Aircraft and Lightning
Recent experimental flights designed to determine probabilities and
causes of aircraft lightning strikes have provided some new information
related to a pilot's chances of receiving an in-flight encounter with
lightning bolts. Although there have been several recent studies, the
bulk of the new information comes from two research projects: a USAF/FAA
study which involved the use of a Convair 580 specially instrumented
transport aircraft which flew for 42 hours and experienced 21 lightning
strikes; and from a NASA Storm Hazards Program, which involved the use
of a specially instrumented F-106B aircraft which made 1,154
thunderstorm penetrations and received 637 lightning strikes. These
1. The majority of strikes (greater than 90 percent) were triggered
by the aircraft itself.
2. The probability of an aircraft triggering a lightning discharge in
a thunderstorm increased with altitude.
3. The probability of a lightning strike to an aircraft flying in a
thunderstorm increased from a minimum at the thunderstorm base to a
maximum at the 36,000- to 40,000-foot level. The temperature at this
level was from -40 degrees C to 45 degrees C. The strike rate
encountered at these high altitudes was two strikes per minute of
penetration time. At 18,000 feet, the frequency was one strike every 20
minutes. An average of only one aircraft strike every 3 hours was
encountered when flying below active thunderstorms.
4. Lightning strikes at high altitudes generally resulted in greater
total charge transfer than strikes at lower altitude; however, the low
altitude strikes sometimes produced greater instantaneous discharge.
5. The entire surface of the aircraft may be susceptible to lightning
attachment even though strikes are more probable to particular areas
such as the aircraft extremities (nose, wingtips, tail) and composite
6. During penetration of thunderstorms at low levels, lightning
strikes were found to occur in areas of moderate or greater turbulence
at the edge of and within large downdrafts. Conversely, lightning
strikes experienced in the upper areas of thunderstorms and in the
vicinity of decaying thunderstorms most frequently occurred under
conditions of little turbulence or precipitation.
It should be remembered that prior to this research it was thought
that an aircraft had to fly into the path of naturally occurring
lightning to get struck, and the altitudes near the freezing level (0
degrees C) were considered the most probable location for this to
happen. The research data, however, seem to conflict with previous
This does not mean that the old rules do not apply any longer. What
it does mean is that we are learning more about the behavior of
lightning and its effects on aircraft. Many of the old rules are still
valid, and several new rules are being developed which we will be able
to apply in the future.
Thunderstorms in Perspective
Actually thunderstorms and lightning are part of a global electric
circuit. According to nature's plan to maintain an electric potential
between the earth's surface and the ionosphere (called the "fair
weather" electric potential), thunderstorms are necessary. They, in
fact, play a key role in maintaining the earth's fair weather electric
potential. The total number of thunderstorms occurring at any given time
around the globe is approximately 2,000. These thunderstorms average
about 100 lightning strikes per second. They act as an electric
generator, maintaining the electric field. From this perspective,
lightning within a thunderstorm cloud helps maintain the earth's
Statistics show that commercial pilots experience an average of one
lightning strike for every 3,000 flight hours, and the commercial
airlines average one hit per aircraft per year. Air Force statistics
show a somewhat lesser rate than civilians, but nonetheless the USAF in
the past has averaged 51 lightning mishaps per year.
Aircraft Damage Caused by Lightning
Aircraft damage from lightning can be caused as a direct or indirect
effect. Direct effects result when the lightning current attaches to and
flows through the aircraft skin. Locations on the aircraft where
lightning strikes occur experience extreme heating which causes burning
and melting damage. Current flowing through the aircraft structure can
result in isolated arcing or sparking and heating. If this occurs in a
fuel tank, explosion and fire can result.
Indirect effects are caused by transient electrical pulses produced
by the changing electric and magnetic fields due to the lightning
current. Unless avionics and other systems are properly shielded, they
are easily damaged by indirect lightning effects. It is also interesting
to note that 57 percent of the mishaps attributed to lightning strikes
to aircraft occur during the months of March through July.
