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Aircraft Performance and Operations This chapter discusses the performance and flight characteristics of airplanes as distinguished from other forms of aircraft, such as helicopters and airships. Most of the analysis cannot be generalized and may not apply to aircraft other than airplanes. Although the hazards of wind shear to helicopters could be large, there are insufficient statistics to determine the extent of the problem. Similarly, the frequency of airship operations is so low that it does not merit special attention at this time. Wind-Shear Warnings Operational information on wind shear originates from meteorological forecasts, pilot reports (PIREPs ) , and/or detection by Low-Level Wind Shear Alerting Systems (LLWSAS ~ at those airports that have such equipment (currently 59 ~ . Meteorological forecasts are the least useful for predicting downbursts because of their short lifetime and random occurrence and location. These forecasts, at best, warn flight crews and controllers of conditions conducive to generating downbursts and wind-shear activity. This information, despite its lack of small-scale detail, alerts pilots to the possibility of a wind shear encounter, reducing their recognition and reaction times. Forecas ts and PIREPs of encounters are the only sources of wind shear information for the thousands of airports that lack LLWSAS. Pilots are not required to submit reports of wind-shear encounters. PIREPs are voluntary weather reports, broadcast by pilots to towers, departure control, approach control, flight service stations, or to an air-route traffic control center to warn of encountered or observed weather phenomena . The Airman 's Information Manual (AIM) tells pilots when and how to report wind shears and other hazardous-weather information. Published quarterly by the FAA, it provides fl ight crews with teas ic £1 ight information and air traffic control (ATC) procedures for use in the National Airspace System. In particular, AIM's Section 523 urges pilots to report wind shear encounters and specifies a particular format to ensure that enough information is furnished to make the report useful. Excerpts from the AIM, Section 523, are contained in Appendix A of this volume. 51
The FAA Air Traffic Controller's Handbook contains instructions for the use of PIREPs and LLWSAS information, including the issuance of wind shear advisories to pilots. The instructions contained in Section 7, are minimal. Under Section 6 the subject of wind-shear advisories is dealt with at some length for those airports that are equipped with LLWSAS. The handbook notes that "LLWSAS is designed to detect possible low-altitude wind shear conditions around the periphery of the airport and that it does not detect wind shear beyond that limitation." The FAA Facility Operation and Administration Handbook (paragraph 1222, dated September 2, 1982) gives the facility chief the option of using the centerfield wind information: "if operationally feasible, facility chiefs may elect to designate wind information derived from remote sensors located near runway thresholds as the wind to be issued to arriving aircraft rather than from the centerfield source, except that the centerfield source will be used during outages of the remote sensor/s." Such procedures as described above require the local facility chief to issue a letter to airmen explaining to arriving aircraft the origin of wind information, if it is obtained from a peripheral sensor. However, controllers are not required to specify the source of remote-wind data used when issuing these data to arriving aircraft, except when an alert occurs. The training material of the Controller Training Academy at Oklahoma City does not incorporate the best-available information on wind shear. Wind shear is treated under the heading ''Turbulence" and comprises, perhaps, three pages of text. Additional, more up-to-date information should be included and given more emphasis in training materials a In response to NTSB Safety Recommendations issued March 25, 1983 (NTS8, 1983), which addressed the need for improvements in the LLWSAS system and procedures for its use, the F. M is in the process of amending its handbook for controllers. Similarly, the F. M plans to emphasize in the AIM the importance of pilots reporting wind-shear occurrences promptly. The FAA is preparing additional material for pilots and controllers in the form of advisory circulars and bulletins. At busy airports, controllers must communicate vital LLWSAS data and other weather information while occupied with their primary task of guiding aircraft and ensuring traffic separation. Not only is it difficult to interpose weather information, in the fast-paced flow of traffic directives, it is also important that the wind shear information be transmitted promptly and clearly without the likelihood f or mis int erpre tat ion . If not given in a standardized format, PIREP can exacerbate the difficulty of the controllers' task. A recent FAA staff study of the FAA weather program in October 1982 (unpubl ished) recognized the 52
problem of handling and distributing PIREPs and recommended revising the current procedures to improve their proper processing and d istribution. This situation calls for an alternative means of transmitting critical information accurately and directly to pilots. During 1977 the FM conducted a study of how to best provide pilots with information on potentially hazardous weather conditions. Among other things, the FM looked at the cost of a system that would allow a meteorologic t at the Center Weather Service Unit (CWSU ~ to tape a hazardous weather advisory covering the center 's geographic area of responsibility. CWSU meteorologists at the ATC center either now have or could acquire al 1 the information they need to prepare a transcribed broadcast of reported or predicted low-altitude wind shears and of any other hazardous weather occurring in or forecasted for the geographical boundaries of the center, including air carrier terminals and other airports. In the proposed system, meteorologists would upda te the taped advisory as required, which would be continuously broadcasted on a discrete frequency. The FAA reported on March 23, 1978 that such a system would require four discrete frequencies in the 25-kilohertz range and 66 transmitters to adequately cover the continental airspace for all aircraft operating above 18,000 feet. The cost of such a system was estimated to be $1 mil 1 ion . As noted earlier, the FAA is developing equipment that will enable meteorologists at their CWSUs to transmit automatically weather messages to towers and control facilities . However, in the near term, pending completion of an automatic data up-link, perhaps the proposed system or some variation, is worthy of another look. In summary, all equipment and procedures now in place can be improved and re fined to make warnings of poss ible wind-shear hazards more effective. It is essential that the aviation system exploit these capabilities to the utmost while more advanced warning systems, such as NEXRAD, terminal radars, airborne radars, and other airborne detection/warning concepts, are being developed. Cockpit Procedures and Training FAA Regulat ions The Federal Aviation Regulations (FAR) applicable to air carriers and certain other categories of operators require these operators to develop and use approved procedures to advise pilots of severe weather conditions, including possible thunderstorms and frontal systems that may cause low-altitude wind-shear conditions at departure and destination airports; flight procedures for operating in potentially hazardous weather conditions; approved airman training programs 53
