Document
NASA/TM—2005-213473
The Implications of Handling Qualities in Civil Helicopter
Accidents Involving Hover and Low Speed Flight
Daniel C. Dugan and Kevin J. Delamer (CDR-USN) National Rotorcraft Technology Center Ames Research Center, Moffett Field, California
November 2005
The NASA STI Program Office . . . in Profile Since its founding, NASA has been dedicated to the • CONFERENCE PUBLICATION. Collected advancement of aeronautics and space science. The papers from scientific and technical confer- NASA Scientific and Technical Information (STI) ences, symposia, seminars, or other meetings Program Office plays a key part in helping NASA sponsored or cosponsored by NASA.
maintain this important role.
• SPECIAL PUBLICATION. Scientific, technical, The NASA STI Program Office is operated by or historical information from NASA programs, Langley Research Center, the Lead Center for projects, and missions, often concerned with NASA’s scientific and technical information. The subjects having substantial public interest.
NASA STI Program Office provides access to the NASA STI Database, the largest collection of • TECHNICAL TRANSLATION. English- aeronautical and space science STI in the world. language translations of foreign scientific and The Program Office is also NASA’s institutional technical material pertinent to NASA’s mission.
mechanism for disseminating the results of its research and development activities. These results Specialized services that complement the STI are published by NASA in the NASA STI Report Program Office’s diverse offerings include creating Series, which includes the following report types: custom thesauri, building customized databases, organizing and publishing research results . . . even • TECHNICAL PUBLICATION. Reports of providing videos.
completed research or a major significant phase of research that present the results of NASA For more information about the NASA STI programs and include extensive data or theoreti- Program Office, see the following: cal analysis. Includes compilations of significant scientific and technical data and information • Access the NASA STI Program Home Page at deemed to be of continuing reference value. http://www.sti.nasa.gov NASA’s counterpart of peer-reviewed formal professional papers but has less stringent • E-mail your question via the Internet to limitations on manuscript length and extent help@sti.nasa.gov of graphic presentations.
• Fax your question to the NASA Access Help • TECHNICAL MEMORANDUM. Scientific and Desk at (301) 621-0134 technical findings that are preliminary or of specialized interest, e.g., quick release reports, • Telephone the NASA Access Help Desk at working papers, and bibliographies that contain (301) 621-0390 minimal annotation. Does not contain extensive analysis. • Write to: NASA Access Help Desk • CONTRACTOR REPORT. Scientific and NASA Center for AeroSpace Information technical findings by NASA-sponsored 7121 Standard Drive contractors and grantees. Hanover, MD 21076-1320
NASA/TM–2005-213473
The Implications of Handling Qualities in Civil Helicopter
Accidents Involving Hover and Low Speed Flight
Daniel C. Dugan and Kevin J. Delamer (CDR-USN) National Rotorcraft Technology Center Ames Research Center, Moffett Field, California
National Aeronautics and
Space Administration
Ames Research Center
Moffett Field, California 94035-1000
November 2005
Available from: NASA Center for AeroSpace Information National Technical Information Service 7121 Standard Drive 5285 Port Royal Road Hanover, MD 21076-1320 Springfield, VA 22161 (301) 621-0390 (703) 487-4650
The Implications of Handling Qualities in Civil Helicopter
Accidents Involving Hover and Low Speed Flight
1 2 Daniel C. Dugan and Kevin J. Delamer (CDR-USN) Ames Research Center ABSTRACT Because of increasing accident rates in Army helicopters in hover and low speed flight, a study was made in 1999 of accidents which could be attributed to inadequate stability augmentation. A study of civil helicopter accidents from 1993–2004 was then undertaken to pursue the issue of poor handling qualities in helicopters which, in almost all cases, had no stability augmentation. The vast majority of the mishaps studied occurred during daylight in visual meteorological condition, reducing the impact of degraded visual environments (DVE) on the results. Based on the Cooper-Harper Rating Scale, the handling qualities of many of the helicopters studied could be described as having from “very objectionable” to “major” deficiencies. These costly deficiencies have resulted in unnecessary loss of life, injury, and high dollar damage. Low cost and lightweight augmentation systems for helicopters have been developed in the past and are still being investigated. They offer the potential for significant reductions in the accident rate.
undertaken to pursue the issue of handling qualities and the INTRODUCTION role that they might play in causing or contributing to mishaps. National Transportation Safety Board (NTSB) Concerned by a trend of increasing accident rates, Key accident summaries were studied and analyzed for the period published the results of a survey of accidents that occurred 1993–2004 (ref. 2). Only hover, hover taxi, and low speed during hover and low speed flight involving four types of flight summaries were considered, as these represented a Army helicopters (ref. 1). The implications of handling significant but bounded portion of the total number of qualities deficiencies in the accidents analyzed in these mishaps (fig. 1). In addition to limiting the scope of the pilot error mishaps were of primary interest. The inquiry to a manageable level, this data set examined a helicopters studied spanned the range from the OH-58D unique aspect of rotorcraft operations which accounted for a Kiowa, a small reconnaissance helicopter, to the large significant portion of the civil rotorcraft mishaps. It also cargo transport CH-47, and included the AH-64 Apache aligned the study with the results of the earlier Key study attack helicopter and the UH-60 Blackhawk. All of these of military accidents. The accidents that were analyzed in aircraft were equipped with a rate command stability this current study occurred, with a few exceptions, on augmentation system (SAS) which provides pitch, roll, helicopters that had no stability and control augmentation and yaw rate damping inputs of small magnitude at and furthermore, in daylight conditions without DVE.