Facts and Myths
Combining new research on lightning with the lessons of the past, we
can learn the following:
It is true that some aircraft are less prone to lightning strikes.
Size, shape, and speed are all aircraft-specific variables which
determine an aircraft's susceptibility to a lightning strike. However,
it is also true that all aircraft are susceptible to a lightning strike.
It is also true that aircraft damage varies with aircraft type. Careful
aircraft design can minimize lightning damage. However, all surfaces are
susceptible to lightning strikes, and all unprotected systems can be
It is true that some pilots are better at avoiding lightning strikes
than others. The wider the berth given to thunderstorms, the better the
chance of avoiding a lightning strike; however, the pilot who tries to
pick his or her way between thunderstorm cells is asking for trouble.
The theory that if you avoid thunderstorms you will avoid all
lightning strikes is false. Statistics show that many triggered strikes
have occurred during flights that did not penetrate thunderstorms.
Aircraft have triggered strikes in cirrus clouds downwind of previous
thunderstorm activity, in cumulus clouds around the periphery of
thunderstorms, and even in stratiform clouds and light rain showers not
associated with thunderstorms.
The statement that if you are greater than 20 miles from
radar-indicated precipitation, you are not susceptible to a lightning
strike is also false. Aircraft have been struck by the proverbial
"bolt from the blue" on more than one occasion. In fact,
aircraft have been struck at distances out to 50 nautical miles from
thunderstorms, particularly when cirrus clouds existed above or at their
altitude, or when there were other developing showers nearby that had
not yet reached maturity. Also, flying through precipitation, volcanic
ash, or heavily polluted air can cause an aircraft to experience
electrostatic discharge or triggered lightning. Usually these discharges
cause only minor aircraft damage; however, there is always the chance
for catastrophic damage if the discharge passes through the vaporized
fuel-air mixture in the fuel tank.
The belief that lightning strikes to aircraft occur only near the
freezing level and are always associated with turbulence and
precipitation is false. Thunderstorm penetration studies show that
lightning strikes can be encountered at all temperatures and altitudes.
In fact, they are most likely to occur in the upper levels of mature or
decaying storms near temperatures of -40 degrees C. In addition, the
studies showed most strikes occurred in regions where turbulence
intensities were light to negligible.
It is true that aircraft flying at altitudes
above the freezing level are more likely to be involved with in-cloud or
inter-cloud lightning flashes, and that aircraft flying at altitudes
below the freezing level are more likely to be involved with a
cloud-to-ground lightning event. It is also true that the more
frequently a thunderstorm is flashing, the lower the probability of
being struck by lightning if the aircraft flies into the storm. However,
the greater the flash rate, the higher the potential for severe
turbulence, heavy rain, and hail. Therefore, this information should in
no way be interpreted as a reason to fly in or near any thunderstorm.
Some Rules to Fly By
The most important thing is
to stay clear of thunderstorms. Do not attempt to "pick your way
through"; deviate around the area on the upwind (non-anvil) side if
The higher the aircraft
altitude, the farther away from a thunderstorm you should fly. Lightning
strikes have been known to occur in the clear air up to 50 miles
downwind from the nearest thunderstorm.
At low levels, avoid flying
close to high surface features (ridge tops, towers, etc.), or between
such features and an overhead thunderstorm.
If you fly above the freezing
level in or near thunderstorms, you can trigger an in-cloud or
cloud-to-cloud discharge. If you fly below the freezing level, you could
be involved with a cloud-to-ground lightning strike. Overall, if you
must penetrate or fly close to a thunderstorm system, you can expect
more strikes penetrating a thunderstorm area well above the freezing
Lightning damage is usually
worse for large total current transfers. At altitudes above the freezing
level, you are more likely to experience longer-lasting lightning
attachments made up of numerous small pulses and a large total current
transfer. Below the freezing level, you are more likely to experience
shorter lightning attachments with a few strong current pulses; however,
the total current transfer is usually less than that above the freezing
Electrical activity generated
by a thunderstorm may exist even after the thunderstorm cell has
decayed; therefore, avoid penetrating the cirrus decks that were
recently associated with thunderstorms.
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