Document
AIAA-92-4092-CP
DESIGN AND CONDUCT OF A WINDSHEAR DETECTION
FLIGHT EXPERIMENT
Michael S. Lewis* Kenneth R. Yenni** Harry A. Verstynent Lee H. Person** NASA Langley Research Center, Hampton, Virginia Summary has been present, though most windshear is not hazardous to an aircraft in flight. For example, an A description of the design and conduct of a series aircraft may descend from the jet stream to low of flight experiments which tested the performance of altitude and experience a shear in excess of 200 candidate windshear detection devices is presented. knots. The direction in which the wind is blowing may With prototype windshear sensors installed, a NASA change through a full 180° during the course of a Boeing 737 test aircraft conducted numerous low flight. Also, the wind field through which an aircraft is altitude penetrations of microburst windshear flying may grow or diminish greatly in strength over conditions. These tests were preceded by extensive time. All of these events can be correctly called preparations which included piloted simulations, 'windshear' but may have no impact on an aircraft's determination of safe operating limits, and the continued safe flight.
development of unique flight test hardware, displays, A certain subset of windshears, however, may be and procedures. of critical impact to flight safety during low-altitude, The test aircraft and more than 50 research and low-speed flight. An aircraft in the takeoff and landing support personnel were deployed to Orlando, phases of flight has minimal excess energy since Florida, and Denver, Colorado, during June and July both altitude and airspeed are low. Moreover, a large, 1991 for field testing. Upon receiving a forecast of transport-type aircraft cannot readily change its developing weather activity, the aircraft crew energy state in this flight phase since deployed high- launched and proceeded to the storm location, lift devices and landing gear result in high drag and jet guided by uplinked ground radar information and engine response to throttle commands can take voice communications with ground weather some time (and the option to trade altitude for personnel. The tests required constant monitoring of airspeed is minimally or not available at all).
numerous factors including aircraft flight parameters, A flight safety hazard exists if a sustained energy ground obstructions, windshear magnitude, reducing windshear (decreasing headwind, lightning, escape routes, ATC coordination, storm downdraft, or increasing tailwind) takes away aircraft · cell development, and others.
energy faster than engine thrust can add it back. In The flight tests were extremely successful, safely such a condition, the aircraft is forced to either reduce recording more than 25 low altitude microburst airspeed or descend Given a low airspeed, low windshear and strong gust front approaches and altitude initial condition, either option may be penetrations, along with completing a full test matrix hazardous. Additionally, late application of full thrust of additional requirements related to windshear by the pilot or, in fact, thrust reductions (in an attempt sensor performance. Data quality from the tests was to initially maintain glide slope speed and altitude) excellent and indicates strong potential for airborne during an energy increasing shear which often remote sensors to accurately predict and warn the precedes hazardous shear can more easily lead to an flight crew of hazardous windshear conditions with accident.
ample time for precautionary crew action.
A weather condition called a microburst can generate hazardous low-altitude windshear. A micro-
burst is formed when a column of air at high altitude f
Notations quickly cools due to evaporation of ice, snow or rain , ..
and, becoming denser than the surrounding atmos- Yp Potential Flight Path Angle phere, falls rapidly to the ground. Upon nearing the T Thrust ground, the downward moving air spreads rapidly in J Drag D all directions away from the descending core (Figure ,
w Weight
1 ). Windspeed changes in excess of 40 meters per ~ Introduction sec-and (80 knots) over 4 kilometers have been [_.·.
recorded in such events. f
Windshear refers to a change in windspeed in a f given direction over a particular distance or length of Copyright 1992 by the American Institute of Aeronautics time. As early as airplanes began to fly, windshear and Astronautics, Inc. No copy rig ht is asserted in the United States under Title 17, U.S. Code. The U.S. Govern- ment has a royalty-free license to exercise all rights under • Windshear Flight Test Project Engineer the copyright claimed herein for government purposes. All ** Research Pilot other rights are reserved by the copyright owner.
t Head, Aircraft Operations Branch, member AIAA
l
windshear conditions allows a pilot to add engine An aircraft flying through the center of a microburst power and fly through even very strong windshear first experiences a performance-enhancing increasing headwind which is rapidly followed by a conditions with minimal altitude or airspeed loss. A performance-degrading sequence of decreasing variety of sensor technologies which could provide headwind, downdraft, and increasing tailwind. A this early windshear warning have been investigated metric termed 'F-factor' has been developed by and developed at NASA Langley Research Center NASA researchers which quantifies the aircraft and by industry over the past 5 years. This research performance loss that a specific windshear has included the study of the basic atmospheric physics and meteorology of microbursts which spawn produces. An added 'atmospheric' term to the windshear conditions, numerical simulation of standard Yp=(T-D)/W performance equation, the F- factor is nondimensional and relates to the equivalent windshear velocity, precipitation and thermal fields specific excess thrust (thrust minus drag divided by and simulation of the potential measurement weight) required to maintain steady flight conditions performance of candidate sensor technologies.
due to the changing winds. Since a typical twin Based upon these studies, Doppler radar, lidar, and engine turbojet transport category aircraft may have passive infrared technologies all showed promise in engines capable of producing a specific excess providing airborne forward-looking windshear thrust of 0.17 (maximum thrust at maximum gross detection.
weight). a microburst which produces a sustained NASA Langley has also developed an advanced shear of greater than 0.17 F-factor exceeds the algorithm formulation which calculates the F-factor performance of the aircraft. This aircraft would then windshear index level due to the shear immediately be forced to either lose airspeed, altitude, or both, surrounding the aircraft. In addition, NASA has regardless of pilot control inputs. developed algorithms which process data transmitted Another way of looking at the F-factor quantity is as to the aircraft from ground-based Terminal Doppler the reduction of potential climb angle a given Weather Radar combined with aircraft measured data windshear takes away from an aircraft's performance to generate an F-factor index.
capability. A sustained shear with an F-factor index of 0.14 results in an approximately 8° reduction in Research Hardware and Aircraft Installations potential climb angle capability (0.14 radians = ~8°).
Since a typical 4-engine transport aircraft at maximum Except for the lidar system which is to be tested in gross weight has a maximum potential climb angle of 1992, research implementations of each of the less than 6°, this shear would again necessarily cause above-mentioned windshear detection systems have an airspeed or altitude loss.
been installed on NASA Langley Research Center's An important consideration in determining the Boeing 737 research aircraft. A brief description of impact of a given shear or F-factor level is the length the background, design, function and aircraft of time over which the aircraft is exposed to the shear.
installation of each system is detailed below: Very quick wind changes which do not persist over a significant distance are categorized more properly as fladar: By applying Doppler processing algorithms to turbulence than hazardous windshear. Although the received signal from an airborne radar, the line-of- turbulence can indeed be a safety issue, this is more sight velocity of the reflecting medium can be so because of controllability or aircraft structural determined. Separating the ground return ('clutter') impact than energy loss considerations. Windshears signal from the desired airborne precipitation velocity which are of importance to the energy state of an signal (from which windspeed is derived) is the chief aircraft are those which result in F-factor values near limiting factor in airborne radar Doppler processing.
the maximum specific excess thrust of a particular Since 1986, NASA Langley has developed and aircraft (Figure 2) and which persist at that average refined a radar and ground clutter simulation model to magnitude over approximately 15 seconds or more.
investigate radar design and signal processing methods to allow an airborne radar to accuratell Background detect and measure hazardous windshear.
Synthetic aperture radar data from multiple airport Inadvertent encounters with low-altitude windshear sites has been stored in a data base to model are a leading cause of transport aircraft accidents and stationary terminal area ground clutter levels and passenger injuries and fatalities. Since 1964, moving clutter targets have been modeled on the winds hear has been a causal factor in at least 26 U.S.
roads and highways surrounding the airport terminals air carrier accidents, resulting in over 500 fatalities and and approach corridors. Against these clutter 200 injuries.2 In 1986, NASA and the FAA signed a sources, parametric variations in radar design features Memorandum of Agreement (MOA) to establish a have been investigated to determine the feasibility joint program to investigate the feasibility of remote and potential design of an airborne radar windshear airborne windshear detection and measurement. In detection system.
1990, this MOA was expanded to include the Based upon these research simulation studies, integration of both airborne and ground-based Rockwell Collins, Inc., modified a Model 708 X-band windshear measurement information.
weather radar system to NASA specifications, which Piloted simulation tests have shown that as little as allow research variation and output of basic radar 20 seconds of advanced warning of hazardous parameters. NASA then designed and integrated a has developed an advanced in situ windshear complete radar operation, processing, display, and measurement algorithm. Fully described in reference data recording station for airborne research. 7, this algorithm provides the vertical, horizontal, and The components of this system are shown in total F-factor shear index value of an aircraft's Figure 3. The research radar receiver/transmitter immediate environment based upon airspeed, accelerometer, angle of attack, groundspeed, and (R/T) unit was installed in the forward galley area of the test aircraft in parallel with a standard Collins other aircraft sensor inputs. The algorithm includes Model 708 R/T installed in a lower electronics bay filtering equations to reduce turbulence feed- forward of the nose landing gear. Both systems used through.
The in situ algorithm has been extensively tested in a common flat plate antenna accessed through a wave guide switch. Additionally, a 2,000 watt high both piloted simulation and in a hot bench laboratory power amplifier could be connected via a second utilizing flight software code. Following this development effort, the software was implemented wave guide switch to increase the output power of on the research aircraft microvax computers for real- the research radar. The radar control pallet (Figure 4) was located in the rear of the aircraft and operated by time operation. A display of various algorithm values and outputs was also designed and implemented on two research engineers.
the research aircraft to allow real-time monitoring of The research radar typically operated with a ±30 windshear levels encountered during microburst degree azimuth scan and a variety of antenna penetrations (Figure 9).
elevation tilt control strategies. The signal processor produced multiple research display formats including range/azimuth reflectivity, velocity, and F-factor shear Terminal Doppler Weather Radar (TDWR): The FAA is hazard maps (Figures 5-7). When the research radar currently implementing a program to develop and was in operation, the standard weather radar was not install powerful ground-based Doppler radar systems operable, though the aircraft pilot could readily switch for windshear detection at major terminal areas off the research radar system and return to operation around the country. This TDWR program is now in and display of the standard system if required.
the final testing stage under the direction of MIT Lincoln Laboratories and utilizes a prototype radar Infrared: Since a microburst is formed by a column of system installed near Orlando International Airport for cool air rapidly descending through warmer ambient field testing. Additionally, a similarly capable TDWR- air, a warm/cool/warm temperature sequence is type research radar is operated by the National typically experienced by an aircraft penetrating Center for Atmospheric Research and provides through a microburst. A forward-looking infrared windshear alert support to Stapleton International device which can sense temperatures well ahead (~5 Airport in Denver, Colorado. As presently km) of an aircraft may be able to identify this thermal configured, both radars produce a display used by air signature, the magnitude of which tends to correlate traffic control personnel which identifies areas of wind with the total windspeed change across the divergence above a given threshold in proximity to microburst. Important to the success of such an runway approach and departure paths. ATC instrument is the uniqueness of the thermal personnel then include windshear caution and signature--that is, whether non-hazardous strength information as part of takeoff and landing atmospheric conditions present similar temperature clearances.
differences--and the degree to which a temperature As part of the NASNFAA joint program in wind- difference which does exist in a microburst accurately shear sensor research, NASA is investigating meth- correlates with the actual windshear hazard.
ods of automatically transmitting and displaying An instrument developed by Turbulence TDWR-derived windshear measurements to an Prediction Systems of Boulder, Colorado (with partial aircraft via radio data link. In addition, further airborne support from a NASA Small Business Innovative processing of the TDWR wind divergence information Research (SBIR) contract) has been installed on the with other aircraft sensor data allows for the research aircraft. The device is mounted in a forward computation and display of a TDWR-based, F-factor left side cabin window and receives atmospheric index.
infrared energy through a small periscope assembly An automatic data link using VHF packet radio exterior to the aircraft (Figure 8). A hazard index equipment has been implemented on the NASA based upon the differential between long range (3 - 5 research aircraft. Wind divergence location, km) and ambient temperatures is computed in real magnitude, areal extent and other information are time internal to the device and, along with numerous transmitted to the aircraft for further processing and other infrared system parameters, monitored, onboard display (Figure 10). This information is displayed (Figure 9), and recorded on the aircraft's updated approximately once per minute as the TDWR data system.
radar completes a full scan sequence.
In Situ: As both an independent research Research Facility development to improve current generation reactive windshear alerting systems and to provide the The test aircraft is a Boeing 737-100 pre-production forward-look research sensors an accurate 'truth' model modified for experimental purposes with a fly- measurement during research flight testing, NASA by-wire research cockpit in the passenger cabin and evolution that an accurate remote measurement 3-5 an extensive suite of sensor and data recording km in front of the aircraft is a good estimate of actual equipment (Figure 11). A diagram of the location of shear magnitude 30 to 60 seconds in the future.
the research systems is shown in Figure 12. The Both of these conditions are required for the success aircraft is powered by two Pratt and Whitney JTBD-7 of a forward-looking windshear detection system.
engines and operated with a standard 737 control The specific goals of the 1991 flight test program system from the forward flight deck (FFD). The aircraft may also be controlled by the research flight deck were threefold. First, the operational feasibility of TDWR/aircraft data communication and the (RFD) located in the aircraft cabin. RFD control inputs performance of an airborne algorithm to process are made through variable-feel sidearm controllers TDWR data into windshear information was to be and modified by advanced automatic flight control evaluated and demonstrated. Second, clear air software operating in one of two general purpose airborne radar ground clutter measurements were to microvax computers. The second microvax primarily controls inputs to eight multi-function color displays in be collected at multiple airport locations along the RFD, with presentation formats including Primary different runway approach paths to assess moving and fixed ground clutter suppression techniques.
