What happens when lightning strikes an aircraft

01

How often is an aircraft struck?[1]

About once or twice per aircraft per year for commercial airliners. Strikes are routine enough that they are handled as a maintenance inspection item rather than an emergency, and the aircraft usually completes the flight normally before inspection.

Aircraft do not only receive strikes, they trigger them. A plane flying through a strongly electrified region distorts the local field enough to initiate a discharge that would not otherwise have happened at that moment. A substantial share of in-flight strikes are triggered this way, which is part of why they cluster in specific altitude and temperature bands rather than only near active storm cores.

The typical strike attaches at an extremity such as a nose, wingtip or fin, then sweeps rearward along the skin as the aircraft moves through the stationary channel, and exits from another extremity. Damage, when there is any, tends to be small burn marks and pits at those attachment and exit points.

02

Why does the current not reach the cabin?[1]

Because the aluminium skin acts as a Faraday cage. Current flows on the outside of a conductive enclosure rather than through its interior, so it travels across the fuselage exterior and leaves without entering the volume where people and most systems are.

The cage only works if it has no electrically isolated gaps, which is why bonding is as important as the skin itself. Metal straps and connections keep every component tied to the fuselage, so no panel, hinge, control surface or fairing ends up floating at a different potential and becoming somewhere for current to arc to.

Composite airframes complicate this, because carbon fibre conducts far worse than aluminium and epoxy barely conducts at all. Composite aircraft embed conductive mesh or foil in the skin to restore the path the metal used to provide, and the certification work for that is a significant part of designing a composite airliner.

03

What are the small spikes on the wing trailing edges?

Static wicks, also called static dischargers. They bleed off the static charge an aircraft accumulates from flying through air, precipitation and ice, giving it a controlled place to leave the airframe. They are not lightning arrestors, and they do not reduce the chance of being struck.

Their real job is radio reception. Accumulated static discharges from the airframe as corona, and that corona generates broadband radio noise that interferes with navigation and communication equipment. The wicks give the charge a controlled place to leave from, positioned well behind the antennas so the noise leaves with it.

They do help incidentally during a strike, since current preferentially leaves through a discharger rather than through structure at the trailing edge. That is a side effect of the design rather than its purpose.

04

What about the fuel tanks?

Fuel vapour ignition is the failure mode that certification is most concerned with, and it is addressed in two ways. Structural joints, fasteners and hinges are designed so current crossing them cannot spark, and fuel tank inerting replaces the vapour above the fuel with nitrogen.

The sparking requirement is demanding because current has to cross every joint between skin sections without producing a spark anywhere near a fuel vapour space. Fastener design, sealing and bonding at those joints are all part of it, and a single non-compliant fastener is a legitimate airworthiness finding.

Inerting attacks the same problem from the other end. A nitrogen-rich vapour space cannot support combustion regardless of what happens electrically, so the ignition question becomes moot rather than merely unlikely. Modern transport aircraft carry systems that generate nitrogen-enriched air in flight and feed it into the tanks.

The combined result is a strong safety record. