Appendix 1
2.3.2 ENCOUNTER DURING TAKEOFF - ON increasing tailwind shear however, the RUNWAY airplane did not reach VR As the until nearing the end of the runway.
Analysis of a typical accident where airplane lifted off, the tailwind conan increasing tailwind shear was en- tinued increasing, preventing any furcountered during takeoff ground roll ther airspeed increase. The airplane showed that initial indications ap- contacted an obstacle off the deparpeared normal (Figure 17). Due to the ture end of the runway.
Microburst Runway Figure 17. Windshear encounter during takeoff on runway. (1) Takeoff initially appeared normal. (2) Airspeed buildup slowed due to windshear. (3) Airplane reached Vp near end of runway, lifted off but failed to climb. (4) Airplane contacted obstacle off departure end of runway.
Appendix 1 windshear encounter, ift Less-than-normal airspeed, resulted in re- due to takeoff attitude (Figure 18). turn, inability lift off soon In duced available at normal enough to clear obstacles resulted.
Runway Normal Takeoff Microburst* Lift Runway Windshear Encounter Figure 18. Windshear effects on liftoff. Microburst reduces airspeed and lift at normal attitude that results in inability to lift off.
11/25/88 AC 00-54 Appendix 1 An additional factor is the difficulty If there is insufficient runway left of recognizing deteriorating airplane to accelerate to normal takeoff speed, performance. Timely recognition of a and inadequate runway to stop, liftoff windshear encounter on the runway may and safe climb may require rotation at be difficult since the only indication speeds less than normal rotation speed may be a slower than normal airspeed (Vp). In this case, additional pitch increase. The presence of gusts may attitude may De required to achieve mask abnormal airspeed build-up. Time sufficient lift (Figure 19). In traavailable to respond effectively to a ditional training, crews are frequentwindshear may be as little as ly cautioned not to rotate at speeds seconds from the initial encounter. less than vp to avoid high pitch at- Effective crew coordination, particu- titudes that could result in aft bod larly standard callouts, is essential contact. In a windshear encounter in routine operations to develop habit otation toward normal takeoff pitch patterns required to ensure timely attitude at lower than normal airspeed recognition of degrading performance. may be required to lift off in the remaining runway. This may result in Full thrust may be required to provide additional performance, particularly advertent aft body contact. To deal with an in- windshear encounter, tech- the if reduced thrust takeoff procedures pilot must be prepared to apply have been used. niques which differ from those ordinarily used.
Lift Microburst• Runway Normal Takeoff Attitude Lift Microburst" Runway Increased Pitch Attitude Figure 19. Pitch attitude effects on liftoff. Increased pitch attitude generates lift required for liftoff.
Appendix 1
2.3.3 ENCOUNTER ON APPROACH along the approach flight path (Figure
20). airplane lost airspeed, Analysis of a typical windshear en- dropped below the target glidepath, counter on approach provided evidence and contacted the ground short of the of an increasing downdraft and tailwind runway threshold.
Microburst Glide Slope Instruments Visual Runway Figure 20. Windshear encounter during approach. (1) Approach initially appeared normal.
(2) Increasing downdraft and tailwind encountered at transition. (3) Airspeed decrease combined with reduced visual cues resulted in pitch attitude reduction. (4) Airplane crashed short of approach end of runway.
Appendix 1 Reduced airspeed, as the airplane encountered the windshear, resulted decreased lift. This loss of lift increased the descent rate (Figure 21).
The natural nose-down pitch response of the airplane to low airspeed caused additional altitude loss. Pitch attitude increase and recovery initiation were not used soon enough to prevent ground contact.
Lack of timely and appropriate response--affected by weather conditro limited recognition time was a inadequate crew coordination significant factor in delaying re- Gradual applicathrust during have masked the initial decreasing airspeed trend.
complicated the approach.
from instruments to exterior references may have detracted from instrument scan. Inadequate crew coordination may have resulted in failure to be aware of flight path degradastabilized approach clearly defined callouts is essential to aid in recognition of unacceptable flight path trends and the need to initiate recovery.
