|
Lightning strikes to aircraft
while in flight 
Pilots are taught to avoid thunderstorms by 20
miles or more, especially when the thunderstorm is exhibiting severe
characteristics such as heavy or extreme rain, hail, high winds or
tornadoes. One of the other inherent dangers of a
thunderstorm is being struck by a bolt of lightning while in flight.
Certainly the threat of a lightning strike increases the
closer you are to the thunderstorm. So keeping your distance
makes good sense. But what about aircraft-induced lightning?
There are two kinds of lightning
encounters. The first one can simply be described as being in the wrong place at the
wrong time. Literally the flight path of your
aircraft intercepts
a lightning bolt that is already in progress. While this is
rare, it normally occurs down low when climbing out after
takeoff as was the case of this aircraft (shown in the
upper-right) on takeoff from the Komatsu Air Force Base off the coast
in the Sea of Japan.
The other more likely encounter is one
induced by your aircraft. There are many observed cases of lightning
strikes to aircraft inside or near clouds that had not previously
produced natural lightning. Studies show that about
90-percent of the lightning strikes to aircraft are thought to be
initiated by the presence of the aircraft itself. The scary
statistic, however, is that 40-percent of all discharges involving
airborne aircraft occurred in areas where no thunderstorms
were reported. The two separate lightning strikes to Apollo
12 shortly after launch were thought to be initiated by the Saturn V
rocket in a region of high instability. Other than these two
lightning strikes to the rocket, there were no other lightning strikes
in the region six hours prior or six hours after the launch.
While aircraft-induced lightning is still being
actively researched, there are a few important characteristics to
consider. From the information above, we know that it
doesn't take a thunderstorm to initiate a lightning strike.
The presence of the aircraft in an environment conducive to
an electrical discharge is all that is necessary.
Most of the aircraft-induced lightning discharges
during the warm season occur when the aircraft is at an altitude close
to the melting level. As shown on the left, the preferred temperatures include a range of +5°C to -10°C,
with the highest number of incidents occurring right at the melting
level (0°C). Click here to view a larger image.
In general, natural lightning in deep,
moist convection doesn't form until the tops of the storm build well
above the melting layer. For natural lightning to form, ice
crystals, graupel and supercooled liquid water need to be simultaneously
present.
If any one of these three ingredients is missing in
sufficient quantities, lightning doesn't generally occur. So
it is not a surprise that an aircraft-induced lightning strike must be
within local air mass instability producing deep, moist convection that
tops out well above the melting level. The conditions,
however, are not conducive for natural lightning.
As
mentioned above, most of the lightning strikes down low are by aircraft
intercepting the lightning. Most aircraft-induced lightning
strikes occur between 10,000 feet MSL and 16,000 feet MSL as seen in
the chart below. Click here to view a larger image. 
In the summer the melting level in the mid-latitudes generally
fluctuates between 12,000 feet and 17,000 feet.
An overwhelming number of lightning
strikes occur within the cloud itself; only a very small percentage of
strikes occur outside of the cloud boundary or below the cloud.
A majority of the strikes occur within precipitation (rain)
and in-cloud turbulence. This includes a mixture of rain,
snow, ice pellets or hail.
In order to avoid an encounter with
lightning, the best advice is to remain in cloud-free air whenever
possible, especially when the atmosphere is unstable and capable of
producing deep, moist convection. Even when thunderstorms
(natural lightning) are not occurring or expected to occur, an
aircraft-induced lightning strike can still be a risk.
Operate outside of areas of precipitation and be especially
careful operating in clouds near the melting level.
|