relatively high frequency to the control system. These Referencing the Cooper-Harper Handling Qualities Rating helicopters had been flight tested and their handling Scale (ref. 3), it is the authors’ opinion that the handling qualities were assessed as marginal. It was Key’s qualities of a large number of the helicopters involved contention that the modification of the rate command could be categorized as having from “very objectionable” systems to provide attitude command response would to “major” deficiencies. Furthermore, it implies that significantly reduce the accident rates. With an attitude extensive pilot compensation is required for adequate command system the control augmentation generally has performance and, in the worst cases, considerable increased authority, and attitude changes are proportional compensation is required to maintain control of the to stick displacement. Motivated by these results and helicopter under adverse conditions.
conclusions, a study of civil helicopter accidents was Deputy Director, National Rotorcraft Technology Center, Ames Research Center, Moffett Field, CA 94035.
CDR-USN, Navy Liaison Officer, National Rotorcraft Technology Center, Ames Research Center, Moffett Field, CA 94035.
In 1998, a Final Report of the joint Department of Defense, Federal Aviation Administration, and the 24.3% National Aeronautics and Space Administration Helicopter Hover and Low Airspeed Accident Analysis Team was published (ref. 6). It was surprising to note throughout the report that there was no Forward Flight mention of poor handling qualities, nor the resulting 75.7% failure of the pilot to maintain control of the helicopter, as Data extracted from NTSB Aviation Accident Database, accessed At http://www.ntsb.gov/ntsb/query.asp [25 June 2005] factors in any of the accidents reviewed from NTSB data.
The FAA’s National Aviation Safety Data Analysis Center (NASDAC) reports were not included, nor were Figure 1. Hover and Low Airspeed Helicopter Mishaps there recommendations in the Safety Investments section 1993–2004 (as a Percentage of All Helicopter Mishaps).
of the report (ref. 6) to improve the designs of control systems to enhance the handling qualities of the helicopters. As a result of the Accident Team’s report, a With few exceptions, the accidents reviewed resulted in workshop was held in July of 1998 in response to the substantial damage, or in some cases the destruction of the recommendations of that body. Subsequently, a report helicopter. This is not surprising since any time the main entitled “Near Term Gains in Rotorcraft Safety—Strategies rotor or tail rotor of a helicopter contacts the ground, for Investment” was published in February, 1999 (ref. 7).
substantial damage will result. The NTSB does not assign Although statistics were discussed which tied many a dollar value to damage classified as “Substantial,” and mishaps to “loss of control,” once again there were no the definition is very broad in scope: “substantial damage recommended actions to improve civil rotorcraft safety by means damage or structural failure that adversely affects the improvement of helicopter handling qualities.
the structural strength, performance, or flight character- Emphasis was placed on training, obstacle protection, risk istics of the aircraft, and that would normally require major management, accident analyses, and even the use of night repair or replacement of the affected part.” Large sums may vision goggles.
be spent to repair damage from accidents that do not meet the NTSB Part 830 definition of “substantial damage.” ANALYSIS BACKGROUND The assessment of the accident reports reviewed has, by necessity, involved qualitative and subjective judgments A study of recent (1963–1997) helicopter accidents in the by the authors. The obvious cases that did not involve U.S. (ref. 4), attributed 1,114 accidents, or 13.2% of the handling qualities, such as attempting to takeoff to a hover accidents studied, to loss of control. In the categories of with one skid tied down, were easily eliminated from single engine piston and turbine helicopters, these consideration. There were also cases where pilots ran out accidents resulted in 247 fatalities, 228 serious injuries, of available power, which often resulted in a loss of main and 319 helicopters destroyed. These categories of rotor RPM and subsequently a loss of control helicopters generally do not have stability and control authority—especially in the yaw axis. Controlled descents augmentation systems installed or available. It would not into the ground, in many cases, also had to be discounted, be possible to speculate, without a detailed study of each as did mechanical failures that resulted in loss of control.
event, that any particular number or percentage of these Others involving loss of tail rotor control power or accidents could have been prevented by the incorporation authority were more difficult. In many cases, hovering of rate or attitude command control augmentation into the downwind or in a stiff crosswind might have resulted in an flight control systems. Many of the occurrences studied in accident by a pilot with low experience; however, the reference 4 undoubtedly were also analyzed in this current experienced pilot could work his way through the study, since the time periods overlapped for some of the condition without losing control. A student or low time years. Isler and De Maio (ref. 5) noted that the chain of pilot will, in many cases, over-control the helicopter and accident events for personal and instructional missions in a use all of the available control authority. There were many large number of helicopter accidents began with loss of cases where precision was required while hovering near control. These accidents, for the low cost category of obstacles. Often in these cases, little or no position drift helicopters, accounted for 48.2% of the total accidents.
could be tolerated. At times, depending on the winds and terrain, a “skids level” attitude was imperative at touchdown to avoid the potential for a rollover or fore/aft The main rotors struck a workman on the pole, the pole pitching which could cause main rotor or tail rotor contact itself, and the helicopter rolled right in a descent and struck with the terrain. the ground. There were no engine or flight control malfunctions and the pilots were highly qualified. Four A total of 547 accidents were thoroughly reviewed from fatalities and one minor injury resulted from this accident.
the years 1993–2004. Initially, the accidents reviewed This was a precision task for an external load hookup in involved only those occurring in hovering or near an OGE hover in an unaugmented helicopter. It is our hovering flight. This review was expanded to include contention that an improved control system, to include many accidents which occurred in low speed flight either stability augmentation, could have enabled the pilot to during takeoffs, approaches to landing, or low speed achieve the high degree of precision required to hold maneuvering. Of the total, 126 (or 23%) could be position and complete the hookup.
attributed to loss of control by the pilot which was caused or aggravated by inadequate or deficient handling qualities. Case 2: A single engine, turbine powered, teetering rotor There were also ten accidents that involved gyroplanes or helicopter crashed in snow covered terrain in daylight under gyrocopters and these were discarded from the matrix. It Visual Meteorological Conditions (VMC) with 1 mile was noted that there were three fatalities, one serious visibility. The pilot was attempting to land next to a injury, and four minor injuries attributed to these ten snow gauging station and he flew past it. With the snow single pilot/occupant accidents. Pilot experience was cover and reduced visibility, the pilot had poor visual typically low and it may be inferred that flying this type references and allowed the skids to contact the terrain.
of hybrid rotorcraft is particularly challenging for Collective pitch was applied, the pilot attempted to inexperienced, though adventurous, aviators. stabilize the helicopter in hover, but it drifted forward and to the left. The left skid contacted the terrain and the Many manufacturers were represented in the accident helicopter rolled on its left side. Although there were no statistics as compared to the four models studied by Key injuries to the three on board, the helicopter was destroyed.
(ref. 1). Fifteen different manufacturers were listed to The whiteout conditions encountered resulted in a classic include homebuilt or kit helicopters consolidated as one loss of control in a degraded visual environment. An category. There were over thirty various models effective stability augmentation system could have allowed represented. the pilot to maintain a level attitude, hold position, and either land level, pull up for another landing attempt, or abort the mission.
MISHAP SUMMARY EXAMPLES Case 3: A single engine (piston) helicopter was to be flown on a maintenance test flight for track and balance of the main rotor. The pilot suddenly applied collective pitch A few summary examples of mishaps reviewed illustrate for takeoff and the helicopter rose to about four feet and the potential improvement in helicopter safety and began to yaw left and right and then pitch forward and aft.
reduction in accidents that could be achieved if helicopter The pilot attempted to stabilize the helicopter, but the handling quality were improved by the addition of stability main rotor blades struck the ground to the front and the and control augmentation systems. While these cases helicopter rolled on to its right side. There were no involve egregious examples of handling qualities injuries to the pilot or mechanic, but the damage incurred deficiencies, they are unfortunately all too representative of was substantial. In this case, abrupt and excessive control the accidents studied: application caused the pilot to lose control of the unstable helicopter in hover. Although the pilot’s control Case 1: This flight involved a surplus UH-1H helicopter manipulation and activity were determined to be the cause operated by the Tennessee Valley Authority in daylight of the accident, perhaps he could have recovered if the under moderate, downwind wind conditions (9 knots helicopter had been a more stable platform through the gusting to 19 reported at an airport 5 miles east of the incorporation of stability augmentation.
mishap site). It had flown for 1.2 hours before the pilot attempted to come to an OGE hover over a power pole for Case 4: On a dual instructional flight, a Student Pilot the attachment of an external load. Commands were given (SP) lifted the single engine, piston engine, two-place to the pilot to “ease the aircraft down and hold position; helicopter to a three-foot hover. The SP then “over move left and move right.” The helicopter drifted to the controlled” the helicopter and caused it to drift to the left left and forward. After this, a command to back off and try in a descent. The left skid contacted the ground and the again was followed by repetitive “move left” commands.
helicopter rolled on to its left side. There were no injuries, but the helicopter sustained substantial damage. From In the late seventies the Helicopter Association reading many similar accounts, these Student Pilots could International (HAI), then the Helicopter Association of use more stable helicopters that are more forgiving of their America (HAA), held a convention in Mission Valley, inexperience. California, near San Diego. The convention was located in a hotel adjacent to a golf course and helicopters were Case 5: In another single engine, piston engine helicopter allowed to land and takeoff in a clear grassy area next to on a dual instructional flight, the student pilot developed a the parking lot. It was apparent from operations observed high rate of descent at slow speed. A rapid application of at this temporary heliport that a particular Bell 206 was collective resulted in an uncommanded rotation to the right very stable while hovering, landing, and taking off near which the instructor was unable to arrest. In the course of the other operating helicopters. The pilot’s control of the attempt to regain rotor speed lost during that recovery, pitch and roll attitude was precise with very little the aircraft impacted the ground and sustained significant oscillation. From an orientation flight in the helicopter, it damage. was determined that it was equipped with a Sfena “Mini- Stab.” This package had been optimized for the pitch and Case 6: A high-time fixed-wing pilot with limited roll axes, and the yaw axis augmentation was still being helicopter experience, flying a single turbine-engine developed. For the hover and in-flight operations, the helicopter, encountered an uncommanded rotation to the improvements in handling qualities were immediately right. He failed to reduce the engine throttle to idle and apparent. The pilot workload was reduced significantly and impacted the ground after multiple rotations. The probable precision was improved considerably. It was an impressive cause determined by the NTSB was a loss of tail rotor accomplishment for this small helicopter which might effectiveness (LTE) coupled with the pilot’s failure to otherwise be described as “squirrelly” by some operators initiate a timely correction. for some tasks—to include hover and hover taxi in a crosswinds and low speed flight in turbulence.
A call to the Bell 206 Product Support division in RESULTS Mirabel, Canada, revealed that a stability augmentation package is not offered as an option for the Bell 206 series of helicopters. This stability augmentation system is The issues which were causal factors, and which also installed in the Navy TH-57C helicopters used for illustrated deficiencies in handling qualities, could be advanced flight training and was required to meet military grouped into four broad categories as follows: standards for certification for single-piloted instrument flight. This package was designed in the late seventies and early eighties and today’s technology should permit the Stability Augmentation design of an inexpensive, lightweight stability augmen- tation system for even the low-end helicopters on the Helicopters are inherently less stable than airplanes. If an market. Hydraulic servo technology, where applicable, has unaugmented helicopter in hovering flight is disturbed also progressed to permit this integration at a lower cost from equilibrium, its attitude continues to diverge until and weight.
corrective control inputs are made by the pilot. Most airplanes, however, are inherently stable and when they are More recently, an Attitude Command/Attitude Hold disturbed from equilibrium restoring moments are augmentation system, HeliSAS, has been developed and generated. Only so much can be done with the fundamental tested (ref. 8). The system weighs only 12 pounds and the handling qualities of the basic helicopter and manufacturers projected cost is $30,000. It was demonstrated on an R-44 generally design the low end helicopter to meet the test aircraft and was evaluated by pilots from the Robinson minimum requirements of FAR 27. Additional factory—as well as a NASA test pilot and one from the aerodynamic modifications to the airframe to improve National Test Pilot School. “Very favorable” comments handling qualities, such as fins or strakes, are often costly, were received. According to the FAA, a low cost SAS has add weight, and can produce additional drag with resulting never been certificated, considerable work needs to be done performance penalties. Many helicopters in use today do to establish standards, and current rules are not adequate. If not have to meet even the minimum standards for the these are reasons for not proceeding, then the FAA needs same class of helicopter being built under the current to move ahead with the formulation of standards and revise requirements.
the current rules. Lightweight, low cost systems such as the one described, have the potential to significantly reduce accidents attributed to poor handling qualities.
A recent simulation study at Ames Research Center twenty-one helicopters involved that did have stability (April–May 2001) began the process to look at the lowest augmentation (rate command). Any augmentation would levels of stability and control augmentation that could be be a step in the right direction for the helicopters involved used to safely operate a “low-end” helicopter single pilot in the remaining accidents. It was sobering to calculate the under Instrument Flight Rules (IFR). It would follow that toll from these external load accidents as a matter of the application of even minimum levels of augmentation information. There were 38 fatalities, 26 serious injuries, for instrument flight to this category of helicopter could and 20 minor injuries. Twenty-six helicopters were make a significant reduction in accidents in hover and low destroyed and all of the others, with one exception, speed flight under VMC. incurred substantial damage. It follows that this operation can be classified as hazardous regardless of handling Attitude or even rate stabilization can reduce the qualities. Precise handling qualities are dictated by the possibility of a pilot induced oscillation (PIO) developing tight tolerances required for these tasks. This highlights by providing a more crisp response to control inputs about the benefits accrued by incorporating stability all axes and by providing the damping that permits augmentation in helicopters performing this mission.
smooth control of the helicopter’s attitude and position.
Overshoot and over controlling are reduced, if not eliminated, even for the novice pilot. While manufacturers Directional Control and operators strive to hold down the manufacturing, acquisition, and operating costs of the lower end The FAA Advisory Circular, AC-90-95, “Unanticipated helicopters, perhaps it would be appropriate for operators Right Yaw in Helicopters” (ref. 10), states that the loss of to re-examine the costs of these “loss of control” tail rotor effectiveness (LTE) is a critical, aerodynamic accidents. As previously stated in this review of hover, flight characteristic which can result in an uncommanded near hover, and low speed accidents, it was common to right yaw rate which will not “subside” (damp) of its own find substantial damage or destruction of the helicopter.
accord. If not corrected, it can result in the loss of aircraft Substantial damage equals large expenses which the low control. The AC also states that “there is a greater profit margin operator cannot afford to sustain without his susceptibility for LTE in right turns. This is especially business being threatened. By investing additional dollars true during flight at low airspeed since the pilot may not up front in the acquisition of at least a minimal stability be able to stop rotation. The helicopter will attempt to and control augmentation system, the helicopter could be yaw to the right and correct and timely response to this flown more safely by pilots at all experience levels with uncommanded yaw is critical. The yaw is usually more precision and less pilot fatigue.
correctable if additional left pedal is applied immediately.
If the response is incorrect or slow, the yaw rate may One particular type of operation that requires precise rapidly increase to a point where recovery is not possible.” positioning by the pilot is the external load or sling load mission. It involves pick-up and placement of loads suspended beneath the helicopter. Often they are conducted in close proximity to obstacles and they require a high degree of precision and demand a high workload of the pilot. In this survey, 85 (or 15%) of the accidents occurred during external load operations, far exceeding the FAA determined helicopter utilization rate of 6% for this mission area (ref. 9), as shown in figure 2. Although most of these accidents could not be attributed to poor handling Percentage of Total qualities, pilots involved in these operations need a stable platform for this precision task. Only six of the mishap Flight Hours Mishaps aircraft in external load operations were equipped with an attitude command system. An attitude command system Figure 2. External Load Operations.
would have been a significant improvement for the Teetering Rotors 15% LTE In a significant number of the accidents reviewed, the helicopter’s design incorporated a teetering rotor system.
All Other Of 547 accidents, 308 (or 56%) of the helicopters had Causes teetering rotor systems (fig. 4). The numbers may, on the 85% face of the data, be misleading. Teetering rotor systems are incorporated in helicopters that have been produced in large numbers for many years. Two manufacturers today are very successful in supplying this teetering rotor system Figure 3. LTE Mishaps (as a Percentage of All design to the world market. One series of helicopters is Mishaps Studied).
relatively inexpensive (approximately $175,000 and up), powered by a piston engine, and is frequently utilized in pilot training. However, flight training by its nature For the accidents surveyed, 82 (or 15%) involved LTE accounts for a disproportionate number of mishaps. By (fig. 3). Recognizing the conditions that can lead to LTE contrast, the rigid or hingeless rotor systems tend to be is very important and the pilot’s situational awareness is incorporated in more sophisticated aircraft, employed in critical at all times. What direction is the wind with specific missions such as medical transport, and flown by respect to the helicopter’s heading? How much tail rotor more experienced pilots.
control margin remains in terms of pedal applied? How much collective pitch can be applied before maximum The semi-rigid or teetering type of rotor accomplishes available power is commanded and main rotor and tail blade flapping by (as the name describes) a teetering rotor RPM start to decay? For the student or low time motion of the two bladed rotor system. This gives the helicopter pilot, these in-flight judgments are often gained helicopter a balanced lift distribution between the only through experience—which comes at a high price in advancing and retreating blades and prevents a rolling too many cases. The Army and Air Force versions of the moment from developing. The teetering system differs H-21 tandem rotor helicopter (unaugmented), used for a from articulated systems which allow individual blades to few years in the early stages of the Vietnam conflict, flap, and hingeless rotor systems which accomplish always acted as though it wanted to swap ends.
flapping by bending of the individual rotor blades to Accordingly, the pilot workload in the yaw axis was very change their angles of attack as the blades advance and high.
retreat. The teetering rotor, in particular, has a significant, built-in phase lag from the pilot’s control input to the Anyone who has flown or spent much time as a passenger main rotor, until the pitching or rolling moment is in any of a long list of small, unaugmented helicopters, is generated. An absence of rate damping only aggravates the aware of their directional instability. A typical maneuver, condition. This alone can cause a student pilot to put in a which illustrates the “squirrel like” response of the larger control input than is required, which eventually machine, is the crosswind hover or hover taxi. The pilot results in a larger roll or pitch rate than desired and can must constantly make high frequency pedal inputs of ultimately lead to PIO. Without an instructor there to varying size to keep the helicopter heading in the desired damp the oscillation, a rollover can result if the main rotor direction. The tail boom dances to the right and left as the or landing gear should contact the surface.
position is held or as the taxi proceeds. It is a high workload task and precision is lacking. With a capable yaw SCAS—especially one designed for attitude command—the pilot’s workload and precision and the 300 Semi-rigid (Teetering) passenger’s comfort are dramatically improved.
Articulated A less attractive alternative might entail building in more Rigid directional control power by improving the tail rotor 100 (Hingeless) design to produce more thrust, increasing engine power Number of Mishaps available to provide that thrust, and making suitable Mishaps aerodynamic changes to the fuselage and tail boom designs to improve the yaw stability of the helicopter. All of these Figure 4. Mishaps by Type of Rotor System.
changes, where applicable and practical, could result in significant weight, cost, and performance penalties.
The teetering rotor is susceptible to conditions which can instrumentation and stability augmentation improvements result in the rotor hub contacting the mast. This is known had to be incorporated in this twin engine helicopter.
as mast bumping and unless it is just a very light tap, the Since that time, many high end helicopters have been result can be a sheared mast in flight which is usually fatal certificated for IFR operation to include single pilot to all onboard. Unless, of course, the helicopter is at a operation. Contrast that with the long time operation of very low altitude and airspeed (hover IGE, for instance). In the venerable UH-1 “Huey” by the Army under Instrument the early design versions of the OH-23 (UH-12), the hub Flight Rules (IFR) flying the airways and making had a circular cutout for the mast. It wasn’t long before a approaches in IMC to both military and civil airports.
few fatal accidents took place and investigation proved that This was done in a helicopter whose total stability mast bumping occurred when excessive blade flapping augmentation consisted of the stabilizer bar mounted angles allowed hub to mast contact. The fix was quite above and 90 degrees to the main rotor. It produced a simple. The hub cutout was elongated under the blades to slight damping of the main rotor displacements by its allow additional degrees of flapping and the serious design gyroscopic and inertial effect. Flying under IMC, flaw was resolved. especially in turbulence, was a high workload task in these military helicopters and it required a high degree of In this survey, only three of the hover or low speed training and proficiency. For the civil version of these accidents were attributed to mast bumping with subsequent helicopters (Bell 204, 205), flight under instrument shearing and separation of the main rotor mast. These conditions is prohibited.
accidents did result in four fatalities, however. A single engine helicopter accident in August, 2000, near During the Vietnam War, the demand for helicopter pilots Watsonville, CA, which resulted in two fatalities, was was so great that the practice of fully qualifying Army attributed to mast bumping by an operator, demonstration pilots for instrument flight was discontinued. The pipeline team leader, and dealer. The NTSB report also stated that for pilots had to be filled as attrition and rotation to other the probable cause was mast bumping. Although the event assignments after the one year tour of duty depleted the occurred in cruise flight, and not hover or low speed, the “inventory” of pilots. As a stopgap measure, a few hours teetering rotor design is susceptible to this type of accident of instrument training were put in the syllabus and the from excessive, uncoordinated, and abrupt cyclic control pilots were awarded “Tactical Instrument Tickets.” This inputs. minimal training was tailored to permit the pilot to get himself out of encounters with inadvertent IMC or, in For all of these disadvantages, the teetering rotor other words, perform the 180 degree turn to fly back to helicopter has one distinct advantage over many visual conditions. This did not always work and many helicopters that use an articulated rotor system with young pilots were not proficient. Add the additional flapping or “droop” stops. That is the ability to start or complication of night conditions and the stage was set for stop the rotor in much higher winds and turbulence—with many non-combat deaths which occurred with some or without a rotor brake. In addition, the design of the regularity. Many days and nights were IMC, especially teetering rotor system’s hub is not complex and it is quite during the long rainy seasons, but the war had to go on rugged and reliable with low maintenance requirements. It and missions had to be flown. Many helicopters, such as does not have the susceptibility to ground resonance which the OH-6A, OH-58A, and the AH-1G, were not designated helicopters incorporating articulated designs can encounter. for instrument flight even by fully qualified pilots. The Blade attachment failures for this simple design, with the entire fleet of Army helicopters at that time would not resulting catastrophic loss of a blade, are rare, as compared meet FAA standards for instrument flight. The handling to articulated rotor designs. These features make it a choice qualities were generally inadequate, most had no stability for many low end helicopters and also for the homebuilt augmentation, and if so equipped, rate command was the market. system installed.
There have been attempts to “fix” poor handling qualities Instrument Flight of some helicopters with the addition of avionics such as Flight Directors. While these additions can reduce the workload of the pilot and permit greater precision during The first civil helicopter certificated for flight in the instrument approach task, they cannot improve the Instrument Meteorological Conditions (IMC) in the fundamental flying qualities of a helicopter or any aircraft.
United States was the Bell 212. In order to achieve the They are effectively used in combination with stability level of handling qualities imposed by the Federal augmentation to provide the pilot with the required Aviation Administration, an expensive package of precision and a manageable workload. The FAA imposes standards for handling qualities as defined in the Federal integration of stability augmentation systems into the Aviation Regulation (FAR) Part 27; however, these control systems of the lower priced helicopters. Where this require only minimal standards. Military helicopters must investment has already been made in the higher priced meet the requirements of ADS-33D-PRF which are more machines, the benefits of a more sophisticated stringent than those of the old MIL-H-8501. Even so, augmentation system (Attitude Command) could be some military helicopters have had extremely poor substantial in terms of helicopter safety.
handling qualities in the past and many of these helicopters are still in military or civilian service today.
RECOMMENDATIONS Very few of the accidents reviewed occurred at night. Night conditions, on a cloudy or moonless night where there is no visible horizon, can often be likened to instrument The feasibility of designing or incorporating a low cost, conditions. It can be speculated that similar conditions can lightweight stability augmentation system should be result in many accidents due to spatial disorientation.
explored by the helicopter airframe manufacturers. Today’s Many helicopters in use today do not have the minimum technology may provide the means to accomplish a goal instrumentation, much less the stabilization, to be flown of significantly improving the handling qualities of their under real or quasi-instrument conditions—even in helicopters. Where a hydraulic system is not practical for emergencies. The lack of any rate or attitude stabilization inclusion in the design, the technology exists to provide in the design of helicopters was continued for decades in the secondary or automatic flight control system functions many helicopters, both military and civil, and continues to with small electrical actuators. A reduction in the accident this day.
rates will surely follow. When the other safety investment strategies are implemented, the goal of reducing the accident rate by as much as 50% may be realistic.
CONCLUSIONS In 1966, NASA published the results of handling qualities evaluations of seven General Aviation aircraft From the analysis made of the accidents occurring in hover manufactured in the United States (ref. 11). This or low speed flight for this current study, it can be inferred investigation revealed that the handling qualities of the that a significant number of previously documented class of general aviation aircraft, although generally accidents could have been prevented if the mishap satisfactory for flight under VMC and instrument flight in helicopters had improved handling qualities. smooth air, were degraded when atmospheric turbulence was encountered. Although the particular aircraft From the accidents reviewed, and the other statistics on designations and manufacturers were not stated, the civil helicopter accidents attributed to loss of control, it is illustrations of the various types of light airplanes puzzling why poor handling qualities have not been evaluated effectively identified the selected models.
pinpointed as causes or factors in the accidents.