Glide Path Lift Flight on Glide Path Runway Normal Approach Glide Path Pitch Down Microburst* Descending Below Glide Path Runway Windshear Encounter Figure 21. Windshear effects on flight path on approach. Microburst reduces airspeed and lift at normal attitude which results in pitch down tendency to regain airspeed.
Appendix 1
2.3.4 WINDSHEAR EFFECTS ON AIRPLANES headwind shears must not be relied AND SYSTEMS upon to provide early indications of subsequent tailwind shears. Be prepared.
Windshear Effects on Airplanes In contrast to shears which increase o Headwind/Tailwind Shear Response airspeed, an increasing tailwind (or decreasing headwind) shear will de- The various components of windshear crease indicated airspeed and performhave unique effects on airplane per- ance capability. Due to airspeed for the shear depts ot the flight loss, the airplane may tend to pitch of the shear depends down to regain trim speed.
path through the microburst.
• Vertical Windshear Response an increasing headwind (or decreasing tailwind) shear increases indicated Vertical winds exist in every microairspeed and thus increases perform- burst and increase in intensity with ance. The airplane will tend to pitch Such winds usually up to regain trim airspeed. An addi- peak intensity at heights greater than tional consideration is that this type 500 feet above the ground.
of shear may reduce normal decelera- speeds greater than 3,000 feet tion during flare which could cause per minute can exist in the center of overrun. a strong microburst. The severity of the downdraft the airplane encounters Any rapid or large airspeed increase, depends on both the altitude and latparticularly near convective weather eral proximity to the center of the conditions, should be viewed as a possible indication of a forthcoming airspeed decrease. Thus a large airspeed Perhaps more critical than sustained increase may be reason for discontinu- downdrafts, short duration reversals ing the approach. However, since mi- in vertical winds can exist due to the crobursts are often asymmetric and the horizontal vortices associated with headwind may not always be present, microbursts. This is shown in Figure 22.
500 ft Microburst Approx Series of Horizontal Scale Vortices 500 ft Microburst Flight Path Figure 22. Encounter with microburst horizontal vortices. Rapid updraft downdraft variations due to horizonta/ vortices can cause uncommanded pitch changes and may result in momentary stick shaker activation well above normal stick shaker speeds.
11/25/88 AC 00-54 Appendix 1 An airplane flying through horizontal input to counteract the roll and mainvortices as shown on the previous page tain aircraft control experiences alternating updrafts and downdrafts causing pitch changes with- • Turbulence Effects out pilot input. These vertical winds result in airplane angle-of-attack Turbulence may be quite intense in Fluctuations which, if severe enough, weather conditions associated with may result in momentary stick shaker windshear. Effects of turbulence can actuation or airframe shudder at mask changing airspeed trends and speeds well above normal. delay recognition of severe windshear. Turbulence may also tend to vertical winds, like those associated discourage use of available airplane vortices the recovery pitch attitude during a recovery by sidered in development of causing random stick shaker activity.
significant These effects can significantly increase pilot workload and distraction.
to increase pilot workload The higher work- • Rain Effects load results from attention to momentary stick shaker actuation and uncom- Accident investigations and the study manded pitch of windshear have shown that some rapid changes in vertical wind. forms of windshear are accompanied by high rates of rainfall. NASA research
• Crosswind Shear Response is underway to determine if high rainfall rates contribute to a loss of A crosswind shear tends to cause the airplane pertormance. The results airplane to roll and/or yaw. Large available to date are inconclusive.
crosswind shears may require large or However, because rain may serve as a rapid control wheel inputs. These warning of sevent wie avoided! areas of shears may result in significantly in- heavy rain should be High creased workload and distraction. In rates of rainfall also cause signifiaddition, if an aircraft encounters a cant increases cockpit noise horizontal vortex, severe roll forces levels, making crew coordination and may require up to full control wheel pilot concentration more difficult.
Appendix 1
• Basic Aerodynamics tudinal stability characteristics tend to pitch the airplane up with increas- The pilot has direct control over air- ing airspeed and down with decreasing plane pitch attitude which in turn airspeed. Thrust changes in response acts to change the flight path. The to these airspeed variations may also angle-of-attack will change with pitch affect stability. In airplanes with attitude resulting in modified underwing-mounted engines, there is an flight path angle. These three angles additional tendency to pitch up as are related as follows: thrust increases, and pitch down as thrust decreases. This tendency may Attitude = Angle of * path Angle Pitch Flight become more pronounced at low speeds.
Attack o Stall and Stall Warning
• Airplane Performance Airplane stall occurs when further in- Transport category airplanes have con- creases in angle-of-attack produce no siderable climb capability at speeds further increase in lift. In order to below normal reference values. As prevent inadvertent _entry. into this seen in Figure 23 for typical takeoff flight region, stall warning devices climb performance, a rate-of-climb of (stick shakers, stick pushers, stall 1500 FPM is possible even at stick warning horns) are installed on airshaker speeds. planes. In addition, having clean, smooth wing leading edges will help o Airplane Stability prevent both early onset of stall buffet as well as rolloff tendency prior As mentioned earlier, typical longi- to stall.
2500 Rate-of- Climb (ft/min) 1500 Stick Shaker 130 150 Airspeed (kt) Figure 23. Typical takeoff rate-of-climb capability. Significant climb capability exists even at stick shaker airspeeds.
Appendix 1 Windshear Effects or. Systems provide accurate vertical speed information. Due to instrument lags,
• Altimeters indications may be several seconds behind actual airplane rate of climb/ During callouts and instrument scan in descent and, in some situations, may a windshear, use of radio and/or baro- indicate a climb after the airplane metric altimeters must be tempered by has started descending (Figure 24).
the characteristics of each. Since vertical speed indicators driven by an radio altitude is subject to terrain Inertial Reference Unit (IRU) show contours, the indicator may show a significant improvement over other climb or descent due to falling or type instruments but still have some rising terrain, respectively. The 1ag• barometric altimeter may also provide distorted indications due to pressure In addition, gust-induced pitot static variations within the microburst. pressure variations within the microburst may introduce further VSI in-
• Vertical Speed Indicators accuracies. Due to such lags and errors, a11 vertical flight path The vertical speed indicator (VSI) instruments should be crosschecked to should not be solely relied upon to verify climb/descent trends.
1200 Actual Vertical Speed ,, inertial- Instruments Referenced VSI Lag and 800 - Air Data Indicate Climb Erroneousiy System VSi Vertical 400 Speed (ft/min) Takeoff Roll -400 Airplane Is Descending 008- 8 12 16 20 24 28 32 36 40 44 48 Time (sec) Figure 24. VSI error during takeoff windshear encounter. VSI may lag actual flight path changes.
AC 00-54 Appendix 1
• Stick Shaker Stick shaker is activated by angle of attack. Consequently, rapidly changing vertical winds or maneuvering will vary the attitude and airspeed at properly functioning system alpha vanes, stick shaker will normally activate providing a warning prior to stall. below the stall angle of attack thus
• Cockpit Angle of Attack Indicators useful of attack indicators do provide indications of margin to stick shaker; however, they may not provide effective guidance in a windshear ensince angle of attack is attitude adjustments where controlled indirectly through pitch In an actual winaging veerical winds cause padly angle of attack fluctuations independenter pilet fangte the attack direts its usefulness as a guiding parameter.
11/25/88
2.3.5 DEVELOPMENT OF WIND MODELS The lessons learned from windshear accident investigations, engineering analyses, and flight simulator studies have provided insight for development of simulator windshear models for pilot training. Through these efforts it was determined that the essential elements which must be taught include:
1) Recognition of windshear encounter,
2) Flight at speeds significantly less than those speeds typically exposed to in training, and
3) Use of pitch attitude rather than airspeed control to recover.
A simple model presenting a significant windshear threat requiring use of prompt corrective attitude control is sufficient to teach these elements.
Once the basics of recognition and recovery are understood, more complex models may be useful.
Appendix 1
2.4 MODEL OF FLIGHT CREW ACTIONS dealing with the situation. The model of flight crew actions must be incor- Due to the serious threat imposed by porated into day-to-day operations to infrequent windshear encounters, an ensure such actions are available and orderly set of actions is necessary to easily recalled when needed.
increase flight crew awareness of weather conditions that produce wind- The recommended procedures in this shear (Figure 25). Additionally, cer- section have been developed for tain operating practices have been de- Boeing, Douglas, and Lockheed jet veloped that improve the chances of transport airplanes. Use of these surviving a windshear encounter. procedures for other types of airpast windshear accidents, certain of planes is not appropriate until verithese actions were frequently missing, fied with the respective airframe thereby reducing crew effectiveness in manufacturer.
Evaluate the Weather No Any Signs o Windshear?
Yes Avoid Known Windshear Is It Safe No ro Continue?
Yes Consider Precautions Follow Standard Operating Techniques Windshear Recovery Technique (Report the Encounter Figure 25. Model of flight crew actions.
Appendix 1
2.4.1 EVALUATE THE WEATHER 1) Terminal Forecasts The weather evaluation process that An examination of terminal forefollows was developed after careful casts is recommended. Study o.
analyses of several windshear-related the following two terminal foreaccidents. accident casts demonstrates what to look occurred, several potential windshear for:
indicators were present, but a clear, definitive choice to divert or delay YNG FT AMD 1 101815 1820Z C45 BKN was not made. The windshear indica- 80 OVC 2018G35 LLWS ONL C20 OVC tors are meant to be cumulative. The SEAL TO OVE 08z" C25 OVC 3214G25 more indicators present, 2RW-/TRW-. 09Z IFR crews should consider delaying depar- CIG RA TRN WND.
ture or approach. Only through an increased awareness of potential wind- FT AMD 2 COR 101815 1745Z C30 shear indicators and a proper weather" BKN 2020G35 OCNL C12 OVC 3TRW CHC evaluation routine will flight crews C4 X 1/2T+RW+AG55. - 22Z C18 OVC be best prepared to avoid microburst 2420G30 windshear. (1/2TRW+G50. 01Z C16 OVC 2918G28 04Z C18 BKN 3016 BKN Every windshear accident has occurred OCNL SCT. 09Z VFR..
on a typical day, to a typical crew, on a typical takeoff or approach. TOO Note that for Youngstown, Onio the many times a number of clues were forecast is calling for LLWS--1 OW present, acknowledged by the crew, but level (altitude) windshear. While not accepted as potential of windshear. The weather evaluation indicators cast, tional "LLWS" clue. given an addi- thunderstorms exist in the fore- pilots are process must continue during the takeoff and climbout and throughout the approach and landing. If some of the In the terminal forecast for Terre indicators are present, this continual Haute, Indiana, LLWS does not searching may lead to early recogni- However, the chance of tion of the potential for a windshear severe heavy rain encounter.
55 knots, suggest the potential If convective cloud conditions are for microbursts if actual thunderpresent and/or if thunderstorms appear storm conditions are encountered.
likely, the potential for windshear and microburst activity exists. Dry microbursts are somewhat more if there are only subtle signs of con- difficult to recognize.
vective weather, such as weak cumulus ing in regions of low humidity riy if the near the surface, any convective particularly cloud is a likely microburst producer. Examination of the terminal forecast for convective activ- The following weather information ity--rain, thunderstorms, etc.--is should be examined for any potential good practice.
windshear conditions affecting the flight: