Chapter 22, Thunderstorms 22-16
Increased pitch attitude generates lift needed for lift-off.
Figure 22-20. Pitch Attitude Effects on Lift-Off
22.7.3.3 Encounter on Approach
Analysis of a typical wind shear encounter on approach provided evidence of an increasing downdraft and
tailwind along the approach flightpath ( see Figure 22-21). The airplane lost airspeed, dropped below the
target glidepath, and contacted the ground short of the runway threshold.
(1) Approach initially appears normal.
(2) Airplane encounters increasing downdraft and tailwind at transition.
(3) Airspeed decrease combined with reduced visual cues results in pitch attitude reduction.
(4) Airplane crashes short of approach end of runway.
Figure 22-21. Wind Shear Encounter During Approach
Chapter 22, Thunderstorms 22-17
Reduced airspeed, as the airplane encountered the wind shear, resulted in decreased lift. This loss of lift
increased the descent rate (see Figure 22-22). 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 conditions, inadequate crew coordination ,
and limited recognition time—was a significant factor in delaying recovery initiation. Gradual application
of thrust during approach may have masked the initial decreasing airspeed trend. Poor weather conditions
caused increased workload and complicated the approach. Transition from i nstruments to exterior visual
references may have detracted from instrument scan. Inadequate crew coordination may have resulted in a
failure to be aware of flightpath degradation. A stabilized approach with clearly defined callouts is essential
to aid in the recognition of unacceptable flightpath trends and the need to initiate recovery.
Microburst reduces airspeed and lift at normal attitude that results in pitch-down
tendency to regain airspeed.
Figure 22-22. Wind Shear Effects on Flightpath on Approach
22.7.3.4 Wind Shear Effects on Airplanes and Systems
Several terms are used when discussing low-altitude wind variations with respect to aviation. These terms
are defined as follows:
• Increasing Headwind Shear: Wind shear in which headwind increases , causing an airspeed
increase.
• Decreasing Headwind Shear: Wind shear in which headwind decreases , causing an airspeed
decrease.
• Decreasing Tailwind Shear: Wind shear in which tailwind decreases, causing an airspeed increase.
• Increasing Tailwind Shear: Wind shear in which tailwind increases, causing an airspeed decrease.
Chapter 22, Thunderstorms 22-18
22.7.3.4.1 Headwind/Tailwind Shear Response
The various components of wind shear have unique effects on airplane performance. In addition, the
magnitude of the shear depends on the flightpath through the microburst.
An increasing headwind (or decreasing tailwind) shear increases indicated airspeed and thus increases
performance. The airplane will tend to pitch up to regain trim airspeed. An additional consideration is that
this type of shear may reduce normal deceleration during flare, which could cause overrun.
Any rapid or large airspeed increase, particularly near convective weather conditions, should be viewed as
a possible indication of a forthcoming airspeed decrease. Thus, a large airspeed increase may be reason for
discontinuing the approach. However, since microbursts are often asymmetric and the headwind may not
always be present, headwind shears are not to be relied upon to provide early indications of subsequent
tailwind shears.
In contrast to shears that increase airspeed, an increasing tailwind (or decreasing headwind) shear will
decrease indicated airspeed and performance capability. Due to airspeed loss, the airplane may tend to pitch
down to regain trim speed.
22.7.3.4.2 Vertical Wind Shear Response
Vertical winds exist in every microburst and increase in intensity with altitude. Such winds usually reach
peak intensity at heights greater than 500 ft above the ground. Downdrafts with speeds greater th an
3,000 fpm can exist in the center of a strong microburst. The severity of the downdraft the airplane
encounters depends on both the altitude and lateral proximity to the center of the microburst.
Perhaps more critical than sustained downdrafts, short duration reversals in vertical winds can exist due to
the horizontal vortices associated with microbursts. This is shown in Figure 22-23 below.
Rapid updraft/downdraft variations due to horizontal vortices can cause uncommanded pitch
changes and may result in momentary stick shaker activation, well above normal stick
shaker speeds.
Figure 22-23. Illustration of an Encounter with Microburst Horizontal Vortices
An airplane flying through horizontal vortices as shown in Figure 22-23 experiences alternating updrafts
and downdrafts causing pitch changes without pilot input. These vertical winds result in airplane
angle-of-attack fluctuations that, if severe enough, may result in momentary stick shaker actuation or
airframe shudder at speeds well above normal.
Chapter 22, Thunderstorms 22-19
22.7.4 Convective Turbulence
See Section 19.2.1 for information on convective turbulence.
22.7.5 Convective Icing
See Section 20.3.9 for information on convective icing.
22.7.6 Hail
See Section 14.4.5 for information on hail.
22.7.7 Rapid Altimeter Changes
Pressure usually falls rapidly with the approach of a thunderstorm. Pressure then usually rises sharply with
the onset of the first gust and arrival of the cold downdraft and heavy rain, falling back to normal as the
thunderstorm passes. This cycle of pres sure change may occur in 15 minutes. If the pilot does not receive
a corrected altimeter setting, the altimeter may be more than 100 ft in error.
22.7.8 Static Electricity
Static electricity (a steady, high level of noise in radio receivers ) is caused by intense corona discharges
from sharp metallic points and edges of flying aircraft. It is encountered often in the vicinity of
thunderstorms. When an aircraft flies through clouds, precipitation, or a concentration of solid particles
(e.g., ice, sand, or dust), it accumulates a charge of static electricity. The electricity discharges onto a nearby
surface or into the air, causing a noisy disturbance at lower frequencies.
The corona discharge is weakly luminous and may be seen at night. Although it has a rather eerie
appearance, it is harmless. It was named “St. Elmo’s Fire” by Mediterranean sailors, who saw the brushy
discharge at the top of ship masts.
22.7.9 Tornado
A tornado is a violently rotating column of air in contact with the ground, either pendant from a cumuliform
cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud. The most
violent thunderstorms draw air into their cloud bases with great force. If the incoming air has any initial
rotating motion, it often forms an extremely concentrated vortex from the surface well into the cloud.
Meteorologists have estimated that w ind in such a vortex can exceed 200 kt; pressure inside the vortex is
quite low. The strong winds gather dust and debris, and the low pressure generates a funnel -shaped cloud
extending downward from the cumulonimbus base. If the cloud does not reach the surface, it is a “funnel
cloud”; if it touches a land surface, it is a “tornado”; and if it touches water, it is a “waterspout.” When
tornadoes do occur without any visible funnel cloud, debris at the surface is usually the indication of the
existence of an intense circulation in contact with the ground.
Tornadoes can occur almost anywhere in the world but are most common in the central and eastern United
States during spring and autumn months. They typically last only a few minutes and travel a few miles, but
can persist much longer (e.g., more than 90 minutes ) and track much farther (e.g., more than 100 mi), in
extreme cases.
On a local scale, the tornado is the most intense of all atmospheric circulations. Its vortex is typically a few
hundred yards in diameter but can range in width from less than 10 yards (yd) to over 2 mi. Wind speeds
are typically estimated on the basis of wind damage using the Enhanced Fujita (EF) Scale (see Table 22-1).
Chapter 22, Thunderstorms 22-20
Table 22-1. Enhanced Fujita Scale for Tornado Damage
EF-Rating Class
3-Second Wind Gust
Description Relative Frequency
mph km/h
EF-0 Weak 65–85 105–137 Gale 53.5%
EF-1 Weak 86–110 138–177 Weak 31.6%
EF-2 Strong 111–135 178–217 Strong 10.7%
EF-3 Strong 136–165 218–266 Severe 3.4%
EF-4 Violent 166–200 267–322 Devastating 0.7%
EF-5 Violent >200 >322 Incredible <0.1%
Note: The EF Scale is a set of wind estimates (not measurements) based on damage. The 3-second
gust is not the same wind as in METAR/SPECI surface observations, which is a 2-minute average.
Note: Confirmed tornadoes with no reported damage (i.e., those that remain in open fields) are always
rated EF-0.
Tornadoes occur with both isolated and squall line thunderstorms. However, o ver 80 percent of all
tornadoes in the United States are produced by supercell thunderstorms. Multiple tornado occurrences
associated with a particular large-scale weather system is termed a “tornado outbreak.” On rare occasions,
one supercell can produce multiple tornadoes over many hours. In addition, families of tornadoes have also
been observed as appendages of the main cloud extending several miles outward from the area of lightning
and precipitation. Thus, any cloud connected to a severe thunderstorm may contain hidden vortices.
An aircraft entering a tornado vortex is almost certain to suffer loss of control and structural damage. Since
the vortex extends well into the cloud, any pilot inadvertently caught on instruments in a thunderstorm
could encounter a hidden vortex.
22.7.10 Engine Water Ingestion
Turbine engines have a limit on the amount of water they can ingest. Updrafts are present in many
thunderstorms, particularly those in the developing stages. If the updraft velocity in the thunderstorm
approaches or exceeds the velocity of the falling rain drops, very high concentrations of water may occur.
It is possible that these concentrations can be in excess of the quantity of water that turbine engines are
designed to ingest. Therefore, severe thunderstorms may contain areas of high water concentratio n, which
could result in flameout and/or structural failure of one or more engines.
22.8 Thunderstorm Avoidance
22.8.1 Airborne Weather Avoidance Radar (Aircraft Radar)
Airborne weather avoidance radar is, as the name implies, for avoiding severe weather—not for penetrating
it. Whether to fly into an area of radar echoes depends on echo intensity, spacing between the echoes, and
the capabilities of the pilot and the aircr aft. The ability of airborne weather radar to detect weather
Chapter 22, Thunderstorms 22-21
phenomena is limited in both direction and range. Some airborne radars are fitted with a turbulence display
mode, which is based on the Doppler effect. These Doppler radars can detect turbulence associated with
precipitation (sometimes referred to as wet precipitation), but these radars are unable to detect clear-air
turbulence (CAT). The radar display also does not provide assurance of avoiding instrument weather
conditions from clouds and fog. A phenomenon called attenuation (see Section 15.2.5) may exist when a
cell absorbs or reflects all of the radio signals sent by the radar system (see Figure 15-5). Attenuation may
prevent the radar from detecting additional cells that might lie behind the first cell. This is sometimes
referred to as a radar “shadow.” For aircraft equipped with airborne weather radar, pilots are expected to be
familiar with the operating techniques and limitations of the specific system.
It is important to note that while hail always gives a radar echo, it may fall several miles from the nearest
visible cloud, and hazardous turbulence may extend to as much as 20 mi from the echo edge.
22.8.2 Thunderstorm Avoidance Guidance
Never regard any thunderstorm lightly, even when radar observers report the echoes are of light intensity.
Avoiding thunderstorms is the best policy. The following is guidance for avoiding thunderstorms:
1. Do not land or take off in the face of an approaching thunderstorm. A sudden gust front of low-level
turbulence could cause loss of control.
2. Do not attempt to fly under a thunderstorm , even if you can see through to the other side.
Turbulence and wind shear under the storm could be hazardous.
3. Do not attempt to fly under the anvil of a thunderstorm. There is a potential for severe and
extreme CAT.
4. Do not fly without airborne radar into a cloud mass containing scattered embedded thunderstorms.
Scattered thunderstorms that are not embedded usually can be visually circumnavigated.
5. Do not trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm.
6. Do not assume that ATC will offer radar navigation guidance or deviations around thunderstorms.
7. Do not use data-linked weather radar (i.e., NEXRAD) mosaic imagery as the sole means for
negotiating a path through a thunderstorm area (tactical maneuvering).
8. Remember that the data-linked NEXRAD mosaic imagery shows where the weather was, not where
the weather is. The weather conditions may be 15 –20 minutes older than the age indicated on the
display.
9. Listen to chatter on the ATC frequency for PIREPs and other aircraft requesting to deviate or divert.
10. Ask ATC for radar navigation guidance or to approve deviations around thunderstorms, if needed.
11. Use data -linked weather NEXRAD mosaic imagery (e.g., FIS-B) for route selection to avoid
thunderstorms entirely (strategic maneuvering).
12. Advise ATC, when switched to another controller, that you are deviating for thunderstorms before
accepting to rejoin the original route.
13. Ensure that after an authorized weather deviation, before accepting to rejoin the original route, the
route of flight is clear of thunderstorms.
14. Avoid by at least 20 mi any thunderstorm identified as severe or giving an intense, heavy, or
extreme radar echo. This is especially true under the anvil of a large cumulonimbus. Such echoes
should be separated by at least 40 mi before flying between echoes. Separation distances may be
reduced for avoiding weaker echoes.
15. Circumnavigate the entire area if more than half the area is covered by thunderstorms.
Chapter 22, Thunderstorms 22-22
16. Vivid and frequent lightning indicates the probability of a severe thunderstorm.
17. Regard as extremely hazardous any thunderstorm with tops 35,000 ft or higher , whether the top is
visually sighted or determined by radar.
18. Give a PIREP for the flight conditions.
19. Divert and wait out the thunderstorms on the ground if unable to navigate around an area of
thunderstorms.
If unable to avoid penetrating a thunderstorm, the following is guidance for before entering the storm:
1. Tighten the safety belt, put on the shoulder harness (if installed), and secure all loose objects.
2. Plan and hold the course to take the aircraft through the storm in a minimum time.
3. To avoid the most critical icing, establish a penetration altitude below the freezing lev el or above
the level of -15 °C.
4. Verify that pitot heat is on and turn on carburetor heat or jet engine anti -ice. Icing can be rapid at
any altitude and cause almost instantaneous power failure and/or loss of airspeed indication.
5. Establish power settings for turbulence penetration airspeed recommended in the aircraft manual.
6. Turn up cockpit lights to highest intensity to lessen temporary blindness from lightning.
7. If using automatic pilot, disengage Altitude Hold Mode and Speed Hold Mode. The automatic
altitude and speed controls will increase maneuvers of the aircraft; thus, increasing structural stress.
8. If using airborne radar, tilt the antenna up and down occasionally. This will permit the detection of
other thunderstorm activity at altitudes other than the one being flown.
9. Keep eyes on the flight instruments. Looking outside the cockpit can increase danger of temporary
blindness from lightning.
10. Do not change power settings; maintain settings for the recommended turbulence penetration
airspeed.
11. Maintain constant attitude. Allow the altitude and airspeed to fluctuate.
12. Do not turn back once in the thunderstorm. A straight course through the storm most likely will get
the aircraft out of the hazards most quickly. In addition, turning maneuvers increase stress on the
aircraft.