Improvements in handling qualities were not even It is recommended that representative classes of light, recommended, within the scope of this research, as a piston engine and turbine powered helicopters be similarly means or investment in safety to reduce the frequency of evaluated to assess their handling qualities and to such accidents. document the deficiencies. In our opinion, strong recommendations would be forthcoming to improve the It can be inferred that a significant reduction in accidents, handling qualities of this class of helicopter.
injuries, and property damage could be achieved by the This technology, applied to the civil variants of many APPENDIX A Army helicopters, could extend these safety benefits to larger segments of the private and corporate sectors.
Amendments to FAR Parts 27 and 29 require that a Comments on Adjunct Safety Issues crashworthy fuel system be included in the design of any helicopter for which a new type certificate has been Post Crash Fires: Many helicopters involved in applied. It does not, however, apply to helicopters survivable accidents have experienced post crash fires that certificated prior to the effective date of the amendments.
have caused thermal injuries and fatalities to crew and These helicopters, which number in the thousands, will passengers. The Army pioneered the development of not afford the crews and passengers the vital protection crashworthy fuel systems based on the incidence of post from, and the reduced likelihood of, post crash fires.
crash fires in their fleet of helicopters and the resulting Retrofit kits could be designed and marketed for the civil injuries and deaths to crews and passengers. An early sector.
historical example involved the OH-13 helicopter. Until the OH-13H was introduced, the incidence of post crash The application of this technology should also be fires in hover accidents was nominal for this particular considered for retrofit in the huge fleet of General Aviation helicopter. The “H” Model configuration had two “saddle” fixed wing aircraft. It is true that it would involve fuel tanks mounted high behind the cockpit bubble. As considerable expense and result in a small reduction in fuel operational time was accumulated on this new model, the capacity. This should be factored against the many lives incidence of post crash fires and thermal injuries increased that would be saved in otherwise survivable accidents. The dramatically in fully survivable accidents. A study, which disfiguring and incapacitating burns that result in the included the use of a tethered helicopter deliberately crashed survivable “crash and burn” accidents should also be from hover, revealed that components of the rotating considered when tallying up the true costs of the swashplate (swashplate driver) struck the tanks and slashed modifications versus the human misery that could be them open. The resulting fine fuel spray was easily prevented.
ignited. As a quick fix for this problem, the Army devised a system to wrap the tanks with a tough nylon fabric The occurrences of post crash fires for the hover and low impregnated with a resin to seal it to the tanks after speed cases studied were not excessive, but the Army’s wrapping. The objective was to provide some tear experience, prior to the installation of the crashworthy resistance to the tanks and reduce the loss of fuel in the systems, was sobering.
event of swashplate contact. Many of these helicopters are still flying today, without any added protection, as Bell 47 Teetering Rotor Systems: As noted in the main body of models.
the report in the Results section , the military services had many fatal accidents attributed to mast bumping in the The expansion of Army Aviation during the years of the UH-1 series of helicopters that were employed by the United States’ involvement in the Republic of Vietnam thousands. Although a significant number of mast led the Army to develop the Crashworthy Fuel System for bumping accidents were not noted in this report, it was its fleet of UH-1 and other helicopters. The design was considered important to describe the conditions conducive simple, but it prevented many post crash fires and the to mast bumping for those not familiar with this usually injuries and fatalities which, in many cases, would have fatal occurrence.
resulted. The fuel bladders were constructed of a heavier, tear resistant rubberized fabric and break away fuel fittings The problem was accentuated and aggravated when the were incorporated in the fuel lines and hoses. These Army began to use nap-of-the-earth and terrain following fittings would break loose on impact and seal the lines and tactics to avoid the shoulder fired missile threat in the tanks to contain the fuel. On the downside, the thicker Republic of Vietnam. For many years in Vietnam, the bladders weighed more and reduced the fuel capacity of the helicopters flew at approximately 1500 feet above the aircraft slightly. For example, a UH-1H (Bell 205) terrain to avoid much of the effective small arms fire original tank held 220 gallons of jet fuel. After the (7.62mm). With the introduction of the Russian Strela modified tanks were installed, fuel capacity was 211 heat seeking missiles into the combat zone, the tactics gallons or a 4% reduction. This was not too great a price changed and put the helicopters “down on the deck.” Mast to pay for the significant reduction in thermal injuries and bumping accidents began to occur more frequently and it fatalities that resulted from the incorporation of this was learned that they were due to a combination of technology into this large fleet of Army helicopters.
circumstances. When the pilot pushed the nose of the helicopter down rapidly to stay near or get close to the The intent was to be able to sustain a light mast bump terrain, such as flying over a ridge, the helicopter rotor without shearing. Hub springs were also incorporated in experienced less than 1g conditions. This caused large the design of these teetering rotor helicopters to aid in rotor flapping angles. When aggravated by a combination controlling the flapping of the main rotor.
of abrupt control inputs, aft center of gravity, or sideslip, a fatal mast bump could occur. Immediate training While the lessons of this conflict only bear peripherally remedies were used to make pilots aware of the conditions on the discussion at hand, they do begin to apply in the that caused mast bumping and how to avoid them. In low airspeed regime and would likely find even greater addition, a thick walled mast was developed to replace all significance in extending this work to an analysis for the of the rotor masts in the entire fleet of UH-1 helicopters. forward flight regime.
6. Final Report of the Helicopter Accident Analysis REFERENCES Team. DOD, FAA, and NASA, July, 1998.
7. Near Term Gains in Rotorcraft Safety Strategies for 1. Key, David L.: Analysis of Army Helicopter Pilot Investment. A Workshop sponsored by NASA Error Mishap Data and the Implications for Ames Research Center, Moffett Field, CA, Handling Qualities. Paper presented at the Feb., 1999.
Twenty-Fifth European Rotorcraft Forum, Rome, Italy, Sept. 14–16, 1999.
8. Hoh, Roger: Attitude Command Attitude Hold Augmentation as a Means to Enhance Safety for 2. NTSB Helicopter Accident Summaries, 1993–2004.
Light Helicopters. Presentation, Hawthorne, CA, Apr. 2, 2004.
3. Cooper, G. E.; and Harper, R. P., Jr.: The Use of Pilot Ratings in The Evaluation of Aircraft 9. FAA Aerospace Forecasts, Fiscal Years 2004–2015.
Handling Qualities. NASA TN D-5153, U.S. Department of Transportation, Federal Apr., 1969.
Aviation Administration, Office of Aviation Policy & Plans, Mar., 2005.
4. Harris, Franklin D.; Kasper, Eugene F.; and Iseler, Laura E.: U.S. Civil Rotorcraft Accidents, 10. FAA Advisory Circular AC 90-95, Unanticipated 1963 Through 1997. NASA/TM-2000-20957, Right Yaw in Helicopters. Feb., 1995.
USAAMCON-TR-A-006, Dec., 2000.
11. Barber, Marvin R.; Jones, Charles K.; Sisk, Thomas 5. Iseler, Laura E.; and De Maio, Joe: Analysis of U.S.
R.; and Haise, Fred W.: An Evaluation of the Civil Rotorcraft Accidents from 1990 to 1996 Handling Qualities of Seven General Aviation and Implications for a Safety Program.
Aircraft. NASA TN D-3726, Nov., 1966.
Proceedings of the American Helicopter Society 57th Annual Forum, Washington, D.C., May 9–11, 2001.
Form Approved REPORT DOCUMENTATION PAGE OMB No. 0704-0188 The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.
1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To) 04-11-2005 Technical Memorandum 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER The Implications of Handling Qualities in Civil Helicopter Accidents 5b. GRANT NUMBER Involving Hover and Low Speed Flight 5c. PROGRAM ELEMENT NUMBER 5d. PROJECT NUMBER 6. AUTHOR(S) Daniel C. Dugan and Kevin J. Delamer (CDR-USN) 5e. TASK NUMBER 5f. WORK UNIT NUMBER 21R-745-60-7513 8. PERFORMING ORGANIZATION 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) REPORT NUMBER National Rotorcraft Technology Center Ames Research Center A-050003 Moffett Field, CA 94035-1000 10. SPONSORING/MONITOR’S ACRONYM(S) 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) NASA National Aeronautics and Space Administration Washington D.C. 20546-0001 11. SPONSORING/MONITORING REPORT NUMBER NASA/TM-2005-213473 12. DISTRIBUTION/AVAILABILITY STATEMENT Unclassified : Unlimited Distribution: Nonstandard Subject Category: 59 Availability: NASA CASI (301) 621-0390 13. SUPPLEMENTARY NOTES Point of Contact: Daniel C. Dugan, NASA Ames Research Center, M.S. 219-3, Moffett Field, CA 94035-1000 (650) 604-2968 14. ABSTRACT Because of increasing accident rates in Army helicopters in hover and low speed flight, a study was made in 1999 of accidents which could be attributed to inadequate stability augmentation. A study of civil helicopter accidents from 1993–2004 was then undertaken to pursue the issue of poor handling qualities in helicopters which, in almost all cases, had no stability augmentation.
The vast majority of the mishaps studied occurred during daylight in visual meteorological condition, reducing the impact of degraded visual environments (DVE) on the results. Based on the Cooper-Harper Rating Scale, the handling qualities of many of the helicopters studied could be described as having from “very objectionable” to “major” deficiencies. These costly deficien- cies have resulted in unnecessary loss of life, injury, and high dollar damage. Low cost and lightweight augmentation systems for helicopters have been developed in the past and are still being investigated. They offer the potential for significant reductions in the accident rate.
15. SUBJECT TERMS Helicopter accidents, Helicopter handling qualities, Mast bumping, Teetering rotors, Stability augmentation, Directional control 18. NUMBER 17. LIMITATION OF 16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON OF ABSTRACT Daniel C. Dugan a. REPORT b. ABSTRACT c. THIS PAGE PAGES 19b. TELEPHONE (Include area code) Unclassified Unclassified Unclassified Unclassified (650) 604-2968 Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39-18