AC 00-54

Appendix 1 In fact, closer examination of the derstorm activity and a number of United States (Figure 6) shows a cor- accidents.

relation between areas of high thun- 10 20 30 40 30 30 20 10 10 20 30 40 40 80

• Microburst-related accident/incident 30 80 30 80 Figure 6. Average annual continental U.S. thunderstorm days. Correlation exists between regions of higher thunderstorm activity and windshear-related accidents incidents.

11/25/88 AC 00-54 Appendix 1 o The Microburst as a Windshear Threat Downdrafts associated with microbursts are typically only a few hundred to Identification of concentrated, more 3,000 feet across. When the downdraft powerful downdrafts--known as micro- reaches the ground, it spreads out bursts--has resulted from the inves- horizontally and may form one or more tigation of windshear accidents and horizontal vortex rings around the from meteorological research. Micro- degion is typically 6,000 to 12,000 downdraft (Figure bursts can occur anywhere convective weather conditions (thunderstorms, feet across. The horizontal vortices rain showers, virga) occur. Observa- may extend to over 2,000 feet AGL.

tions suggest that approximately five percent of all thunderstorms produce a microburst.

Cloud Base 1000 ft Approx Virga or Scale Rain 1000 ft

- Downdraft Outflow Front - Horizontal Vortex Outflow Figure 7. Symmetric microburst. An airplane transiting the microburst would experience equal headwinds and tailwinds.

Appendix 1 Microburst outflows are Therefore, not not always may be much less than the subsequent symmetric (Figure 8). airspeed loss experienced when exiting significant airspeed increase may the microburst.

occur upon entering the outflow, or Cloud Base 1000 ft Approx Virga or Scale Rain • • 1000 ft Downdraft Horizontal Vortex Wind Outflow Front- Outflow Figure 8. Asymmetric microburst. An airplane transiting the microburst from left to right would experience a small headwind followed by a large tailwind.

Appendix 1 More than one microburst can occur in the same weather system. Pilots are therefore cautioned to be alert for additional microbursts if one has already been encountered or observed.

If several microbursts are present, a series of horizontal vortices can form near the ground due to several microbursts being embedded in one another (Figure 9). Conditions associated with these vortices may produce very powerful updrafts and roll forces in addition to downdrafts.

500 ft Approx Series of Horizontal Scale Microburst Vortices 500 ft Microburst Figure 9. Horizontal Vortices. A series of horizontal vortices can form near the ground due to microbursts imbedded in one another.

Appendix 1 Wind speeds intensify for about 5 minutes after a microburst initially contacts the ground (Figure 10). An encounter during the initial stage of microburst development may not be considered significant but an airplane following may experience an airspee change two to three times greater.

Microbursts typically dissipate within 10 to 20 minutes after ground contact.

Maximum Initial Ground Shear Dissipation Downdraft Contact Intensity Begins Cloud base as high as 15,000 ft 10 15 Elapsed Time (min) Figure 10. Evolution of a microburst. Microburst winds intensify for about 5 min after ground contact and typically dissipate about 10 to 20 min after ground contact.

AC 00-54 Appendix 1 Doppler radar wind measurements indicate that the wind speed change a pilot might expect when flying through the average microburst at its point of peak intensity is about 45 knots (Figure However, microburst windspeed differences of almost 100 knots have been measured. In fact, a severe event at Andrews Air Force Base (Camp Spring, Maryland) on August 1, 1983 11/25/88 indicated headwind/tailwind differential velocities near 200 knots.

IT IS VITAL TO RECOGNIZE THAT SOME EStEPED WITH ANY KNOWN SUCCESSFULLY MICROBURSTS CANNOT TECHNIQUES!

Note that even windshears which were within the performance capability of the airplane have caused accidents.

• Maximum windshear capability of jet transports at heavy weight, for a shear encounter at a critical location, is 40 to 50 knots windspeed change Number of Measured Microbursts (500 ft JAWS Doppler Radar Data) 20 30 40 50 60 70 80 90 100 JFK MSYDENOÃD DEN DFW DEN - Accident/ Incident Windspeed Maximum Horizontal Windspeed Change (kt) Changes Figure 11. Microburst frequency versus intensity. Accidents have occurred in windshears within performance capability of airplane. Some windshears cannot be escaped successfully!

Appendix 1 Microbursts can be associated with In this example, air below _a cloud both heavy rain, as in thunderstorm base (up to approximately 15,000 feet conditions, and much lighter precipi- tation associated with convective higher AGL) is very dry. convective clouds Precipitation from falls into relatively virga clouds. Microbursts have occurred in dry conditions of light evaporative cooling causes the air to low humidity air and evaporates. This rain or (precipitation that plunge downward. As the evaporative evaporates before reaching the earth's Tore cautione not to fly bene there- cooling process continues, downsurface). The formation of a dry microburst is illustrated in Figure 12.

vective clouds producing virga conditions.

Cloud Base (As high as 15,000 ft) 1000 ft Approx Virga or Scale Dry Light Rain - 1000 ft Air - Downdraft Outflow Front / Cold Horizontal Air ||| Vortex Plunge Outflow Figure 12. Dry microburst formation. Evaporation of rain below cloud base (virga) causes intense cooling of rainshaft air and subsequent cold air plunge.

Appendix 1

2.3 LESSONS LEARNED FROM WINDSHEAR Windshear Recognition

ENCOUNTERS

• Recognition of windshear encounter Analysis of past windshear accidents is difficult and is usually comand incidents has taught valuable les- plicated by marginal weather regarding windshear recognition and flight path control. Engineering • Time available for recognition and and flight simulator recovery is short (as little as 5 tions have been conducted as well to seconds) additional intormation.

resulting lessons learned form a basis o Flight crew coordination is essenrecommended precautions and tial for prompt windshear recognitechniques in this document. tion and recovery The primary lesson learned is that the Pilot Technique best defense against windshear is to avoid it altogether. This is espe- o Flight path must be controlled cially important because shears wil with pitch attitude (unusual stick exist which are beyond the capability forces may be required as a result) of any pilot or airplane. In most windshear accidents, several. clues-- o Lower than normal airspeed may LLAAS alerts, weather reports, visual have to be accepted to counter signs--were present that would have lift loss alerted the flight crew to the presence of a windshear threat. In all In reaching these conclusions, three instances, however, these clues were types of windshear encounters which either not recognized or not acted have resulted in an accident or inciupon. Flight crews must seek and heed dent were examined: an encounter dursigns alerting them to the need for ing takeoff after liftoff, an encounavoidance. ter during takeoft on the runway, and an encounter during approach. Details Other lessons were also learned re- of these encounters and the lessons garding windshear recognition Tearned are described in Sections pilot technique should the avoidance 2.3.1 through 2.3.3. Following this process fail. These lessons are sum- Section 2.3.4 presents the lessons marized as follows: learned regarding effects of windshear on airplanes and airplane systems.

Section 2.3.5 describes the impact of these lessons on simulator training.

Appendix 1

2.3.1 ENCOUNTER DURING TAKEOFF - AFTER off the runway (Figure 13). For the LIFTOFF first 5 seconds after liftoff the takeoff appeared normal, but the air- In a typical accident studied, the plane crashed off the end of the runairplane encountered increasing way about 20 seconds after liftoff.

tailwind shear shortly after lifting Microburst Runway Figure 13. Windshear encounter during takeoff after liftoff. (1) Takeoff initially appeared normal. (2) Windshear encountered just after liftoff. (3) Airspeed decrease resulted in pitch attitude reduction. (4) Aircraft crashed off departure end of runway 20 sec after liftoff.

Appendix 1 In many events involving after-liftoff required on some airplanes). correcwindshear encounters, early trends in tive action, however, was too late to airspeeand altitude appeared normal. pitch attitude, vertical prevent ground contact since the downspeed ward flight path was well established.

In this example, the airplane encountered windshear before stabilized Reducing pitch attitude to regain lost climb was established which caused airspeed, or allowing attitude to dedifficulty in detecting onset of crease in response to lost airspeed, shear. As the airspeed decreased, is the result of past training emphapitch attitude was reduced to regain sis on airspeed control. Successful trim airspeed (Figure 14). By reduc- recovery from an inadvertent windshear ing pitch attitude, available perform- encounter requires maintaining or inance capability was not utilized and creasing pitch attitude and accepting the airplane lost altitude. As ter- Tower than usual airspeed. Unusual rain became a factor, recovery to ini- and unexpected stick forces may be retial pitch attitude was initiated. quired to counter natural airplane This required unusually high stick pitching tendencies due to airspeed force (up to 30 pounds of pull may be and lift loss.

Lift Normal Flight Path Normal Climb Pitch Down Descending Microburst * Lift Flight Path →--^ Figure 14. Windshear effects on flight path. Microburst reduces airspeed and lift at normal attitude which results in pitch down tendency to regain airspeed.

Appendix 1 To counter the loss of airspeed and flight path degradation be prevented lift resulting from windshear, pitch (Figure te from the intended flight Once the airplane begins attitude must not be allowed to fall to deviate below the normal range. Only by prop- path and high descent rates develop, erly controlling pitch attitude and it takes additional time and altitude accepting reduced airspeed can to change flight path direction.

Pitch Down Descending Flight Lift Path Microburst- Lack of Proper Pitch Control Improved Flight Path Microburst Controlled Pitch Attitude Figure 15. Pitch control effects on flight path. Control of pitch attitude and acceptance of reduced airspeed results in improved flight path.

Appendix 1 In the windshear encounter cited ear- Only 5 to 15 seconds may be available Capability may not have been used bee available airplane performance to recognize and respond to a windshear encounter (Figure 16). It is cause of two factors: lack of timely therefore of great importance that a recognition and inappropriate or in- windshear encounter be recognized as adequate response. Rapidly deterio- soon as possible. Timely recognition rating climb performance may not De of windshear requires effective crew apparent to the crew unless all appro- coordination and appropriate callouts priate vertical flight path instru- by the pilot not flying.

ments are closely monitored.

300 200 Normal Altitude (ft) Typical Accidents/Incidents 100 5 10 15 20 25 Time From Liftoff (sec) Figure 16. Time available to respond to windshear encounter. Takeoff initially appeared normal.

Additional time is required to arrest descent. Result: only 5 to 15 sec may be available for recognition and recovery.