covering all aspects of normal and emergency operations; and manuals including "instructions and information necessary to allow the personnel concerned to perform their duties and responsibilities with a high degree of safety." [FAR 121.135(a)~13~. This is in accordance with the Federal Aviation Act of 1958 [Sec. 601(b)], which prescribes that: "the Administrator shall give full consideration to the duty resting upon air carriers to perform their services with the highest possible degree of safety in the public interest." _ ¢~ __= a_ ___ A__ _ for airplane type certification by the FAA need not include a discussion of wind-shear effects on the airplane. The flight operations manual for each airplane used by an airline, however, is required to include a detailed discussion of piloting procedures to be followed in the event of a wind-shear encounter during takeoff or landing. In addition, air carriers are required to include procedures for coping with wind shear as part of their FAA-approved training programs. These operational and training programs are monitored in the field by FAA operations inspectors, who have the responsibility to evaluate the adequacy of the air carriers' training programs and line operations. The airplane flight manna 1 required The FAA issued Advisory Circular AC 00-50A, entitled Low Level Wind Shear, on January 13, 1979. It is the primary source of FAA guidance to pilots for recognizing the more significant meteorological phenomena that may cause wind-shear hazards. It also contains procedures for pilots to use in detecting the presence of wind shears and for flying airplanes safely in the event of an encounter. Since its publication in 1979, much more has been learned about the characteristics and hazards of wind shear, especially from Projects NIMROD and JAWS. Advisory Circular AC 00-50A should be revised and updated to include new information. In particular, the FAA should describe the structure of downbursts and enhance the discussion of airplane performance and piloting techniques in wind-shear conditions. Operating Procedures Airline flight operations manuals were reviewed by the committee as to the procedures specified for use by airline flight crews in the event of low-altitude wind-shear encounters. All of the instructions appear to be based on the recommendations in FAA Advisory Circular AC 00-50A, and on recommendations developed by the Boeing Company, and published in the January 1977 and January 1979 issues of Boeing Airliner magazine (Higgins and Roosme, 1977; Higgins and Patterson, 1979~. But there is considerable variance among airlines in the extent of coverage and in details on flight operations procedures. Some manuals provide very extensive instructions, while others are far less complete. Additional emphasis needs to be placed on the potential severity of and hazards from downbursts and strong wind shears and the importance of early recognition of and immediate reaction to them. In addition, the manuals should discuss flying at high angles of attack 54
or at stick-shaker speed* and the need for timely PIREPs phrased in standard terminology. It is noted that some airlines include material relevant to wind shear in their training manuals rather than in their flight operations manuals. In these cases, the training materials and flight operations manual materials are complementary. Airline flight operations manuals are organized differently from one company to another, as are their methods of presenting procedures for dealing with wind shears. Typically, all procedures related to takeoff and initial climb appear in one section of the manual and all procedures applicable to approach and landing appear in another. One major airline presents extensive material on wind shear in its flight operations manual. It includes the following warnings in the part of the manual dealing with takeoff (normal operations): If significant wind shear is suspected, consider the alternatives of taking off in a different direction or delaying the takeoff until conditions are more favorable. If shear is suspected, use full takeoff thrust; do not use reduced thrust. If the takeoff is not obstacle limited, a speed in excess of V2+10** may be used for the initial climb to provide additional protection from decreasing headwinds or downdrafts. If significant wind shear or downdrafts are encountered at low altitude after takeoff and airspeed has decreased to below normal climb speed, apply go-around thrust and adjust pitch attitude to climb out at the existing airspeed. Do not lower the nose in an attempt to regain speed until reaching a safe altitude. If ground contact is imminent, use the procedure for avoiding imminent ground contact. A similar warning appears in the section of the flight operations manual dealing with landing (normal operations): If wind shear is encountered on final approach, do not hesitate to go around if the approach profile and airspeed cannot be restabilized. It cannot be emphasized too strongly that a go-around is often the profess tonal p i lo t's best course of ac Lion. If ground contact is imminent, use the procedure for avoiding imminent ground c ont ac t. The emergency section of the same manual contains the following: *The "s tick shaker" is a device that vibrates the control column to provide stall warning prior to reaching stall angle of attack. **V2 is defined as the engine-out takeoff safety speed. Normal all-engine initial-climb target speeds vary from V2~10 to V2+20 knots, depending on the model. 55
AVOIDING IMMINENT GROUND CONTACT In the event of imminent contact with the ground, such as during an extreme wind shear or downdraft encounter or unintentional flight toward terrain, it may be necessary to use all available airplane energy by trading airspeed for altitude to avoid or soften impact. Simultaneously increase pitch attitude and apply thrust, if necessary, to the limit of forward throttle movement. The initial rotation should be accomplished sharply, and pitch attitude should then be adjusted to achieve a rate of airspeed decay sufficient to arrest the descent and to climb. Maintain these conditions until reaching a safe height or until the stick shaker activates, whichever occurs first. If the stick shaker activates, lower the nose sufficiently to stop further airspeed decay, maintain attitude and thrust, and continue climb with the stick shaker activated. When safe conditions are achieved, initiate recovery of airspeed. Be aware that in almost all cases, pitch attitudes in excess of 20 degrees will be required in this maneuver. Rate of airspeed decay should not be so great as to decelerate significantly below stick shaker speed. There are some potential problems with a pullup to stick shaker speed that should be considered: 0 If the pullup is accomplished too soon (significantly prior to imminent ground con-t-act) climb rate is actually decreased, since climb rate at stick-shaker speed is less than at V2, V2 + 10, or Vprog o If the pullup is accomplished too soon, and ground contact still occurs, there is no airspeed cushion to use for a "flare" to soften impact. 0 If airspeed decay rate during the pullup is too great, speed will decrease through the stick-shaker speed, and the airplane could stall. When at stick-shaker speed, any turbulence or additional shear could cause the airplane to stall. O It is possible that the effects of heavy rain could cause an increase in the airplane's stall speed. This could cause the airplane to stall before reaching stick-shaker speed. Nevertheless, if ground contact is indeed imminent, this maneuver represents a "last chance" effort to avoid or soften contact with terrain. Early recognition of the flight *Vprog. refers to a preselected or commanded speed, which may be entered into a flight director or autothrottle. 56
condition, either through GPWS [Ground Proximity Warning System] alert or crew awareness of the possibility of wind shear, should allow use of the normal go-around procedure which would avoid the necessity of using the extreme measures described above. Any time engine limits are exceeded in these circumstances, the possibility of severe engine damage exists and a landing at the nearest suitable airport may be required. This discussion of operating procedures during a wind-shear encounter is among the most comprehensive treatments found in manuals used by airlines operating under Part 121 of the FAR. It does not appear that similarly detailed procedures are common among the Part 135 commuter and air taxi or the general aviation communities. No U.S. aircraft operators--airline, commuter/air taxi, or general aviation--are known to have established operational limitations governing the takeoff and landing decision by the pilot-in-command based solely on reported low-altitude wind-shear levels. LLWSAS information is generally treated as advisory in nature. It is clear that the 15-knot vector difference threshold that triggers an LLWSAS alert is not regarded as a serious operational hazard except in the case of a takeoff limited by runway length or by obstacles in the departure flight path. Airline operating policies and procedures typically require pilots to assess all relevant factors in deciding whether to take off or to continue an approach to landing in the event of a potential wind-shear exposure. The wind-shear level indicated by LLWSAS is only one of many factors to be cons idered. Training Programs A review of airline training programs reveals that, as required by the FAA, FAR 121 air carriers train fl ight crews on the nature of wind shear and operational procedures required to cope with its potential threat. But the exact nature and timing of the training vary widely from airline to airline. All airlines appear to cover the subject extensively in ground training. As with operations manuals, the material is based largely on excerpts from the FAA's advisory circular, Boeing Airliner articles, and the technical literature. The FAA has a similar wind-shear ground training requirement in FAR 135, for operators subject to those regulations. However, the committee was unable to assess the scope of coverage of wind-shear hazards in ground training programs of FAR 135 operators. There is no specific requirement for wind-shear training for general aviation pilots other than the very general "aeronautical knowledge" requirement in FAR 61, applicable to issuance of pilot certificates and ratings. The FAA has no spec if ic requirement for flight training of pilots on procedures to be followed in the event of an inadvertent wind-shear encounter. The FARs applicable to training have evolved from a premise 57
that all necessary flight training--for all classes of operators--can be conducted in flight in an airplane. Requirements in the FARs governing use of simulators to conduct flight training are permissive. Simulators may be substituted for airplane training, but there is no requirement for simulator training for any class of carrier, not even for air carriers operating under FAR 121. Inflight training for wind-shear encounters is not feasible. In view of the random and rare occurrences of wind shear in nature and because of the threat posed to flight safety by exposure to severe wind shears at low altitude, air carriers operating under Part 121 conduct such flight training in simulators. These devices are more economical to operate than airplanes and provide superior training capability for a range of abnormal and emergency training problems, including severe wind-shear encounters. However, only those few "advanced" simulators approved under F. M 's Phase II and Phase III criteria in Appendix H of FAR 121 are required to incorporate "representative three-dimensional wind-shear dynamics based on airplane-related data." FAR 135 operators typically conduct a much smaller amount of flight training in simulators. Also, relatively little flight training in simulators is conducted within the general aviation community, although corporate and other operators of sophisticated multiengine turbojets use simulators extensively for training. It is unlikely that simulators will become universally available for flight training in the foreseeable future--not even for air carriers operating under FAR 121. Their use is limited in par t by the FM requirements that s imulators meet demanding technical specif ica- tions (FAR 121.407; FAR 121, Appendix H; FAR 135 .335; Advisory Circular AC 120-40~. Many aircraft, including older transport-category types, have never had an adequate simulator data package developed to enable a simulator to be programmed such that it can meet applicable criteria for FAA approval. The FM has fostered and regulated simulator training since the introduction several decades ago of regulations governing the use of simulators in air carrier training programs. As simulators have been improved, the FAA has systematically recognized and credited their use in pilot training, checking, and certification programs. Advisory Circular AC 120-40, Airplane Simulator and Visual System Evaluation, contains criteria for the approval of simulators. However, these criteria are conservative in the sense that they require simulators with capabilities that make them very complex and expensive to obtain and to operate, in order to be approved as a training device for pilots operating under FAR 121 and 135. The F. M does not specifically recognize simulators as a substitute for airplane training of general aviation pilots, although the FM has granted several exemptions for this purpose. 58
Wind-Shear Modeling for Training Simulators Advanced simulators currently used for pilot training and checking are required to be capable of full-mission, pilot-in-the-loop simulation. They are required to have appropriate visual display and actual cockpit force-feel systems, including instrumentation recording systems, and appropriate dynamic modeling of the wind shear conditions to be encountered. Accurate portrayal of downbursts and wind-shear conditions and related aircraft responses in a simulator with motion base and visual displays presents a complex computer modeling problem. It requires extensive and expensive computation capabilities, available only in costly advanced simulators with large memory capacities, fast compu- tation capabilities, and real-time cockpit controls and displays and response t imes necessary for the realis tic portrayal of wind-shear encount ers. Results of recent wind-shear measurements in the JAWS Project indicated that the wind-shear models now being used by the airline industry for training, for airborne control and display system development, and for certification purposes do not accurately portray actual wind-shear situations. Most models in use today are based on data developed for the FAA during 1976-1979 (Foy, 1979~. The most accurate portrayal of wind-shear fields are the four- dimensional, time-varying models based on JAWS data being proposed by NASA's Langley Research Center. These models are of interest and importance in research. However, their use of time dependence greatly increases computational requirements, which can easily exceed the capabilities of the computers used in simulators. It is possible to use a somewhat simplified representation of wind shears for some purposes without significantly compromising the results. In these cases, very real benefits in time and effort plus substantial savings in costs can be achieved. The increasing complexity and costs associated with the latest developments in computer modeling and simulation may make it prohibitively expensive for many potential users and may serve to stifle the application of new technology and safety advances. Studies should be undertaken to determine when simplified wind-shear models are acceptable for purposes of system design, evaluation and certification, and for training in simulators. The FAA has proposed procedures for approving airborne wind shear systems (Draft Advisory Circular AC 120-XX, ~ ). This proposed advisory circular describes wind fields considered acceptable for various specific applications. They were developed from accident reconstructions, meteorological data, and other sources. The FAA plans to update these wind field definitions as new information from JAWS and other studies becomes 59
available. The new def initions should include "severe" wind profiles that may exceed the performance capability of a specific airplane. Development of these new wind-shear models will take a considerable period of time and much effort. It is essential that industry standards for implementation and use of the resulting models be established to minimize different interpretations of the data. This should be undertaken as a joint government-industry effort. The FAA should publish the results for use in simulator training as well as for evaluation and approval of airborne systems. If possible, these wind-field models should be standardized for specific applications to ensure consistency among users. Particularly promising is a simple empirical model that defines a three-dimensional wind field that may be adapted for use in simulator applications to represent thunderstorm outflow phenomena of the type associated with recent aircraft accidents. This model is being developed at NASA's Ames Research Center. Per formance in Wind Shear Airplane Response to Wind Shear Airplanes generate the aerodynamic forces that make fl ight possible by means of airspeed, which is the velocity of an airplane relative to the surrounding air mass. Thus, a change in velocity of the surrounding a ir, or a wind shear, will cause a change in the aerodynamic forces on an airplane. However, all airplanes have some degree of speed stability and, as a consequence, once they have been disturbed by a wind shear, they will try to return to their original velocity relative to that of the new air mass. This process may take a half a minute or more depending on the airplane and the size of the wind shear (assuming the pilot takes no corrective actions). When an airplane encounters a wind shear, there will be changes in components of wind along each of the airplane's axes of motion. The longitudinal axis runs along the center of the fuselage and the wind component along this axis is a headwind or tailwind. The vertical axis has its associated up- or downdrafts, and the lateral axis has its associated crosswinds from the left or right. Each of these wind components will produce a different response based on the airplane's aerodynamic configuration. A brief description of how an airplane responds to each of these wind shears along individual axes will contribute to an understanding of the wind-shear problem. The airplane's response to each of these wind-shear components assumes no corrective action by the pilot and that the wind is steady before the plane enters and after it leaves the shear field. In actuality, wind-shear components exist in three dimensions, and the net effect of flying through such a wind field imposes disturbances in all six degrees of freedom of the aircraft. Much of the ensuing discussion centers on the control-fixed response. The closed-loop (piloted) responses will, of course, be very different and will represent actual cases that must be studied. 60
Longitudinal Wind Shear. A longitudinal wind shear that increases i. - , . . an airplane's airspeed can arise either from an increase in a headwind or a decrease in a tailwind. In either case, an increased airspeed caused by a shear will cause an increase in lift and drag. The air- plane pitches up and climbs, while its pitch stability causes it to reduce its angle of attack and decrease its lift as it starts to recoverer to its original trim condition. In the free-response case, after s everal osc it let ions, the a irplane wi 11 res tab 1 ize at i ts original airspeed on a flight path that is parallel but displaced above its original one, and its new speed relative to the ground will have decreased by the magnitude of the wind shear. A reduction in engine thrus t is required to regain the original inertial f light path. A decreasing tailwind and an increasing headwind both provide an apparent increase in airplane performance. However, the two cases present different problems to the pilot attempting to fly a glideslope since the decreasing tailwind case, with its higher ground speed, re quir e s 1 e s s power t o fo 1 1 ow th e pr oper appr oach pa th . The opposite situation for a longitudinal wind shear occurs when there is an increasing tailwind or decreasing headwind. In this case, airspeed decreases and lift and drag are reduced. The airplane pitches down and descends, while the airplane 's pitch stability causes it to increase its angle of attack to recover lift. Eventually, the airplane will restabilize at its trimmed airspeed on a flight path parallel to the original one but displaced below it and at a higher ground speed than before the wind-shear encounter. The shear has caused a reduction in altitude, which has been converted into increased ground speed at the original airspeed. To restore the altitude loss, energy must be added to the airplane in the form of increased engine thrust. Updrafts. An updraft disturbs an airplane by increasing its angle of attack. This increased angle of attack increases li ft and drag, which cause the airplane to climb and decelerate. The increased lift causes the airplane to pitch nose-down to reduce the angle of attack and to recover its original value. Again, after several oscillations, the airplane wit 1 return to its original angle of attack and airspeed relative to the air mass, but it will be climbing relative to its original inertial flight path. The opposite situation, a downdraft, decreases an airplane's angle of attack, thus reducing lift and causing it to sink. The decrease in lift causes a decrease in drag and a nose-up pitch to restore angle of attack. Eventually, the airplane will settle out at the original airspeed and angle of attack but will descend inertially within the a or mas s . Updraf ts impart energy to an a irpl ane making it c l imb; downdraf ts absorb energy, making it sink. Pilots must reduce power in updrafts 61
and increase power in downdraf ts to res tore the energy balance of the airplane. l.ateral Wind Shear. Crosswinds and lateral wind shears act on an , airplane by generating side forces plus yawing and rolling moments. The inn' ial response is to "weathervane" into the wind and to roll with the upwind wing rising. In a steady crosswind the airplane, which may or may not be enhanced by a stability augmentation system, will eventually stabilize with the wings approximately level and flying into the wind on a new heading. Basically, lateral wind shears do not cause large changes in altitude or airspeed. However, if large bank angles develop or if large lateral control spoiler deflections are used, a small loss of lift and rate of descent will be generated. During the landing approach, lateral wind changes will increase a pilot's workload by making the directional tracking task more difficult. However, while taking off or during go-around, precise heading control is not required, and some amount of dri ft from the runway centerline is acceptable. In summary, longitudinal and vertical wind shears can add energy to or subtract energy from an airplane. This must be compensated for by the use of throttles and by the appropriate application of longitudinal control by the pilot. Lateral shears do not affect an airplane's energy state as significantly, but they do make flying a precise approach path more difficult, thus adding to a pilot's workload. P ilot ' s Control in Wind Shear. Having discussed the response of an airplane to wind shears in the absence of pilot control, some discussion of pilot actions required to counter wind shear is relevant. The mos t exacting period of a f light is the landing approach because the pilot must fly an inertially fixed flight path to a point on the runway. It is in this context that pilot actions will be discussed. A decreasing headwind or increasing tailwind shear causes a loss in airspeed and a tendency for an airplane to sink below the glide slope. To counter this, the pilot must increase the airplane's angle of attack at the rate of about 1 degree of angle of attack for each 3 to 4 knots of speed lost. Additional pitch attitude is required since the angle of descent increases and the airspeed decreases. Simultan- eously with the required pitch-attitude increase, power must be increased to return the airplane to the glide slope, to overcome the increased drag, and to accelerate back to the intended approach speed or to initiate a go-around if the pilot sees fit. In a strong wind shear, large and rapid pitch-attitude and power corrections are required. The most common pilot mistake is not to make these corrections vigorously and rapidly. Downdrafts require similar pilot reactions. Downdrafts reduce an airplane's angle of attack and cause it to descend, since it is immersed in a column of descending air. To compensate for the effects of strong downdrafts, large nose-up pitch-attitude corrections may be 62
required. At the same time, thrust must be increased with the pitch-attitude increase to return the airplane to the glide slope. Increased thrust will also be required to maintain the flight path. A combination of downdraft and decreasing headwind or increasing tailwind, as is characteristic of a downburst, will require very rapid and large pitch-attitude and power corrections. Simulator studies of flying a landing approach through severe wind shears of this type have shown that unles s an immediate go-around at a high pit ch angle and high thrust is initiated by the pilot, there could be little chance of s urvival . An increas ing headwind shear would appear to be benign since it carries the airplane above the glide slope and increases airspeed. To compensate for this, pilots hold airspeed by reducing thrust and decelerating ;nertially. However, when the airplane emerges from the shear, the airplane can be low on thrust and starting to sink rapidly. If this occurs at low altitude and when transitioning from instrument to visual flight, the increased rate of sink may not be detected by a pilot before it is too late to complete corrective action to avoid ground contact. Lift Characteristics. In the F. M certification process for any airplane, the FAR stall speeds are determined by extensive flight testing. This is important because the stall speeds are the basis for specifying the operating speeds at low altitudes in the vicinity of an airport. The FAA specified stall speed (Vmin FAR) is the minimum speed that is achieved in a stall maneuver that involves a 1 knot per second deceleration with zero thrust. Since the FAR stall speed is a minimum speed at less than lg for most airplanes, level flight cannot be sustained at this speed. The minimum speed at which level flight can be sustained is the lg stall speed (Vmin la) ~ which is typically 5 to 7 percent faster than the FAR stall speed. The lg stall speed is affected by engine thrust, since the angle of attack is sufficiently high that there is a component of thrust in the lift direction that augments the wing 's lift. Maneuvering capability and speed margins in a wind-shear situation should, therefore, be referred to as the lg stall speed. Figure 12 shows this comparison. The height of the bars indicates the speed margin and maneuverability, and the width indicates the spread in speeds of different aircraft types. Stall buffet and/or stick-shaker actuation occurs as a warning slightly above the lg stall speed and provides a warning of the impending loss of lift. Flying at incipient stick-shaker speed or on the edge of stall buffet is the practical lower airspeed limit for sustaining flight. It provides a small operating margin over the lg stall speed for turbulence, rain, instrumentation errors, and the like. Commercial and general aviation aircraft operate at similar percentages of their FAR stall speeds during takeoff and landing. The 63
