Chapter 22, Thunderstorms 22-9
Figure 22-9. Illustration of a Symmetric Microburst
Figure 22-10. Illustration of an Asymmetric Microburst
Chapter 22, Thunderstorms 22-10
Figure 22-11. Illustration of a Dry Microburst
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 (see Figure 22-12). Conditions associated with these vortices may produce very powerful
updrafts and roll forces in addition to downdrafts.
Figure 22-12. Illustration of a Series of Vortices Due to Microbursts Being Embedded in One Another
A downburst or microburst is dangerous to aircraft when climbing from takeoff or approaching to land.
During these phases, the aircraft is operating at slow speeds. A major change of wind velocity can lead to
loss of lift and a crash. During landing, if the pilot has reduced power and lowered the nose in response to
the headwind shear (see Figure 22-13) this leaves the aircraft in a nose-low, power-low configuration when
the tailwind shear occurs, which makes recovery more difficult. It can cause the airplane to stall or land
short of the runway.
Chapter 22, Thunderstorms 22-11
Figure 22-13. Landing in a Microburst
Pilots should be alert for indications of a microburst early in the approach phase, and ready to initiate a
missed approach at the first indication. However, it may be impossible to recover from a microburst
encounter at low altitude.
Pilots should be aware of asymmetrical microbursts (see Figure 22-10), since a significant airspeed increase
may not occur upon entering the outflow or may be much less than the subsequent airspeed loss experienced
when exiting the microburst.
It is vital for pilots to recognize that some microbursts cannot be successfully escaped with any known
techniques. Some wind shears that are within the performance capability of the aircraft have caused
accidents.
22.7.3.1 Encounter During Takeoff—After Lift-Off
In previous accidents studied, the airplane encountered an increasing tailwind shear shortly after lifting off
the runway (see Figure 22-14). For the first 5 seconds after lift -off, the takeoff appeared normal, but the
airplane crashed off the end of the runway about 20 seconds after lift-off.
Chapter 22, Thunderstorms 22-12
(1) Takeoff initially appears normal.
(2) Aircraft encounters wind shear just after lift-off.
(3) Airspeed decreases resulting in pitch attitude reduction.
(4) Aircraft crashes off departure end of runway 20 seconds after lift-off.
Figure 22-14. Wind Shear Encounter During Takeoff After Lift-Off
In many events involving after -lift-off wind shear encounters, early trends in airspeed, pitch attitude,
vertical speed, and altitude appeared normal. In this example, the airplane encountered wind shear before
stabilized climb was established, which caused difficulty in detecting onset of shear. As the airspeed
decreased, pitch attitude was reduced to regain trim airspeed (see Figure 22-15). By reducing pitch attitude,
available performance capability was not used and the airplane lost altitude. As terrain became a factor,
recovery to initial pitch attitude was initiated. This required unusually high stick force (up to 30 lb of pull
may be needed on some airplanes). Corrective action, however, was too late to prevent ground contact since
the downward flightpath was well established.
Reducing pitch attitude to regain lost airspeed or allowing attitude to decrease in response to lost airspeed,
is the result of past training emphasis on airspeed control. Successful recovery from an inadvertent wind
shear encounter necessitates maintainin g or increasing pitch attitude and accepting lower -than-usual
airspeed. Unusual and unexpected stick forces may be needed to counter natural airplane pitching
tendencies due to airspeed and lift loss.
Chapter 22, Thunderstorms 22-13
Microburst reduces airspeed and lift at normal attitude, which results in
pitch-down tendency to regain airspeed.
Figure 22-15. Wind Shear Effects on Flightpath
To counter the loss of airspeed and lift resulting from wind shear, pitch attitude must not be allowed to fall
below the normal range. Only by properly controlling pitch attitude and accepting reduced airspeed can
flightpath degradation be prevented ( see Figure 22-16). Once the airplane begins to deviate from the
intended flightpath and high descent rates develop, it takes additional time and altitude to change
flightpath direction.
Control of pitch attitude and acceptance of reduced airspeed results in
improved flightpath.
Figure 22-16. Pitch Control Effects on Flightpath
Chapter 22, Thunderstorms 22-14
Only 5 to 15 seconds may be available to recognize and respond to a wind shear encounter ( see Figure
22-17). Therefore, it is of great importance that a wind shear encounter be recognized as soon as possible.
Takeoff initially appeared normal. Additional time is needed to arrest descent.
Result: Only 5 to 15 seconds may be available for recognition and recovery.
Figure 22-17. Time Available to Respond to Wind Shear Encounter
22.7.3.2 Encounter During Takeoff—On Runway
Analysis of a typical accident where an increasing tailwind shear was encountered during takeoff ground
roll showed that initial indications appeared normal (see Figure 22-18). Due to the increasing tailwind shear,
however, the airplane did not reach rotation speed (VR) until nearing the end of the runway. As the airplane
lifted off, the tailwind continued increasing, preventing any further airspeed increase. The airplane
contacted an obstacle off the departure end of the runway.
(1) Takeoff initially appeared normal.
(2) Airspeed buildup slowed due to wind shear.
(3) Airplane reached VR near end of runway, lifted off but failed to climb.
(4) Airplane contacted obstacle off departure end of runway.
Figure 22-18. Wind Shear Encounter During Takeoff on Runway
Chapter 22, Thunderstorms 22-15
Less-than-normal airspeed, due to wind shear encounter, resulted in reduced available lift at normal takeoff
attitude (see Figure 22-19). In turn, the inability to lift off soon enough to clear obstacles resulted.
Microburst reduces airspeed and lift at normal attitude that results in
inability to lift off.
Figure 22-19. Wind Shear Effects on Lift-Off
An additional factor is the difficulty of recognizing deteriorating airplane performance. Timely recognition
of a wind shear encounter on the runway may be difficult since the only indication may be a
slower-than-normal airspeed increase. The presence of gusts may mask abnormal airspeed buildup. Time
available to respond effectively to a wind shear may be as little as five seconds from the initial encounter.
If there is insufficient runway left to accelerate to normal takeoff speed, and inadequate runway to stop,
lift-off and safe climb may require rotation at speeds less than VR. In this case, additional pitch attitude may
be needed to achieve sufficient lift (see Figure 22-20). In traditional training, crews are frequently cautioned
not to rotate at speeds less than V R to avoid high pitch attitudes that could result in aft body contact. In a
wind shear encounter, rotation toward normal takeoff pitch attitude at lower -than-normal airspeed may be
needed to lift off in the remaining runway. This may result in aft body contact. To deal with an inadvertent
wind shear encounter, the pilot should be prepared to apply techniques that differ from those
ordinarily used.