strike Accidents During 1994 Experience (hours) 0-200 201-500 501-1000 1001-2000 >2001 Number 7 1 6 4 23 The majority of the accidents involved pilots between the ages of 40 and 59 (78%) and who had more than 2000 hours (56%) of rotorcraft flight experience. The average age of the pilots was 47.3 years and the average flight experience was about 3575 hours. For the period 1970-1979, Tuomela and Brennan [1] found that 95% of the wire strike accidents occurred in weather conditions with clear skies and unlimited visibility. The average age of the pilots involved in (all) wire strike accidents was about 34 years, and the average flight experience of the pilots was over 2200 hours. Figure 21 shows the distribution of civil helicopter fatal wire strike accidents during 1994-2003 by type of operation. Helicopters operating under general aviation (14 CFR Part 91) account for 65% of all fatal wire strike accidents. Agricultural operations (14 CFR Part 137) and rotorcraft external load (14 CFR Part 133) account for 14% and 12% of the fatalities. 28 0 10 20 30 40 50 60 70 General Aviation (65%) Agricultural (14%) Air Taxi & Commuter (7%) Rotorcraft External Load (12%) Public Use (2%) Figure 21. U.S. Civil Helicopter Fatal Accidents by Operation (1994-2003) Figure 22 shows a comparison of the proportion of all wire strike accidents (as percentage of the total) with the fatalities in each type of operation. Operations under 14 CFR Part 91 general aviation account for about 65% of wire strike accidents and for about 60% of the fatalities. Agricultural operations account for about 27% of wire strike accidents and about 14% of fatalities in wire strikes (14 CFR Part 137). 0 10 20 30 40 50 60 70 All accidents Fatalities Rotorcraft External Load Air taxi and Public Usage Agricultural General Aviation (% of total) Figure 22. Comparison of Accidents and Fatalities by Type of Operation (1994-2003) Table 11 shows the helicopters involved in fatal wire strike accidents. Bell 206 models were involved in nearly 26% of the fatal wire strikes, Robinson R22 models were involved in about 21% of the fatal accidents. The MD 369 and Bell 47 models accounted for 14% and 12% of the fatal accidents, respectively. 29 Table 11. Helicopters Involved in Fatal Wire Strike Accidents Type 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Total Percent Bell 206 1 3 1 1 1 2 1 1 11 25.6 AS-350 1 0 1 1 3 7.0 Tomcat MK5A 1 0 1 2.3 Robinson R22 2 0 2 1 1 2 1 9 20.9 Hiller HU-12E 1 0 1 2.3 BO-105S 1 0 1 2.3 Bell 47 1 1 1 1 1 5 11.6 MD 369 2 1 1 1 1 6 14.0 Bell 407 1 1 2.3 Bell UH 1B 1 1 2.3 Hughes 269 1 1 2.3 Enstrom 280FX 1 1 2.3 Robinson R44 1 1 2.3 Sikorsky S58ET 1 1 2.3 Helicopters flying 14 CFR Part 137 (agricultural) and 14 CFR Part 133 (rotorcraft external load) operations need to fly close to the ground in order to execute their missions. It is natural to expect that these helicopters would be involved in more wire strikes than helicopters involved in 14 CFR Part 91 (general aviation) operations. However, it is surprising to note from figures 21 and 22 that the helicopters in general aviation account for 60% of all wire strikes and for 65% of fatalities in wire strike accidents. Note that most wire strike accidents occur under visual meteorological conditions, and that the most often cited probable cause for these accidents is the pilot’s “inadequate visual lookout.” It appears that most of the wire strike accidents in general aviation operations were potentially preventable. 3.3.6 Summary of Civil Helicopter Wire Strike Incidents. The total number of accidents of civil helicopters shows an increasing trend from 1997 onwards. There were 212 accidents involving 67 fatalities in 2003. The number of accidents per 100,000 flight hours also shows an increasing trend from 1996 onwards. There were ten accidents per 100,000 flight hours in 2003. In 2003, the number of accidents for fixed-wing aircraft was slightly above 0.3 accidents per 100,000 hours. Thus, the rate of accidents per 100,000 hours for fixed-wing aircraft is 3% of the accident rate for helicopters. Helicopters conducting general aviation operations were involved in 70% to 80% of all the accidents to civil helicopters. 30 Helicopters involved in agricultural application operations accounted for 10% of all the accidents. Between 1994 and 2004, there were 124 wire strike accidents involving 41 fatal accidents. The accidents resulted in 65 fatalities, 45 serious injuries, and 42 minor injuries. There was a 40% decrease in the number of wire strike accidents during the period 1994-2003 compared to 1970- 1979. However, there is an increase of 76% in the number of fatalities, but the number of serious injuries decreased by 13% in 1994-2003. There were 1.24 fatalities and injuries per wire strike during 1994-2003 compared to 0.84 during 1970-1979. For the period 1994-2004, general aviation operations accounted for approximately 60% of all wire strike accidents while agricultural operations accounted for approximately 27% of the accidents. During 1970-1979, general aviation operations accounted for 42% of the wire strike accidents while agricultural operations accounted for about 48%. The wire strike percentage in general aviation operations has been increasing since 1998. The Bell 47 helicopter and its variants account for about 20% of all the wire strike accidents while the wire strike accidents to the Bell 206 and its variants come close, accounting for about 18.6%. The Robinson R22 helicopter (including three R44 models) accounts for another 15.3% of the wire strike accidents. Wire strike protection kits could potentially have been fitted on 32 of the 124 helicopters (25.8%) involved in wire strike accidents, leaving nearly 74% of the helicopters without wire strike protection. The probable causes for most of the accidents are one or more of the following three reasons: inadequate visual lookout (38 accidents), failure to maintain sufficient clearance with the obstacle (59 accidents), and failure to maintain proper altitude (9 accidents). Other reasons (for 19 accidents) include improper judgment (e.g., decision to continue flying VFR under IFR conditions), inadequate preflight planning, and failure to see and avoid wires, intentional buzzing (low-altitude flight), and selection of unsuitable area for landing. Thirty-seven (86%) of the fatal wire strike accidents occurred in day visual meteorological conditions. Two accidents occurred in day instrument flying conditions and two occurred in night instrument flying conditions. The majority of accidents involved pilots between 40 and 59 years old (with 78% being over 40) and who had more than 2000 hours of flight experience (with 56% having over 2000 hours). The average age of the pilots was 47.3 years and the average flight experience was about 3575 hours. Helicopters operating under 14 CFR Part 91 general aviation accounted for 65% of all fatal accidents. Agricultural operations (14 CFR Part 137) and rotorcraft external load (14 CFR Part 133) account for 14% and 12% of the fatalities, respectively. Because most wire strike accidents occur under visual meteorological conditions and the most often cited probable cause for these accidents is the pilot’s “inadequate visual lookout,” it appears that most of the wire strike accidents in general aviation operations could have been potentially prevented. 31 3.4 WIRE STRIKE PROTECTION AND WARNING SYSTEMS. A number of devices are currently available that can provide helicopter pilots warning of the proximity of wires. In addition, a combination of wire deflectors and wire cutters has been available for some time. A description of these devices are discussed in the following sections. The data for each system were collected from the manufacturer’s websites and from discussions with their personnel. The performance figures for each system have been given as stated by the respective manufacturers but have not been independently verified in this research. 3.4.1 Wire Strike Protection System. Wire impacts have serious consequences to helicopters. For example, a power transmission cable can slice through the windshield if the helicopter impacts the cable at sufficient forward speed. One system that can provide a degree of protection to the helicopter in frontal impacts is the WSPS, manufactured by Bristol Aerospace Limited. A typical installation consists of a roof- mounted cutter and one or more cutters mounted on the fuselage of the helicopter. A deflector running vertically along the middle of the windshield guides the cables into the cutters. Figure 23 shows the system installed on a Bell 206. Wire cutter Figure 23. The WSPS Installed on a Bell 206 [13] The effectiveness of the WSPS was verified by the U.S. Army through pendulum-swing tests conducted at the National Aeronautics and Space Administration Impact Dynamic Test Facility at Langley, Virginia. Table 12 lists the models and costs of WSPS for helicopters that have FAA approval for fitment of WSPS. The installation of WSPS can be performed by a qualified helicopter mechanic in about 40 hours. 32 Table 12. Price of WSPS for Helicopter Models Helicopter Price (U.S. $) Bell 204, 205, 212, 412 8,515 Bell 206A, B, L, L-1, L-3, L-4 6,870 Bell 222, 230, 430 34,000 Bell 407 7,370 Bell 427 13,995 MD Helicopters 500, 500C, 500D 7,450 MD Helicopters 500E, 500N, 530FF 7,450 MD Helicopters 600N 12,995 Eurocopter AS-350 13,995 Eurocopter AS-355 13,995 Sikorsky S-76 20,000 For the cutters to be effective, the helicopter must be flying at speeds greater than 30 knots [14]. The manufacturer states that if a helicopter with a WSPS impacts a wire at an angle less than 60° to the wire, the WSPS may not cut the wire (figure 24). In addition, the maximum pitch angle at which the WSPS should strike the wire and still be effective is ±5°. The system is designed to cut a 3/8-inch steel cable with a breaking strength of 12,000 lb [14]. Figure 24. Maximum Wire Strike Angle [15] If wires are contacted during a flight, the manufacturer suggests that the cutter blades be immediately replaced [16]. No measurements of the additional drag created by the WSPS have been made. 33 3.4.2 Powerline Detector. The Powerline Detector, manufactured by Safe Flight Instrumentation Corp., is a system that senses the electromagnetic fields surrounding power lines and uses this information to alert the pilot to the proximity of wires. The system consists of an electronics unit mounted in the cockpit (figure 25) and a whip antenna mounted on the fuselage. The antenna can be placed on the nose, roof, or tail boom of the aircraft. The electronic unit measures 1.66″ x 2.42″ x 5.51″, and the total system weight is approximately 3 pounds [17]. Figure 25. Powerline Detector Electronics Control [17] If a power line is detected in the flight path, the system gives the pilot an audible warning starting from a distance of 1800 feet from the hazard. The warning sound increases in frequency as the power line gets closer. If the pilot continues to approach the power line, a red warning light illuminates in the cockpit. The system can be temporarily muted when flying in an area where there are a number of power lines [17]. The Powerline Detector is compatible with all civil and military helicopters, and Safe Flight suggests that the system be installed by a certified avionics shop or by the manufacturer. The installation takes approximately two days to complete. The unit requires 28 Volts direct current (Vdc), which is provided by the aircraft, and detects the power line signals at 60 Hz. Safe Flight has tested the lifespan of the detector to be greater than 10,000 flight hours. The retail price of the product is $11,800, not including installation [17]. The aerodynamic drag of the whip antenna is not known, but it is likely to be negligible, especially if it is mounted on the tail boom. This detector only senses active power lines, and the range of detection depends on the electrical power in the lines. The system will not detect other types of wires such as guy wires, weak telephone lines, and nonactive power lines. Also, the 34 pilot does not get an indication about the direction of the power lines with reference to the aircraft. 3.4.3 The OASys Radar. The OASys radar (Amphitech Systems) uses a radar mounted on the nose of the helicopter to transmit a 35-GHz radio frequency for detecting obstacles in the flight path. The radar constantly searches for obstacles in its field of view. At a given instant of time, the system uses data from the aircraft’s GPS receiver to calculate the aircraft’s flight path for the succeeding 19 seconds. Three zones of increasing potential risk are then designated by the system. If the obstacle is located in any of these three zones, the pilot is given a level of alert that depends on the proximity and zone of the obstacle and is notified by a small display in the cockpit that illuminates in accordance with the distance and direction to the obstacle. OASys also adapts to the requirements of the flight. For instance, during takeoff, the system scans a wide angle around the helicopter, but only a short distance in front of it. However, when the aircraft is cruising at high speeds, the scan narrows, but looks far ahead in the flight path [10]. These zones are shown in figures 26 and 27. Figure 26. Zones of Scan, OASys (Hover to 5 knots) Figure 27. Zone of Scan at 100 Knots, OASys The OASys radar has a maximum range of 1600 meters, an optimal range of 800 meters, and a minimum range of 2 meters. This system is also effective in rain and fog. The physical unit consists of an electronics box mounted inside the helicopter and a sensor mounted on the nose of 35 the helicopter (figure 28). The sensor dimensions are 15.48″ x 16.78″ x 22.44″, and the total weight is approximately 54 pounds. Figure 28. The OASys Scan Head [19] OASys can currently be installed on the Bell 212, Bell 412, and Eurocopter AS-350 models, with Amphitech installing the system. Eventually, Amphitech plans to sell the radar system as a kit that any helicopter mechanic can install in less than 24 hours. Amphitech is currently working on obtaining approval for installation of OASys in all helicopter models in the U.S. and Europe [18]. The expected lifespan of the OASys radar is 20,000 flight hours or 20 years with proper maintenance. A certain part of the system must be replaced every 2500 flight hours, but this process can be completed by a mechanic in less than 1 hour. The OASys costs $170,000, not including installation. Installation can cost somewhere between $5,000 and $10,000 [18]. Though the mounted sensor may be large, it produces little added drag on the helicopter (see figure 28). The manufacturer, Amphitech, estimates that there will be a loss of 1 knot when cruising at 140 knots. OASys does not detect objects behind or directly above the aircraft. Thus, the system will not warn the pilot of wires near the tail rotor while hovering or taking off. 3.4.4 Laser Obstacle Awareness System. The LOAS (Goodrich Sensor Systems) detects small obstacles, such as wires, and feeds these data into the aircraft’s targeting, control, display, or navigation systems. It uses an eye-safe laser radar to sense the obstacles. The system is composed of a scan head mounted on the fuselage and an electronics control box inside the cockpit. The system weighs about 35 pounds, and the cylindrical scan head measures 8.66″ in diameter and 15″ high [20]. The LOAS requires 28 Vdc from the helicopter and uses less than 300 watts of power. Goodrich states that the mean time between failures for the electronics and optical components of the system is greater than 1000 hours. However, the laser component’s mean time between failures 36 is lower than this. Goodrich is currently re-evaluating this part of the system to improve its performance. The LOAS is an adaptable system that can be implemented in naval ships and airplanes as well as in rotorcraft. Thus, the system is intended to be compatible with all commercial rotorcraft and has been successfully tested on the Eurocopter EC155, Bell 412, and Bell UH-1H. The system has the capability of being pointed in the direction desired by the pilot [21]. The range of the system varies from 2 meters (minimum) to 2 kilometers (maximum). The scan head has a horizontal field of view of 180° and a vertical field of view of +30° to -90° [20]. The scan head and electronics box are shown in figure 29. Figure 29. The LOAS Scan Head and Electronics [20] The LOAS can be installed by Goodrich or by the helicopter manufacturer. Goodrich is working with third-party companies to design installation mounts that can be purchased separately so that the LOAS can be installed by the consumer on various helicopter models. The mount and sensor can be placed anywhere on the fuselage, but the system will perform best if it is located at the nose of the helicopter. Goodrich does not provide an exact price for the system, but an estimated cost would be approximately $100,000, without installation [21]. No drag measurements have been made, but Goodrich states that the drag associated with LOAS is similar to the drag produced by current forward looking infrared mounts [21]. Because of its weight (35 lb), the LOAS can be considered for fitting on larger helicopters. 3.4.5 Laser Radar Visual Display. EADS Dornier’s Helicopter Laser Radar (HELLAS) is an obstacle detection system that can sense objects as thin as wires, thus making it useful for wire strike prevention. The system uses an eye-safe laser that is mounted on the fuselage of the helicopter to give the pilot information about the surrounding environment. Both an optical display and aural warning are used for this purpose. The optical display depicts the airspace surrounding the helicopter and marks the objects that lie in the flight path of the helicopter. The aural signal is used if the pilot moves the 37 rotorcraft too close to the obstacle [22]. The system is connected to the helicopter by a standard interface, which requires 28 Vdc [23]. Figure 30 shows the optical display box installed on the fuselage. HELLAS Figure 30. HELLAS Installed on UH-60 Helicopter HELLAS is useful as a pilot aid under low visibility conditions. The optical display gives the pilot a view of the environment at night or during inclement weather so that it is possible to pilot the aircraft solely by images from the display. The range of the system decreases in adverse weather. EADS states that the range decreases from 3280 feet in VFR to 1312 feet in bad weather [23]. The system is also useful when sun glare prevents a pilot from visually acquiring the location of power lines. The effectiveness of HELLAS has been proven in its employment by the German Federal Border Guard. EADS Dornier was contracted to deliver 43 units installed on Eurocopter EC 135 and EC 155 helicopters [23]. The Canadian Defense Research Establishment has also tested and purchased the HELLAS unit [24]. The U.S. Army has performed more than 20 tests on the system. EADS says that a more accurate HELLAS unit has been created for the Army’s nap-of-the-earth operations, which requires extreme accuracy during flight maneuvers [23]. Because of the high level of integration into the helicopter, this system is currently installed by EADS Dornier only. HELLAS is said to be compatible with all helicopters and has been successfully implemented on a few models, i.e., the Eurocopter BK 117, EC 135, EC 145, EC 155, Sikorsky CH-53, and Bell UH-1D. EADS Dornier does not list a price for the HELLAS system, but the price estimated is above $100,000 [24]. 38 The HELLAS system has several attractive features that make it useful to the pilot, especially in bad weather (rain and snow) conditions. However, its high cost makes it uneconomical for most light helicopters. 3.4.6 Ground Proximity Warning System. Honeywell’s EGPWS uses a database, maintained by Honeywell, of all obstacles in North America that are 30 feet AGL or higher [25]. The helicopter needs to be equipped with a GPS antenna and an EGPWS computer unit. When the aircraft approaches an obstacle or wire, the pilot is given an audible warning. If the pilot continues on the flight path, a louder aural warning is given. Together with a light inside the cockpit, the audio warning tells the direction towards which the pilot should fly to avoid an obstacle strike. For instance, if a pilot is slowly descending while cruising forward, the EGPWS will aurally tell the pilot to “pull up” to avoid a set of power lines. The system also provides other features such as excessive descent rate call- outs and altitude call-outs [26]. EGPWS is designed to work with all helicopters built in North America. The system consists of a GPS antenna located on the fuselage and an electronics unit located inside the cockpit. The total weight of the EGPWS is approximately 3.9 pounds. The system requires 28-Vdc power. Honeywell states that the life of the EGPWS units is more than 10 years. The system can be installed at a qualified avionics shop [26]. EGPWS is limited in its alerts to prevent wire strikes. The database of obstacles includes only those which are 30 feet AGL or higher. Therefore, any wires that are lower than 30 feet off the ground are not automatically made known to the pilot. Furthermore, Honeywell is constantly updating the database, and acknowledges that all wires above 30 feet are not in the database. The EGPWS costs about $45,000, excluding installation [25]. 3.4.7 Passive Tower-Based System. The Obstacle Collision Avoidance System (OCAS) is manufactured by OCAS AS of Norway and is different from the other systems because it is not installed on the helicopters. The system has been refined in formal cooperation with the Royal Norwegian Air Force, Norwegian Civil Aviation Administration, Civil Aviation companies, and Norway’s utility company. According to OCAS AS, the system can be installed nearly anywhere because the OCAS is relatively small. The unit has a modular mast design, which consists of one to three 9-foot, 375- pound tubes. The number of tubes varies, depending on the topography of the region. The unit is powered by either a 220/110-volt alternate current (Vac) connection or self-powered by solar panels and a battery bank. If it is not being operated on solar power, the system can be connected to the nearby power lines. OCAS was originally designed for rural areas of Norway where there is very little sunlight during the winter, and is intended to use as little power as possible. The target detection range is 1.7 to 3.0 miles, depending on the target size, and the very high frequency (VHF) range is approximately 19 nautical miles, depending on radio service area [27]. 39 Typically, the OCAS is located on utility and power line towers and detects all air traffic entering a predefined warning zone and activates warning lights that illuminate the tower. If no evasive action is taken by the pilot, a voice/audio warning is transmitted on all channels in the airborne VHF or optional ultra-high-frequency (UHF) communication bands. This system also allows the lights on large utility towers to be switched off unless needed [27]. The OCAS also records each encounter with a nearby aircraft. The system’s computer documents the speed, position, and size of the aircraft, among other details, and sends this information to a central database, which is usually located at the utility company or tower owner. The records can be used to create historical data of all air traffic in the area [27]. OCAS is not currently approved for use in the U.S. The FAA plans to begin testing the unit in September 2004. The target price for the unit is $50,000, and the cost is likely to be covered by the utility companies [27]. The helicopter users only need a functioning VHF radio in their aircraft to use this system. The system is beneficial to adjoining property owners because the towers are illuminated only when an aircraft penetrates the warning zone. Note that the towers are illuminated but not the wires. Therefore, there is no certainty that the pilot will be able to see the unlit wires. There are also questions about the effectiveness of OCAS if multiple aircraft are in the detection zone. 3.4.8 Aerial Markers. SpanGUARD Helimark Aerial markers, manufactured by P&R Technologies, are colored spherical balls that are attached to wires and power lines to make them more visually prominent to aircraft pilots. The spheres are available in orange, yellow, and white, and in varying diameters. The markers use a wire rod that wraps around the line to grip the wire. The construction includes the rods connected to the outside of the marker and lock nuts for easier live line installation. These markers use a clam shell design so that they can be installed easily by hand from a helicopter or installed before the power lines are strung up. Aerial markers should last many years. They are designed to minimize color degradation caused by the sun. P&R Technologies does not estimate an official life span for the markers, but cites several counties in the U.S. where the markers have been in place for over 20 years [28]. Figure 31 shows a marker being installed from a helicopter. The weight and cost of some spherical markers are given in table 13. 40 Figure 31. SpanGUARD Installation [29] Table 13. SpanGUARD Helimark Specifications [28] Diameter Weight With Rods (lb) Weight Without Rods (lb) Price (each) 9 inches 4 20 inches 6 5 $120 24 inches 11.5 10 $186 36 inches 19 17 $378 Aerial wire markers are a passive method of wire strike protection and rely on the pilot’s ability to spot them during flight. Even though wires with markers are more visible than unadorned wires, they may be of little assistance to an inattentive or sun-blinded pilot. Some spherical markers mounted on power lines have been designed to glow as a result of the electrical field surrounding the power line. It is recommended that 36-inch-diameter markers be used over rivers, canyons, and lakes, while 20-inch-diameter markers be used within 1500 feet of airports on wires less than 50 feet AGL. Markers can be spaced a maximum of 200 feet apart. If multiple wires are strung between towers, then the markers should be placed on the highest wire [30]. The total cost can vary greatly when installing the aerial markers, depending on the number of markers that are necessary for the length of power line and the time it takes to install them. 41 3.4.9 Summary of the Systems. Table 14 summarizes some of the main data on the systems described in the previous sections. In this table, “Install by” refers to whether the system can be installed by the helicopter owner or mechanic, “Compatibility” refers to whether the system is available on all helicopters, and “Range” refers to detection range. Table 14. Summary of Wire Protection and Detection Devices System Cost Weight (lb) Install by Helicopter Compatibility Power Life Range (feet) WSPS $6,870-34,000 20-35 Owner/mechanic See table 8 N/A Until wire strike N/A Powerline $11,800 3 Owner/mechanic All 28 Vdc >10,000 hours 1,800 OASys $170,000 54 Manufacturer Few, currently 28 Vdc 20 years 5,249 LOAS <$100,000 35 Manufacturer Few, currently 28 Vdc >10,000 hours 6,561 HELLAS >$100,000 60 Manufacturer All 28 Vdc 3,280 EGPWS $45,000 4 Owner/mechanic All 28 Vdc 10 years 600 OCAS $50,000 375 N/A All 220 Vac Not known 15,840 SpanGUARD $120-$378 6-19 Utilities N/A N/A 20 + years N/A N/A = not applicable 4. SUMMARY. Data on wire strike accidents involving civil helicopters were presented and analyzed in sections 3.2 and 3.3. It was noted that a majority of the accidents were potentially preventable. Devices that have the potential to protect helicopters from damage after impacting wires or devices that warn the pilot about impending wire strikes were described in section 3.4. The potential of each of these devices to prevent and/or to minimize the number of fatalities and injuries in potential wire strike situations are discussed in the following sections. 4.1 WIRE STRIKE PROTECTION SYSTEM. As discussed in section 3.1, there was a significant decrease in the number of wire strike accidents in U.S. military helicopters after the installation of WSPS. In addition, there were no fatalities in wire strike accidents after installing WSPS. While the reduction in the number of fatalities is due to the WSPS, it is conjectured that the reduction in the number of wire strike accidents is because the WSPS constantly alerts the pilot to possible wire strikes. WSPS is most effective when the helicopter impacts the wires nearly perpendicular to the wires in a level altitude and at flight speeds of more than 30 knots. WSPS is not available for all currently flying helicopters. As discussed in sections 3.2 and 3.3, WSPS can only be installed in about 25% of the helicopters. The other helicopters (e.g., Robinson R22, Robinson R44, Bell 47, and MD 369) cannot be equipped with WSPS. 42 During the years 1994-2004, there were 65 fatalities in wire strike accidents, in which 21 fatalities (32%) involved the Bell-206 helicopter. The AS 350 and the Bell 407 helicopters each had 4 fatalities (6%). Thus, assuming that the trend of the past 10 years continues in the coming years, and assuming that all the wire strike accidents occur in conditions when it is most effective, equipping helicopters with WSPS can reduce about 44% of the fatalities. However, this is an overestimate because most of the accidents occurred during the climb and descent phases of flight and involved wire strikes of the main rotor system, the tail boom, or the vertical tail. 4.2 POWERLINE DETECTOR. The Powerline Detector System (Safe Flight Instrumentation Corporation) senses the electromagnetic fields surrounding power lines. This detector only senses active power lines, and the range of detection depends on the electrical power in the lines. The system will not detect other types of wires such as guy wires, telephone lines, and nonactive power lines. Also, the pilot is not alerted to the direction of the power lines with reference to the aircraft. Since the retail price (not including installation) is $11,800 and because it needs a 28-volt power source, this system may be best suited for more expensive helicopters. This system has the potential to be developed for use in light helicopters if it can be manufactured at a lower cost and use less power. 4.3 HELICOPTER-INSTALLED RADAR AND LASER-BASED SYSTEMS. Devices that use lasers or radar to scan the surroundings for the presence of obstructions such as wires are OASys (Amphitech System), LOAS (Goodrich Sensor System), and HELLAS (EADS Dornier). These are comparatively heavy (between 35 and 60 lb) and expensive devices (more than $100,000) for most civil helicopters. Developing less expensive devices that can be fitted in light helicopters, such as the Robinson R22, can be helpful to about 75% of the helicopters. 4.4 HONEYWELL EGPWS. The EGPWS can warn pilots about obstacles that are over 30 ft AGL using GPS and a database that is maintained by Honeywell Aerospace. The EGPWS weighs about 4 lb and costs $45,000, which makes the system too expensive for most civil helicopters. 4.5 AERIAL MARKERS. Spherical markers mounted on wires enhance their visibility and help to avoid wire strikes. The MOWAT Work Group [9] recognized the importance of wire markers in preventing wire strikes. 43 4.6 OBSTACLE COLLISION AVOIDANCE SYSTEM. The OCAS is located on utility and power line towers and detects all air traffic entering a predefined warning zone and activates warning lights that illuminate the tower. If the pilot does not take evasive action, a voice/audio warning is transmitted on all channels in the airborne VHF or optional UHF communication bands. Because the OCAS does not require installation in the helicopter, there is no expense for the helicopter operators. In spite of its cost ($50,000 per installation), it can be more economical for the utility company because the utility towers are lighted only when the system detects an aircraft entering the warning zone. 4.7 HUMAN FACTORS/PILOT TRAINING. It has been noted that most of the helicopter wire strikes occurred at daytime with good visibility and experienced pilots. Specialists have stated that a variety of actions by helicopter pilots can prevent wire strike accidents. Robert Feerst, President of Utilities/Aviation Specialists, teaches courses in wire strike avoidance. He recommends reviewing aeronautical charts and conducting a reconnaissance flight at a higher altitude before conducting low-altitude operations [5]. Wires are difficult to see, partly because of the way the human eye functions, and partly because of the effects of camouflaging. The movement of wires in sunlight and changing sunlight patterns can also obscure wires. Older wires may be difficult to see because, as they age, their color often changes. For example, copper wires oxidize with age, turning a greenish color that makes them difficult to distinguish from grass and trees in the background. The exact location of specific wires may change throughout the day because of fluctuating ambient temperatures, which may cause wires to sag or tighten. Sagging wires may also be blown by the wind [5]. The pilot’s view of the outside can be affected by the canopy and the vibratory environment of the cockpit. The canopy may not always be clean, which contributes to lower visibility. It is known that human performance deteriorates after prolonged exposure to certain vibrations. For example, the human eyeball and intraocular structures have natural frequencies in the 20- to 90- Hz range [31]. Most helicopters have structural frequencies in the 20-Hz range. More human factors research is needed to understand how the human eye perceives obstacles, such as wires, in this environment. Pilots need to be trained to recognize different types of wires, including power and guy wires, and how to anticipate the location of wires. They need to know the power grid system and how to determine wire direction from the location of insulating connectors on various towers. The MOWAT Work Group [9] recognized the importance of training in preventing wire strikes and has made two recommendations to the FAA. (1) A modification to the PTS to state that “Applicant should be aware of low level hazards and be knowledgeable in recognizing wires, towers and other low level hazards.” (2) one-word change in Section 15(a) of 14 CFR Part 77. This recommendation deals with objects that are shielded by existing structures or by natural terrain of equal or greater height. Both recommendations are under consideration by the FAA (in 2004). 44 4.8 RECOMMENDATIONS. Recommendations based on the study are as follows: a. All helicopter pilots flying 14 CFR Part 91 (general aviation) should avoid cruising below 750 ft, unless required to do so by the mission. b. Encourage pilots to review aeronautical charts and to conduct a reconnaissance flight at a higher altitude before conducting low-altitude operations. c. Develop less expensive wire proximity warning devices that can be fitted to light helicopters such as the Robinson R22. These less expensive wire proximity warning devices can be helpful to about 75% of the helicopters. d. Install mechanical wire cutters on applicable helicopters. e. Implement the recommendations made by MOWAT on wire marking and on pilot training. f. Conduct human factors studies to develop an understanding of wire identification and avoidance used by pilots in the cockpit environment. 5. REFERENCES. 1. Tuomela, Clyde H. and Brennan, Mark F., “Civil Helicopter Wire Strike Assessment Study, Volume I: Findings and Recommendations,” NASA-CR-153289, October 1980. 2. Tuomela, Clyde H. and Brennan, Mark F., “Civil Helicopter Wire Strike Assessment Study, Volume II: Accident Analysis Briefs,” NASA-CR-152390, October 1980. 3. Hart, Sandra, “Analysis of Civil Helicopter Accidents,” HeliExpo ‘98, February 15, 1998. 4. Harris, Franklin D., Kasper, Eugene F., and Iseler, Laura E., “U.S. Civil Rotorcraft Accidents, 1963 through 1997,” NASA/TM-2000-209597, December 2000. 5. Harris, Joel S., “Helicopter and Wire Strike Accidents,” Helicopter Safety, Volume 28, No. 4, July-August 2002, pp. 1-5. 6. Moorman, Robert W., “Now You See It, Now You Don’t,” Aviation Today, June 2, 2004. 7. Walker, Mal and White, Sue, “Wire Strike,” Flight Safety Australia, July-August 1999, pp. 37-38. 8. Wimsatt, William, H., “Update: Wire Strike,” www.mcmc.com/wirestrikes.html. Last visited 2004. 45 46 9. Final Report, Marking of Wires and Towers (MOWAT) Work Group, March 17, 2003. (Copy provided by Richard M. Wright, Helicopter Association International). 10. U.S. Army Safety Center (http://safety.army.mil). Last visited 2004. 11. Feerst, R., “Wire Strike Stats,” Private communication, December 5, 2003. 12. Helicopter Association International, Accident Database, www.rotor.com/cgi.bin. Last visited 2004. 13. http://www.crescentair.com/spares/wskits.htm 14. E-mail correspondence with Bernie Adelman, Mechanical Engineer and Product Assurance Manager for Bristol Aerospace Limited, June 9, 2004. 15. Bernie Adelman, Bristol Aerospace Limited. 16. Wire Strike Protection System Maintenance Manual, Bristol Aerospace Limited, http://www.bristol.ca/Downloads/WspsManual.pdf. 17. E-mail correspondence with Adrian Rosenberg, Safe Flight Engineer and Pilot, June 1, 2004. 18. Telephone interview with Martin Bissionette, Engineer and Executive Vice President, Amphitech Systems, June 10, 2004. 19. http://www.nrc-cnrc.gc.ca/images/photos/200308_iar2.jpg. 20. “Laser Obstacle Awareness System Brochure,” Goodrich Sensor Systems, 2002. http://www.sensors.goodrich.com/literature/lit_pdfs/4109_LOASYS.pdf. Last visited 2004. 21. E-mail correspondence with Bob Syring, Remote Sensing Marketing Manager at Goodrich Sensor Systems, June 22, 2004. 22. Defense Daily Network News Website http://www.defensedaily.com/cgi/av/show_mag .cgi?pub=av&mon=0201&fil