- 2.0 4J o c - Z c:~ LU UJ J At: 40- 20- _ 10- ~ 1 O- 40 ~1.8 _ 30 ~ 20- _ 10_ ~ O~ In LAND I NG 1.8 _~ m 1.6 m o 1.4 0 c: 1.2 ~ o .1.0 v !min. FAR -0.8 L 1 1 1 tonne Vmin 1 y = ~00 1 note VREF+~ VR ;: Vstick shaker 90 100 110 120 130 AIRSPEED (knots) cn J 1.6 ~ z 6 14 m O o c' 6 lL a 1.0 0 1.2 TAKEOFF Example: V . = 120 knots min. 1 9 Range of Typical Speeds 1 1 140 150 Target Cl imp 110 120 130 140 150 160 Vstick shaker Vmin, 1 9 ~ , I' min.,FAR r I, -0.8 1 -^ 11 ,l i I . AIRSPEED (knots) _ Range of Typical Speeds FIGURE 12 Maneuver Margins of Transport Aircraft During Takeoff and Landing Approach. minimum takeoff safety speed set by the FAA for commercial transports, V2, is required to be 1.15 VMIN FAR for propeller-driven aircraft and 1.2 Vmin FAR for turbojet-powered aircraft. However, V2 may be set higher for other considerations. Minimum landing approach speed, VREF, is 130 percent of the FAR stall speed in the landing approach configuration. Normal operation of most types of commercial aircraft is at V2 + 10 knots or higher on takeoff and VREF ~ 5 or 10 knots on landing. These speeds provide transport aircraft with approximately a 25-knot margin over the lg stall speed on takeoff and a 30- to 35-knot margin on landing. General aviation aircraft that have lower stall speeds 64
will have proportionately smaller speed margins. For an aircraft with a 50-knot stall speed, the margin could be one-half of the above va lues . Some accidents have occurred in thunderstorms where the airplanes encountered heavy rain and wind shears simultaneously. Some preliminary analytical and experimental data indicate that heavy rain can degrade a wing's aerodynamic performance by reducing its maximum lift capability (Haines and Luers, 1982~. NASA has embarked on a program to develop a technique to measure the effects of rain on airfoils and plans to apply this technique to some representative airplane configurations to determine the effects on performance. Thrust and Drag. The ability to survive large longitudinal and . vertical wind shears depends both on the excess thrust that is available to accelerate an airplane and on the available lift margin. A typical three-engine airplane's thrust and drag characteristics are shown in Figure 13 as the ratios of maximum takeoff thrust over weight (T/W) and drag over lift (D/L). These ratios vary with speed above the FAR stall speed. They also vary for takeoff and landing flap settings at typical takeoff and landing weights . The figure shows that: o The aerodynamic characteristic D/L is a function of angle of attack (or V/Vs for lg flight ~ and flap and gear position only. o Maximum T/W depends on an airplane 's weight and on the engines ' maximum takeoff thrust, which depends on altitude and temperature. The excess T/W, which is the difference between maximum T/W and D/L, defines the acceleration or climb capability of an airplane. o Since the FAR performance requirements specify a minimum climb capability with the critical engine not operating, when all engines are operating the T/W can range from twice the minimum for two-engine configurations to 1.33 for four-engine configurations. Thus, two-engine airplanes have the largest thrust margins. 0 The excess T/W does not vary significantly with V/VsFAR over the normal operating speeds. Below V2 + 10 and VREF there is some decrease in excess T/W, but it is still substantially greater than is available in a climb at V2 or VREF with one . · ~ engine Inoperative. O The acceleration capability shown in Figure 13 for takeoff at V + 1O2 is 2.7 knots per second ~ ~ T/W = 0.14) and is 1.9 knots per second at stick-shaker speed. For landing the acceleration capability is somewhat larger due to the lower 65
airplane weight, although this is partially offset by the increased drag of the landing configuration. Takeoff in Wind Shears During takeoff the critical altitudes for wind-shear encounters are below 400 feet. If a severe shear is encountered above this altitude, an airplane probably has enough altitude and speed margin to traverse the shear and have terrain clearance. As a downdraft approaches the ground, the ground presents a solid boundary that converts the vertical winds of the downdraft into an outflow of horizontal winds. 0.28 0.24 O I ~ CE: 0.20 0.12 _ 0.16 0.08 _ TAKEOFF Jt/Weio:. Stick Shaker 1.0 1.1 0.32 IL ~ ~ O 0.28 I ~( CE: 0.24 own 0.12 T/W d/L (Gear Up) 1.2 1.3 1.4 1.5 1.6 V/Vs FAR LANDING Stick Shaker _ _ , Max. Thrust/Weight D/L (Gear Down) V ~ ~ ,,, _ 1 ~ 1 1 1 1 1 1 1.0 1.1 1.2 1.3 1.4 1.5 1.6 V/Vs FAR FIGURE 13 Thrust and Drag Characteristics for a Typical Three-Engine Transport Aircraft with All Engines Operating. 66
It is relatively easy to determine the magnitude of the critical tailwind that will cause an airplane to lose altitude. Not considering downdrafts, if the rate of shear exceeds the acceleration capability of an airplane for any significant period while the airplane is below 100-200 feet, the airplane will lose altitude, airspeed, and rate of climb. As a result, the airplane will settle into the ground or stall. For instance, if the airplane represented in Figure 13 encountered a tailwind shear that increased to 50 knots over a 10-second interval, the airplane will lose 25 knots of airspeed if flown at a constant altitude. The airspeed loss will be greater if the pilot attempts to climb. This tailwind shear will drive the airplane to stick-shaker speed (see Figure 12~. If significant downdrafts are associated with a shear, it is unlikely that an airplane can successfully penetrate the shear at altitudes below 100-200 feet. Measurements made during the JAWS Project have shown that this large a shear is not unusual in a microburst. The best solution is to devise some means to warn a pilot before takeoff of the hazardous wind-shear condition, so that departure can be delayed. If a severe wind shear is encountered at takeoff, there is little a pilot can do except be sure he has set the throttles at maximum available thrust or overboost the engines and try to conserve altitude until the shear is traversed. Altitude Loss in Wind Shear on Landing Approach Analysis of the flight path of an airplane that penetrates a severe wind shear on landing is a complex problem. Figure 14 presents a generalized view of the altitude loss in transitioning from a 3° glide-slope approach to a go-around in tailwind shears of varying magnitude and intensity. The assumptions of pilot recognition time and the pilot's response in such a study are crucial to the results. This figure assumes two different sets of pilot actions. The lower set of curves is representative of what may be expected when a pilot has no cockpit warning and no wind-shear flight director or guidance information to cope with the wind shear. In this case, however, the pilot visually identifies the hazard in a reasonable time and vigorously executes a missed approach. Here, a 4-second time delay is assumed from the time of the wind-shear encounter to the application of additional power. An additional 2 seconds is assumed to elapse before the pilot initiates an airplane-nose-up rotation to arrest the airplane's descent. The pilot continues to apply power until maximum power is attained (about 2 seconds), and the aircraft continues to rotate until the stick-shaker warning is actuated (about 3 seconds). Thereafter, the airplane is controlled to maintain an attitude of incipient stick-shaker actuation until its descent has been arrested and it has begun to accelerate. The upper curves in Figure 14 show the maximum performance capability of the aircraft allowing a minimum realistic time for wind shear recognition. These curves are representative of the potential 67
