Can lightning cause an airplane crash? Not today. In fact, the last time a commercial airplane crashed in the United States due to a lightning strike was about 50 years ago. On December 8, 1962 lighting hit a Pan American Boeing 707 in a holding pattern over Elkton, Md. The lightning created a spark that ignited jet fuel vapor in a holding tank, triggering an explosion that killed all 81 aboard. Nowadays, the FAA estimates that each commercial airliner will be hit by lightning once a year (some will get hit more than once, some won’t get hit at all). And in some cases, the airplane itself actually triggers the lightning!
So why doesn’t lightning cause commercial airliners to crash? There are three main reasons: highly conductive outer shell, a combination of surge, shielding and grounding protectors on electronic equipment, and newer jet fuel which produces much less explosive vapor.
The first line of defense is the outer skin of a commercial jet which is made of aluminum or a composite with interwoven conductive fibers which provides a path for the electric current. When lightning strikes the skin of an airplane, it travels along the outside of the aircraft and discharges back into the air, much like a Faraday Cage. Passengers on board might not even know the aircraft was struck unless they happen to see the flash or hear the thunder through the roar of the engines. Pilots may notice a short flickering of lights or jumps in their instrumentation, but it should never cause a failure. We can thank NASA for this.
Even though the electric current remains on the outer skin of the aircraft, small electrical transients are induced on wiring inside the aircraft. As a result, regulations were put in place such that all electronic equipment, fuel tanks and fuel lines have their own built-in surge protection, shielding and special grounding systems. Even if the system experiences flickering, it will return to normal in less than a second.
To further reduce the risk of explosion, jet fuel (Jet-A) now has additives that reduce the amount of vapor produced. Although the vapors are still explosive, the fuel tanks are insulated and grounded, which makes it very difficult (dare I say impossible?) for an electrical ignition source to enter the tanks and ignite the fuel.
NASA conducted several tests in the 1980’s in which pilots deliberately flew an F106B jet into thunderstorms to gather data and see what affects lightning would have on the aircraft. Despite being struck over 700 times on 1400 missions the aircraft never sustained any damage. However, it was concluded based on the data collected that on board instrumentation could be affected by small electrical charges caused by the lightning.
As a strange side note, if pilots avoid the large thunderstorms, why are aircraft still being struck by lightning? Well, that’s because smaller clouds can also produce lightning and the airplane itself may actually trigger it. With all the new technological advances, it would be very unlikely that lightning would cause an airplane crash.