Flight Displays (PFD), moving map navigation Third, the most difficult and critical test was to displays, engine parameters, and checklists. Engine evaluate the windshear detection performance of the power is controlled through either fully automatic or IR, radar, and in situ systems in actual atmospheric manual throttle inputs. When the RFD is in operation, the FFD pilots function as flight safety monitors and and operational conditions.
can disengage the research flight system at any time.
Figure 13 shows the standard arrangement of the RFD. Flight Operations and Safety Simulation The aircraft is equipped with two VHF, one UHF, and three intercom voice communication channels. In order to establish windshear flight test operating Guidance, navigation, and control inputs utilize an Air procedures and confirm safety margins for actual Data/Inertial Reference System (ADIRS), GPS, MLS, flight testing, a flight operations and safety simulation multiple augmented control modes, and a variety of was conducted using the NASA Langley Transport other research systems. Approximately 22-30 Systems Research Vehicle (TSRV) fixed-base piloted research and support personnel participate onboard simulation facility. The simulation used the standard during a typical research flight. TSRV 737 math model which incorporates More than 500 parameters are recorded on the performance data in look-up tables based upon wind aircraft's primary magnetic tape data system in flight. tunnel and flight tests. An Attitude Control Wheel Four videotape systems record the PFD and Steering flight control mode only was utilized. The navigation primary displays as well as images from a standard TSRV RFD displays were masked as forward-looking camera mounted in the nose of the appropriate to only simulate display information aircraft and a second camera located in the Research available in the forward flight deck of the research Flight Deck. Three eight-channel stripchart recorders aircraft, where all windshear penetrations were flown.
are available for research use. The research radar and The analytic, symmetric Bowles/Oseguera microburst lidar systems also include additional high-speed, model was used to simulate microburst winds. A magnetic-tape recorders for the high data volume computer-generated image of the Denver area was research requirements of each system.
utilized as a visual scene and included lightning effects and a transition to and from zero visibility upon entering and exiting the microburst. Continuous Flight Test Design moderate turbulence was modeled using Dryden model root mean square velocities of 4 ft/s in all axes.
The objective of NASA's Windshear Airborne A number of parameters were varied to investigate Sensors Flight Test Program is to safely develop, operating limits and procedures. Initial aircraft altitude validate, and demonstrate advanced windshear was set to 500, 750, and 1000 feet above ground sensor technologies over a representative range of level. Since flight operations were to be conducted meteorological and other operational environments. at both Denver and Orlando sites, the Denver site A fundamental philosophy which shaped the design conditions of approximately 5200 ft MSL base and operation of the test flights was the use of the in altitude was used. Initial indicated airspeeds of 150, situ algorithm to be the 'truth' measurement of true 175, 200, and 225 knots were evaluated. A reactive- windshear magnitude. Thus, a forward-looking type shear warning light enabled approximately 5 sensor in continuous operation could compare seconds after shear entry was compared to a no predicted shear hazard values with in situ automatic warning condition. Aircraft gross weight measurements as the aircraft flew through or near a performance effects were evaluated at both 85,000 position in space previously sampled by the remote and 95,000 pounds. Shear penetrations were flown sensor. Close agreement between a forward-looking both directly through the microburst center and offset sensor and an in situ measurement would indicate to the side to evaluate the effects of possible that both the sensor can accurately measure shear additional lateral control requirements. Four shear hazard from a remote distance and, importantly, that levels were tested, varying from an F-factor averaged atmospheric windshears are of slow enough over 1 kilometer of approximately 0.2 to exit pathways. In addition, the RFD personnel a~ways approximately 0.4. These shear magnitudes can be provided the FFD pilots with an 'escape· vector rn the roughly categorized as strong to extremely strong.
event the storm was stronger than expected.
Four research pilots participated in the simulation and each flew the entire test matrix. Each pilot was 2. Establish operational limits and procedures.
briefed on the simulation setup and design. Control Through both piloted simulation and an~lysis, the strategy for the shear penetrations was to approach following limits and procedures were established: the microbursts at a constant initial altitude and a. F-factor: As determined from TDWR ground airspeed and, upon penetrating through the shear, radar and as calculated using a reference airspeed of add power as required while minimizing first altitude 210 knots, the maximum F-factor for penetration was loss and secondly, airspeed loss. (This strategy was 0.15. This limit included consideration of the desired for flight test purposes since in situ possible quick growth of microburst strength measurement comparisons with predicted shear between update intervals of the TDWR ground radar strength was desired at the same altitude, though not (approximately 1 minute apart).
necessarily at the same airspeed.} A number of b. Altitude: Unrestricted with TDWR F-factor familiarization runs with the simulation procedures <0.10. Restricted to > 750 ft AGL with TDWR F-factor began each simulation session. The shear test cases >0.10 and< 0.15 were run in a random order so the shear strength was c. Airspeed: Unrestricted with TDWR F-factor not known to the pilot prior to shear entry. Post-run <0.10 Restricted to >210 KIAS with TDWR F-factor statistical calculations included analyses of minimum >0.10 and< 0.15 and maximum altitude, airspeed, angle of attack, d. Reflectivity. Allowable reflectivity levels within normal acceleration, pitch attitude, engine pressure a storm cell were limited so as to avoid extremely ratio, and F-factor index.
heavy rain rates and, most importantly, hail. The limits Simulation results showed that for complete flight were higher at the Orlando site due to the lower test safety, a minimum initial altitude of 750 feet_ and probability of hail given a certain reflectivity_level. Due initial airspeed of 200 knots were appropriate.
to the availability of both high and low altitude radar Microburst wind variations with altitude did not reflectivity data at Orlando, the two-level hail significantly impact flight performance or control, nor avoidance algorithm described in Ref. 9 was able to did laterally offset penetrations away from the be utilized. The limits used were: microburst core. The onset of the shears was readily Orlando Site: <50 dBz surface; <45 dBz 1400 apparent to the flight crew based upon careful meters above freezing level.
monitoring of standard flight instruments and was Denver Site: <45 dBz surface recognized quicker than the 5 second delayed e. All shear penetrations to be piloted from the automatic warning. For even the most severe FFD of the research aircraft, with groundspeed windshear cases, the maximum angle of attack was callouts from the RFD.
less than that required to reach the stick shaker limit f. Engine air igniters on during shear ( ~ 11 °) for all penetrations which began at 200 knots penetrations (to minimize flameout potential due to or greater airspeed. The highest workload task water ingestion}.
during these tests was in power management, and all g. All ground obstructions near the test sites of pilots anticipated that actual flight conditions which height greater than approximately 200 feet were included a two-person crew would significantly identified and programmed into the moving map improve throttle control. Finally, for each pilot, flight navigation display in the RFD.
control technique and aircraft performance 3. Minimize lightning effects. Though the risk of a management improved with simulation experience, lightning strike to the aircraft was considered low an expected result of 'learning· the simulation, but since the risk of triggered lightning is almost also a desired result in preparation for actual flight negligible below 6000 ft MSL, the test flights were to testing.
be conducted underneath active thunderstorm cells and, thus, would expose the test aircraft to a chance of lightning strike. Limited lightning hardening Flight Jest Safety and Planning modifications were made to the aircraft to improve grounding connections, inspect fuel tank sealants Based upon the conduct and results of the and bonds, and miscellaneous other items. In simulation tests described above, and upon addition, only JP-5, JP-8, Jet A, or Jet A-1 fuel was anticipated research and flight operations allowed. Lower flashpoint JP-4, Jet B fuels were not requirements, six overall guidelines were established to be used.
as follows to assure adequate safety margins for the 4. Maintain communications with ground support, flight tests.
a. All microburst penetration flights required 1. Minimize weather exposure. The type of weather continuous voice communication with personnel events for which data was required was examined to located at the TDWR operations site. These radar minimize aircraft exposure to severe weather. For operators and meteorologists were extremely example, storm cells embedded within strong frontal important in both assessing developing weather activity were not penetration candidates since activity and monitoring shear strength and reflectivity relatively isolated storm cells can produce the same information.
strength microbursts with much clearer approach and b. Continuous coordination with Air Traffic deployments to establish and practice flight Control personnel was also of critical importance operations procedures. Microburst data recorded by since all maneuvering was to be conducted at low the Orlando TDWR system in rn90 was accessed via altitude in and around the Terminal Control Areas of modem, processed, and relayed to the test aircraft to both Orlando and Stapleton airports. simulate live conditions. The timing and internal 5. Flioht crew trainino. Prior to the research test aircraft communications required to maneuver the aircraft from a loiter position, descend to the test flights, the flight crew completed specific training altitude, and penetrate the developing shear on a activities.
radial line from the TDWR site (to maximize Doppler a. The FAA Windshear Training Aid was reviewed measurement data correlation between airborne and for basic background in windshear recovery ground radars) were developed. Along with flight procedures.
tests conducted to finalize the development and b. The flight crew participated in a piloted integration of the IR, radar, in situ, and TDWR simulation which included hundreds of windshear systems, these preparation flights established the penetrations. This simulation accomplished a aircraft's and crew's readiness for field deployment number of objectives, including: a review of the early and actual microburst windshear penetration tests.
recognition of the onset of windshear conditions; the establishment and repeated practice of control strategies for windshear penetrations; and the confirmation of Boeing 737-100 performance Eliobt Operations capabilities in windshears of various sizes and strengths.
Windshear penetration flight operations were c. The flight crew participated in special conducted within an approximately 25 nautical mile windshear recovery training in a 737 airline training range of both Orlando International Airport June 10- simulator. · 20, 1991, and Denver Stapleton Airport July 8-24, 6. Phased approach. A phased approach was 1991. A typical day's flight activity began with a weather briefing the previous evening to determine established to gradually increase the maximum windshear strength limit to the final 0.15 F-factor level the approximate time of day during which favorable weather development might occur. Research system in three steps. First, a microburst with shear of less than 0.1 O (as measured by the TDWR) was to be hardware and software preflight checks were conducted on the morning of the flight day, while penetrated. Second, a shear of F-factor greater than weather information from sounding balloons was 0.10 and less than 0.13 was required. Third, any collected. In Orlando, TDWR personnel from MIT shear with F-factor less than 0.15 was acceptable tor Lincoln Laboratories along with a NASA test measurements.
meteorologist continuously assessed the day's developing weather and microburst potential. At the The RFD was specially configured (Figure 14) for Denver site, personnel from the National Center for these tests as the experiment control center. Pre- Atmospheric Research (NCAR) operated the Mile penetration maneuvering was often flown from the High Radar (equivalent to Orlando's TDWR) and RFD due to the centralized information displays similarly assisted the NASA tests. Based upon an located there, though as mentioned, all penetrations approximately 30-minute prediction of developing were flown from the FFD. The RFD left side displays windshear activity in the test area, the research crew were maintained in the standard ADI, Nav, engine boarded and launched the test aircraft.
monitoring and checklist formats, while the right side utilized all four available displays and two additional The radio uplink, airborne processing, and display in the RFD of TDWR information provided real-time CRTs installed in the upper right 'windscreen' area.
information on developing windshear conditions.
These six displays depicted outputs from the radar, The RFD crew would then assess (and sometimes IR, and in situ research sensor systems, video output control) aircraft positioning requirements so as to from a camera in the nose of the aircraft, two TDWR begin a penetration flightpath from an approximately uplink displays (one specialized for flight operations, five mile range from the microburst along a radial path one for research purposes), and a moving map extending to or from the TDWR site. Often, TDWR navigation display with ground obstacle positions and personnel were able to predict developing microburst heights highlighted.
conditions prior to their identification by the TDWR Prior to the deployment of the research aircraft, automatic wind divergence calculation process.
radio voice and data communications equipment Simultaneous with the approach toward the were installed and checked at each site. Air traffic microburst shear, a number of onboard activities control personnel at both sites were briefed on the occurred. Throughout the maneuvering, the FFD objectives of the research program and cooperative flight and ATC operational procedures were crew coordinated anticipated flightpaths and clearances with air traffic control personnel. TDWR established. Finally, aircraft site basing arrangements ground personnel monitored both low-level and high- were made at Orlando International Airport with a altitude storm cell reflectivity measurements and fixed-based operator and at Buckley Air National relayed the information to the aircraft to satisfy hail Guard Base in Denver.
avoidance limits. The RFD crew continuously Rehearsal flights based at Langley Research communicated maneuvering requirements and safety Center were conducted 2 weeks prior to the
higher levels, but was of short enough duratkm so as I
limits to the FFD crew in order to penetrate the to not be of significant difficulty.
desired portion of the microburst at the appropriate In the Denver area, strong gust fronts were also time. While on final approach to the microburst, the penetrated in a very similar manner. These gust RFD crew increased monitoring and communications fronts were first identified by the ground radar and to the FFD regarding expected shear strength (based their position communicated to the aircraft. The upon processed TDWR data), aircraft groundspeed fronts were typically relatively clear air phenomena (more accurately displayed in the RFD), storm cell (<15 dBz), and were associated with outflows from reflectivity, recommended routes for either aborts very large nearby thunderstorm activity. Very nearly prior to penetration or repositioning following the opposite of a divergent microburst, gust fronts penetration, and the position of any important ground are characterized by converging winds and produce obstacles near intended routes. Other onboard strong performance increasing shear. The fronts communications coordinated research sensor penetrated in Denver also included the greatest operation, and the operation of the aircraft data turbulence levels observed during the flight tests.
system and other aircraft support systems. The FFD crew determined the microburst entry speed (typically between 210 and 230 knots) and altitude (between 800 and 1100 feet) and additionally assessed Results and Conclusions whether lightning activity levels were excessive.