o - cr 7 8 us -200 -100 it: ~ -300 o 111 a' by: TOTAL HOR IZONTAL TAI LWIND SHEAR (knots) 10 20 -400 -500 40 50 NOTE: Altitude Loss on Vertical Scale is for Recovery to Level Flight with the Aircraft accelerating 60 70 80 ~ ~ '1'~ :~ 90 1 00 - Maximum _ Capabil ity with ,' Warning , A/ + , / Gu Ida nce x ~~ 1 ASSUMPTI ONS Initial Speed VREF + 10 Constant 30 Deg. Flaps and Gear Down , I 1 1 FIGURE 14 Transport Aircraft Flare Capability in Wind Shear from a Three-Degree Approach Path. v >~ go-around and flare performance for an aircraft with a system providing a warning of a shear encounter. It assumes an appropriately trained pilot who follows wind-shear guidance and command indicators. Here, the power application and nose-up attitude control are initiated at the end of 2 seconds and, as in the previous case, power application is continued until full power and incipient stick-shaker conditions are attained and the flare maneuver completed. The constant, incipient-stick-shaker angle-of-attack portion of the flare is a very important assumption in the dynamics of the flare maneuver. The asymptote at the top of the upper curves in Figure 14 shows an approximately 80-foot altitude loss. The first 30 feet represent the 2-second delay in recognizing the shear encounter, and the remaining 50 feet are lost during a straightforward flare. For wind shear conditions along the asymptote line, the airplane has sufficient performance to penetrate without further loss in altitude, providing the pilot uses the airplane's pull-up capabilities up to the stick-shaker angle of attack. For more severe shears an aircraft cannot maintain altitude after it has reached stick-shaker condition. In these cases, as it loses altitude the airplane accelerates and develops a normal load factor, which provides the flare. In Figure 14, at 8 knots/second 68
shear gradient and with an 8-second penetration time, there is a 64-knot change in horizontal tailwind. The curve shows a loss of 235 feet in altitude in this maneuver for the maximum performance case. Figure 14 considers only horizontal tailwind shears. The addition of downdrafts is difficult to present in a generalized form. Follow-up studies should consider combined horizontal wind shears and downdrafts with realistic combinations of shear gradients. Figure 14, which presents the results of a simplistic analysis of a very complex problem, shows that under 500 feet above ground level (AGL) the probability of surviving a severe wind shear is greatly enhanced by an immediate recognition and response to the shear. A corollary is that warning devices and equipment that improve the pilot 's ability to respond in this manner also contribute significantly to the probability of success. Thus, pilots must rapidly recognize a severe low-altitude wind shear on approach and immediately execute a missed approach maneuver. This means immediate application of maximum thrust and rotation to arrest the descent of the aircraft. As a last resort, in a severe wind shear the airplane may have to be flown at or slightly below stick-shaker (or other stall-warning) angle of attack until it leaves the shear area and its performance margins are restored. At that point, it should be possible to climb and accelerate to normal climb speeds. Few nonmilitary aircraft are equipped with an angle-of-attack display. Therefore, stick-shaker (or other stall-warning) angle of attack, which is 2° to 3° below the angle of attack for maximum lift coefficient, becomes the upper limit of rotation on aircraft lacking special instrumentation. The combination of energy trade and use of maximum thrus t gives pita ts the tees t chance of preventing ground contact i f they encounter a severe shear without warning at low altitude. Pilots must recognize that flight at stick-shaker angle of attack entails significant risks that can be justified only if the aircraft encounters severe shear at very low altitude. Pilots should react to shears above 500 feet as rapidly and positively as described above, but in this case the extra altitude makes it possible to initially rotate to less severe angles of attack, better preserving speed and lift safety margins. General Aviation Aircraft While extensive research has been conducted on the characteristics of wind-shear encounters of conventional jet transport airplanes, there is only a small amount of analysis (Lehman et al. , 1977) for the large variety of general aviation airplanes, which includes corporate, commuter, and personal aircraft. First, there appear to be few documented cases of wind-shear-related accidents of general aviation airplanes. Second, accidents involving these airplanes are rarely 69
investigated as aggressively as those of air carrier aircraft. However, the hazard of low-altitude wind-shear accidents is present f or genera 1 avi a t ion a ircr a f t . The to 1 lowing s impl if fed analys is provides some 1 imited ins ight into the basic factors influencing the ability of general aviation airplanes to adjust to wind shears. It also indicates an avenue of further study that should be pursued. The analysis assumes that an airplane takes 20-30 seconds to tranverse a downburst, during which period the wind-shear velocities are constant. A measure of the severity of the shear, then, is the peak velocity difference across the microburst. Airplane performance capability can be represented as the ability to increase speed per nautical mile traveled. On this basis, comparisons can be made among the various classes of airplanes. Table 4 presents acceleration capability at the best angle of climb speed in the clean configuration for general aviation airplanes and for a heavily loaded three-engine jet transport in the takeoff configuration, based on airplane flight manual performance data. It shows that many airplanes thought to have low performance capabilities have surprisingly high values of acceleration potential compared with jet transports. While this appears to be a very favorable factor for light general aviation aircraft penetrating wind shears, it is by no means the only factor to be considered with regard to the hazards of severe wind-shear encounters. Also, the acceleration potential values presented in Table 4 are based on a static, instantaneous performance analysis and are only indicative of the relative acceleration capabil- ities of the aircraft types listed. A similar criterion is described by Frost (1983~. TABLE 4 Acceleration Capability at Best Angle of Climb Speed Rate Acceleration Performance Aircraft Type Airspeed of Climb Margin Factor, ~V/^ X V(KN) R/C(FPM) (KN)/(SEC) (KN)/(NMI) Trainer 62 780 2 .37 138 Basic 4-Place 62 660 2.00 116 High-Per formanc e Single Engine 83 1050 2.38 103 Light Twin 78 1140 2.75 127 Cabin Twin 88 1380 2.95 121 Light Turboprop 105 1560 2.79 496 H igh-Per formance Turboprop 106 2100 3.72 126 Light Turbo fan 150 2040 2 .56 61 High-Performance Jet 200 4200 3.95 71 High-Performance Turboprop 180 3300 3.45 69 Typical 3-Engine Je t Transport at T.O. 160 2300 2.7 61 . . . . . . 70
The following additional points should be explored before definite conclusions can be reached on the relative capabilities of general aviation aircraft and jet transports in penetrating severe wind shears. 1. General aviation aircraft usually have lower takeoff and landing approach speeds than do commercial jets. Consequently, a given total wind shear represents a higher percentage of the airplane's flight speed. This, of course, makes it more difficult to penetrate a given wind shear. 2. Since the minimum operating speeds during takeoff and landing approach are defined as a percentage of the FAR stall speed, general aviation aircraft with low stall speeds will have smaller speed margins than do jet transports. Their ability to absorb an airspeed loss in a wind shear would be less than airplanes with higher stall speeds. However, general aviation aircraft generally operate at proport ionately larger speed margins above their minimum operating speeds compared with transport aircraft. Their slower penetration speeds would allow pilots more time for recognition of and response to wind shears. 3. Propeller-driven general aviation aircraft have the benefit of the propeller slipstream over the wing, which provides an added margin of lift not available to jets. 4. Some jet-powered general aviation aircraft have very high thrust-to-weight ratios. This is a very favorable factor in penetrating adverse wind shears. 5. Slow-flying general aviation aircraft will be more sensitive to updrafts and downdrafts than are jet transports, since such drafts produce a larger angle-of-attack change on a slower-flying airplane. 6. General aviation aircraft with small wing spans have less roll damping and are more active in roll than large jet transports. Lateral control could require a large increase in pilot attention in a severe shear that could deteriorate the pilot's flight-path-tracking performance. Thus, there is no justification of an assumption that general aviation aircraft are less susceptible to wind shear than transport aircraft. More research is needed on representative general aviation aircraft to determine their vulnerability to wind shear. NASA has conducted wind tunnel tests on a series of full-scale general aviation airplanes, and this would provide a data base for the required aerodynamic characteristics. The Ames Research Center has performed related simulator investigations of wake vortex encounters on various categories of airplanes, using available aerodynamic data for representative airplanes. The aerodynamic data base that permits a detailed piloted simulation is generally available or could be estimated, and it should be suitable for analytical and simulator investigations of general aviation airplanes in wind shear. 71
Guidance and Control Aids The threat to safe aircraft operation posed by wind-shear effects, as distinct from high levels of turbulence, occurs principally near the ground, when an aircraft is taking off or landing. During these phases of flight, the pilot's workload could be greatly al leviated with guidance and control aids. Documented dangerous encounters with wind shear have lasted for half a minute or less, implying transient forcing of the closed-loop phugoid motion, which provides the basic mechanism for interchanging kinetic and potential energy, i.e., for trading speed and altitude. Guidance and control aids could improve pilot capability in effectively combating wind shear by helping to manage the aircraft's energy level, either automatically or by helping to perform the necessary maneuvers. Other critical elements in the cockpit for coping with wind shear include: o Warning that severe wind shear may be encountered. O Recognition that a wind shear has been encountered . O Prompt initiation of the proper control actions to deal with the effects of wind shear. o Exercise of continuous control actions during an encounter so as to succes s ful ly go-around, complete the takeof f, or successfully land or execute ground contact under circumstances that will maximize the chances for survival. All of these elements need to be addressed to improve flight safety in the presence of wind-shear hazards. In the context of practical flight operation, guidance and control aids must allow an aircraft to complete its nominal flight phase as often as possible, reserving the decision to abort the approach or delay the takeof f as infrequently as poss ible and for only the more severe shear encounters. The "go/no-go" decision must be accurate and must be based on all available information: meteorological, LLWSAS, contra l tower reports, and as a last resort onboard warning or guidance information if a shear is encountered . Wind-shear guidance and control aids must be capable of using the full performance of the aircraft. As distinct from normal conditions, severe wind shears can drive an aircraf t to "the edge of the f light envelope" and beyond. Consequently, these aids mus t be optimal in the sense of generating maximum performance while maintaining safety marg in S . F1 ight guidance displays provided to pilots range from very primit ive one s on light general aviation a ircraf t to very sophisticated displays on advanced commercial transports that incorporate wind-shear guidance features. In between these extremes 72
are the bulk of aircraft that are not adequately equipped with guidance systems to cope with a severe wind-shear encounter. In fact, some flight directors may provide misleading guidance information by commanding nose-down pitch to recover speed in an increasing tailwind, rather than anticipating a loss in altitude and commanding a pitch-up maneuver. Manufacturers should review their systems to determine if misleading commands exist and to warn pilots of this possibility. It is imperative that wind-shear guidance and control systems provide margins in their designs for turbulence and rain effects when these effects have been determined. Specifications for appropriate onboard systems depend on the type of aircraft as well as on the circumstances of installation (new or retrofit). The components of wind-shear guidance and control aids may include sensors, ground-air communication links, cockpit control devices, computers, and displays. Systems can be assembled from off-the-shelf components; in fact, wind-shear warning devices are currently being sold, but only the most advanced fl ight directors contain some degree of wind-shear command capability to provide the required margin of safety and a minimum of schedule interruptions. More research and development must be done, particularly on ground and airborne "predictive" sensors. Airborne Warning Sys tems All airborne radar warning systems based on current technology will likely suffer from the ground-return (clutter) problem. This problem stems from the need for an aircraft on approach to scan the descending fl ight-path volume ahead for evidence of wind-shear activity. A low-altitude look-down capability (ahead of the aircraft) probably is not feasible in the near future because of associated development problems. However, research should be continued on promising technologies that can sense wind shear before it is encountered. The payoff for a practical device would be high. Existing airborne weather radar, and lidar and IR sys tems under development, can look up the proposed flight path without attendant ground-c lut t er re t urn . I f such radar had a Doppl er capab i 1 ity, i t could allow pilots to detect severe low-altitude wind shear in the critical portion of the proposed takeof f path. This information could influence the pilot to delay takeof f until the observed or suspected wind shear had dissipated and, through PIREPs, provide a warning to approaching a ircraf t . Because pilots bear the ultimate responsibility for ensuring safe flight, the information presented to them regarding aircraft state and wind-shear environment must be appropriate, cone ise, and complete . It should be presented in such a way as to enable pilots to make the right decisions. As a minimum, pilots need both a wind-shear warning and a wind-shear command display. The warning is needed to identify a possible fl ight-critical event and a confirmation that abnormal 73
aircraft response and cockpit displays can be expected. Pilots should be apprised of the correct command strategy, either to control manually or to monitor the automatic system. command information is required both for aborting and for completing the intended flight phase. Devices. State-of-the-art equipment appears to be sufficiently well developed that no research is needed specifically for wind-shear components. However, consideration should be given to nonstandard application of flight controls such as spoilers and flaps. Algorithms. The key to effective use of available components is the logic that binds them together. Existing flight computers can implement anti-wind-shear logic, processing, as necessary, a multitude of inputs and outputs. Onboard computers could carry mathematical models of aircraft performance, nonlinear control logic, and optimal estimation algorithms. These can be used to ensure that the aircraft and the pilot perform at their best in a wind shear. Manual and Automatic Control Systems Guidance and control systems can be designed to generate the critical functions and displays required to cope with most wind-shear encounters. The automatic flight control systems on advanced transport aircraft have the capability to satisfactorily perform many of these functions automatically. Such systems and their associated displays constitute a near-term approach to providing adequate guidance and control aids for dealing with wind shear. An example of such a system is discussed in Appendix B. It should be noted that advanced transport aircraft in service today have these capabilities. ~ . Extensive evaluations must be performed to ensure that automatic flight control systems operate satisfactorily during all foreseeable situations. Detailed nonlinear models of automatic flight control systems are investigated on a simulator in a large variety of wind-shear conditions to validate the systems. These evaluations include, for instance, sensitivity analyses with discrete wind-shear models to ensure that landing performance does not vary excessively when wind levels are increased. Finally, flight testing in the terminal area is performed with the express purpose of demonstrating satisfactory performance in the most severe winds available and to validate earlier analytical and simulator test results. These winds, however, do not approach the severity of those evaluated by analysis or simulation. This work is part of FAA's certification process for airplanes and their subsystems. Based on the demonstrated capability of systems such as the one described in Appendix B. current technology and existing guidance and control systems have the inherent capacity to cope with unexpected wind shears up to some level of severity, reflecting the basic 74
aircraft performance limits. However, newer, more complete wind-shear models have not yet been evaluated in connection with such guidance and control systems. This needs to be done to more fully determine acceptable system performance for wind shears used for system design. Furthermore, research should be conducted to ensure that the operational envelope of flight safety provided by automatic systems is near the maximum attainable. Separate Wind-Shear Indication and Alerting Displays. Many existing autopilot and/or flight director sys tems contain the basic sensors and can be upgraded to a wind-shear-certified level, but such modifications would undoubtedly be costly and time-consuming. This would be especially true for automatic system modifications, which would have to be multiple-redundant to achieve dispatch reliability. Therefore, consideration should be given to wind-shear-specific di splays that ran be added to existing cockpit instrumentation. A study sponsored by the FAA (Foy, 1977) explored a number of such possibilities. Most of the reasonably successful displays measured ground speed, which could eas i ly be compared wi th airspeed to deduce wind speed and which was additionally used as the teas is for commands on the fast/slow indicator. Some also used energy rate or acceleration margin for a warning and go-around advisory. However, their relative success was airplane dependent, being considerably higher for the DC-10 and Boeing 727 than for the Boeing 707, perhaps, although not so stated, because of the additional engine-out performance margin of a 3-engine compared with a 4-engine airplane. Successful penetrations for the 3-engine aircraft group were approximately 70-80 percent, whereas succesful penetrations for the 4-engine Boeing 707 dropped to approximately 40 percent. None of the systems permitted the pilot to cope successfully with severe wind shear at takeoff. The relatively poor penetration performance during takeoff was about the same as for the baseline (standard instruments) condition. Additional studies were conducted by Bray at NASA's Ames Research Center (Foy, 1979~. This effort led to a head-up display (HUD) presentation that used complementary-filtered horizontal inertial acceleration and rate-of-change of airspeed and vertical inertial acceleration and rate-of-change of barometric climb rate. The system provided s tatus information with a signal corresponding to potential fl ight path serving as the primary alerting cue. It also presented the angle of attack, developed from measured pitch attitude and computed flight-path angle. The pilots exposed to the HUD thought it was a very good learning tool and that it appeared to improve shear-penetration performance. However, there were a significant number of unsuccessful shear penetrations recorded in the studies. ~ _ . Perhaps encouraged by the degree of wind-shear penetration improvement reported by some of the studies discussed above, various avionics manufacturers have begun marketing products specifically designed to improve pilot performance in wind shears . Of three systems that have reached hardware s sage, 75
those by Smiths Industries and SFENA incorporate new information in an existing display, while Safe Flight is producing a separate instrument. Other devices may be available but were not made known to the committee. The Smiths system utilizes air data computer outputs of vertical speed and airspeed, differentiating the latter and combining the result with vertical speed to give energy rate. The energy-rate information is displayed on a second needle incorporated into the vertical speed indicator. An energy rate that falls below vertical speed (climb rate) implies a requirement for added thrust and vice versa. The Safe Flight system uses an aircraft's existing airspeed and angle-of-attack information from conventional sensors, along with horizontal and vertical accelerometers supplied by their computer to calculate wind-shear components and alert the pilot. At least one airline intends to retrofit its fleet with this device if planned simulations are successful and "if the Government authorizes everyday f 1 eetwide use" (New York Times, June 6, 1983~. Angle-of-attack sensors have long been available commercially and are considered by many to be an obvious and highly desirable aid in the proper utilization of an aircraft's maximum climb performance, so necessary to the successful transit of severe wind shear. An energy-rate sensor described at NASA's Langley Research Center (Ostroff, 1983) is an interesting instrument that, because of its relative simplicity, may be added to existing general aviation aircraft. The probe is simply a hollow round tube with an aft-facing hole near its closed tip. The tube is mounted on the side of the airplane near the nose, protruding at an angle 20° forward of a line perpendicular to the surface. An interior line connects the probe to a climb-rate transducer through a restrictor and a filter volume . Increases in either speed or altitude cause a decreased pressure at the sensing hole (and vice versa), which is transformed to an energy-rate indication by the resulting "leakage" flow. Although it is not described as an anti-wind-shear device, the Hunt ing ton Air Speed Direc tor (Kidd, 1983 ~ could improve the probabilities of a safe wind-shear penetration. The device measures a differential pressure that is proportional to the product of dynamic pressure and angle of attack. It has been concluded that changes in this quantity are proportional, in turn, to vertical acceleration, or roughly to the energy acceleration, which provides a direct indication of hazardous downdrafts and tailwinds. Applicability and Utility. Assuming that the aforementioned systems give correct command and display information to a pilot during all types of wind-shear encounters, they would be a useful addition to present transport and general aviation aircraft to reduce the hazard posed by wind shear s . However, the extent of their individual 76
usefulness for particular aircraft has yet to be established, as does their routine usability. Many airline transport pilots strongly resist the imposition of yet another instrument into their already crowded scan pattern unless it reduces their workload. To the extent that such aids are considered crucial or critical to flight, they must be certified and their reliability to established standards demonstrated if they are to be required for aircraft dispatch. Reliability implies low failure rates; however, the greater concern is the probability of malfunction during a wind-shear encounter. This is the product of the individual probabilities of malfunction and the probability of encounter, both of which should be much less than one. Requirements. On May 3, 1979, the F. M issued an Advance Notice of Proposed Rulemaking (NPRM 79-11, Docket No. 19110~. This notice discussed FAA research and development on wind shear, and it requested comments and recommendations to assist the agency in determining what, if any, regulatory proposals should be developed to amend FAR 121 to require wind-shear detection equipment or other onboard systems to assist in coping with hazardous wind shears. The cogent period closed on August 3, 1979. The FAA docket file includes 35 responses from industry, government organizations, and the public. At the time the notice was issued, the results of the FAA s imulator research program were incomplete and not widely circulated. The consensus of the comments on the NPRM appeared to be that research and deve lopment work should cont inue on ground and airborne wind-shear systems to gain better understanding of the problems and to develop practical solutions prior to any regulatory action by the FAA. No final action has been taken on the NPRM 79-11 and the docket is still open. In view of the advances in anti-wind-shear technology, it may be timely to ask again those questions posed in NPRM 79-11 and to cons ider whether regulatory ac tior1 by the FAA is now appropriate to require anti-wind-shear equipment for air carrier aircraft. 77
