Lightning Effects
Both the occupants of an aircraft and the aircraft itself are subject
to the powerful effects of a lightning strike. The inherent structural
design of an aircraft provides the occupants almost complete
protection despite the massive amount of current involved. This
protection is based on the principle known as the Faraday cage, first
devised by the physicist Michael Faraday in 1836.
A Faraday cage is a hollow enclosure made of conducting material, such
as the hull of an aircraft. In the presence of a strong electric
field, any electric charge will be forced to redistribute itself on
the outside enclosure, but the space inside the cage remains
uncharged. Thus, the metal hull of the aircraft acts as a Faraday
cage, protecting the occupants from lightning.
Some aircraft are made of advanced composite materials, which — by
themselves — are significantly less conductive than metal. To overcome
this resulting safety problem, a layer of conductive fibers or screens
is imbedded between layers of the composite material to conduct the
lightning current.
Regardless of hull material, the direct effects of lightning on the
exterior can also include:
- Burning or melting at lightning strike points;
- Increase in temperature;
- Residual magnetism;
- Acoustic shock effects;
- Arcing at hinges, joints and bonding points; and,
- Ignition of fuel vapors.
Accident data indicate that most of these effects are not serious.
However, an estimated one-third to one-half of aircraft lightning
strikes result in at least some minor damage. Lightning generally
enters an aircraft at one location, usually an extremity, and leaves
at another.
Burn marks are found at the entry and exit point(s) of the strike,
although exit points are not present if the energy was dissipated via
wicks or rods — static dischargers whose primary purpose is to bleed
off into the surrounding air the static charge build-up that occurs
during normal flight.
Because many aircraft fly a distance equivalent to several times their
own lengths during a lightning discharge, the location of the entry
point can change as the discharge attaches to additional points aft of
the initial entry point. The location of the exit points may also
change.
Therefore, for any one strike, there may be several entry or exit
points.
Occasionally, in more severe strikes, electrical equipment or avionics
may be affected or damaged. This potential problem is addressed in
modern aircraft design by redundancy. The functions of most critical
systems are duplicated, so a lightning strike is unlikely to
compromise safety of flight. In most strike events, pilots report
nothing more than a temporary flickering of lights or short-lived
interference with instruments.
The exception is the incidence of positive lightning. Positive
lightning strikes — because of their greater power — are considerably
more dangerous than negative lightning strikes. Few aircraft are
designed to withstand such strikes without significant damage.