When possible, a given microburst would be The 1991 flight test program is considered to have penetrated a second or third time until shear levels been extremely successful. Without any significant dissipated. Microburst lifetime with appreciable shear safety of flight incidents, approximately 19 microburst levels was typically from 5 to 15 minutes. This rela- windshear penetrations were recorded and greater tively short duration necessitated extremely efficient than 30 weaker divergences were also measured.
coordination among aircraft, ground radar, and ATC Approximately eight strong gust front penetrations personnel in order to plan and execute the maneu- were recorded, as were five approaches to storm cells vering required to repeatedly approach and which exceeded flight limitations, but were measured penetrate the microburst cells in minimum elapsed with onboard remote sensors. The maximum in situ windshear measured reached an F-factor index level time.
The visual appearance of microburst cells varied of 0.17, well in excess of an alert threshold for widely. Many were isolated cells with well-defined commercial aircraft reactive sensors. Low reflectivity rain shafts which bowed outwards near the ground, "dry microburst" windshear measurements desired at indicative of the wind profile. Others, however, were the Denver site were not collected, due to part of larger rain cell systems and were not so readily unfavorable weather conditions, though the low identifiable. At times, different approach directions reflectivity strong gust front shears which were resulted in very different visual appearances of the recorded provided nearly equivalent data. The same microburst. Rain rates and rain shaft diameters maximum performance increasing shear penetrated also varied widely from narrow (~0.5 km) with at Denver reached an F-factor level of -0.24.
relatively light rain (35 dBz) to much larger (>2 km) The airborne and ground-based sensor systems with heavy rain (>50 dBz) The expanding gust front acquired outstanding high resolution measurements from the storm cell was also typically characterized by of microburst dynamics and structure. For the first increased turbulence from 0.5 to 1 km or more prior to time ever, an in situ measurement of hazardous shear the storm cell entry. was correlated with other independent Following concurrence by the FFD crew that a measurements. Also for the first time ever, an penetration was warranted, the aircraft entry airspeed airborne radar detected and accurately measured and altitude initial conditions were chosen and the areas of hazardous windshear. The radar ground current groundspeed noted. On penetrations with clutter data collected at both sites are expected to any significant shear, the initial performance- form the basis for eventual national certification enhancing headwind increase was readily apparent to standards the flight crew and provided good warning of the Additional sensor performance and flight test imminent onset of performance-decreasing shear.
operations observations are listed below. Highly The flight crew attempted to maintain groundspeed detailed reports on the results of each one of the constant at the initial value throughout the sensor systems are forthcoming from the research penetration. During the penetrations, the workload groups at NASA Langley.
between the two man FFD crew was split so that the pilot flying controlled aircraft attitude while the other 1. The TDWR ground radar data link, airborne pilot managed the throttles in response to processing and display were definitively groundspeed callouts from the RFD. (At all other demonstrated as both a feasible and extremely useful times during the test flights, the non-flying pilot's automatic windshear communication system.
attention was completely concerned with ATC and 2. All in situ algorithm hazard computations RFD coordination and traffic awareness.) Airspeed appeared to correlate well with aircraft performance.
was allowed to vary as required to maintain constant No false in situ alerts were generated, no nuisance groundspeed. Turbulence levels within the alerts were generated, and two valid hazard alerts microbursts often reached moderate and sometimes were annunciated.
3. The airborne radar detection system identified 4. Proctor, F. H., "The Terminal Area Simulation System. Volume 1 : Theoretical Formulation," and tracked high hazard areas in flight. Ground NASA CR 4046, 1987.
processed data shows multiple alerts generated within storm cells and with significant advance warning (Figure 15). 5. Bowles, R. L. and Hinton, D. A., "Windshear 4. The test procedure was shown to be both safe Detection: Airborne System Perspective," and productive, allowing a transport size aircraft to Windshear One-Day Conference, London, England, Nov. 1, 1990.
maneuver quickly at low altitude in and near hazardous weather conditions. The aircraft did not 6. Britt, C. L., "User Guide for an Airborne experience a lightning strike.
Windshear Doppler Radar Simulation (AWDRS) 5. A short period of light to moderate turbulence prior to entry into, and on exit from, the microburst rain Program," NASA CR 182025, June 1990 shafts was frequently encountered and considered to be associated with the expanding gust front from 7. Oseguera, R. M., Bowles, R. L., and Robinson, the microburst core. P.A., "Airborne In Situ Computation of the 6. Visual indications of windshear strength are not Wind Shear Hazard Index," AIM Paper 92- apparent, though at times a bowing out of the 0291, presented at the 30th Aerospace rainshaft shape due to divergent winds at low altitude Sciences Meeting & Exhibit, Reno, NV, Jan. 6- was observable (Figure 16). However, at other times, 9, 1992.
the microburst windshear was embedded within 8. Oseguera, R. M., and Bowles, R. L., "A Simple, multiple rain cells and a distinct shape could not be Analytic 3-Dimensional Downburst Model observed. Additionally, strong performance increasing shears penetrated in the Denver area were Based on Boundary Layer Stagnation Flow," clear air phenomena with no associated visible NASA TM 100632, July, 1988.
moisture.
7. As expected from the piloted simulations, shear 9. Joss, J. and Waldvogel, A., "Precipitation entry airspeeds of 210-240 knots were sufficient for Measurement and Hydrology," Chapter 29a, the test aircraft to experience the energy loss of the Battan Memorial and 40th Anniversary Radar penetrated shears with little altitude loss. Meteorology Conference, American Additionally, advanced knowledge of the location and Meteorological Society, Boston, MA, 1990.
strength of the shears allowed the pilots to quickly and readily manage engine throttle, airspeed, and altitude control during the penetrations.
The Windshear Program at NASA Langley would like to gratefully acknowledge MIT Lincoln Laboratories, the National Center for Atmospheric Research, Orlando and Denver FAA Air Traffic Control personnel, Buckley Air National Guard, and Page Avjet, for their helpful assistance in conducting this program.
References 1. Bowles, R. L., "Reducing Windshear Risk Through Airborne Systems Technology," 17th Congress of the International Council of the Aeronautical Sciences, Stockholm, Sweden, September 9-14, 1990.
2. "Low Altitude Windshear and Its Hazard to Aviation," National Academy Press, Washington, DC, 1983.
3. Hinton, D. A., "Recovery Strategies for Microburst Encounters Using Reactive and Forward-Look Windshear Detection," AIAA Paper 89-3325, presented at the AIAA Flight Simulation Technologies Conference, Boston, MA, Aug. 14-16, 1989.
~df\{& 0••=••; p~--~y:::'•Y'~;<,"' ' .- WI ND SH Ef,J) E """'~.;,;,,, . ,,-t-,.,_ Figure 1. The windshear problem.
Approximate Maximum # Engines Specific Excess Thrust 2 0.17+ 3 0.13 4 0.11 Figure 3. Airborne radar antenna, receiver/ Figure 2. Turbojet transport aircraft maximum performance at maximum gross weight. transmitter, display, and control units.
Figure 4. Airborne radar research control pallet.
l i/ir 21: : /4 IILl[HED REf. DUL Figure 5. Airborne radar reflectivity display showing storm cell with maximum reflectivity near 50 dBz 5 km ahead of the aircraft.
r
Figure 6. Airborne radar velocity display showing 12 m/s headwinds 5 km from the aircraft and 12 mis tailwinds 8 km from the aircraft. An area of zero Doppler wind is 6.5 km from the aircraft.
0.H1 Figure 7. Airborne radar shear display showing an area of strong shear between 6 and 8 km ahead of the aircraft.
Figure 9. Research display of IR ('AWAS Ill') and Figure 8. Infrared periscope.
in situ sensor data.
244KT 1061FT INSITU F ~ 0.06 171 2045:42 0.04 TRK 022 MAG 237 1.1 Figure 10. Research display of TDWR data showing microburst icons approximately 1 nmi in front of the aircraft symbol, with the tagged icon containing a 36 knot shear with an F-factor hazard index of 0.14. Aircraft airspeed is 244 knots, groundspeed is 237 knots, altitude 1061 feet with a left quartering headwind of 7 knots.
Figure 11. NASA Langley Research Center Figure 12. Research aircraft interior layout.
B-737 Transport Systems Research Vehicle.
Figure 13. Research Flight Deck Figure 14. RFD during windshear research flight.
standard configuration.
i
144 I
I
TIME 21: 27::?4
2.00 ALERT
FRAME tt 1648 '=' 731, . . Af fOIAL 1000M f FACT-OR Figure 15. Airborne radar display showing shear hazard levels above threshold in two areas with average F-factor values as shown.
Figure 16. Orlando, Florida, microburst approximately 1 minute prior to penetration.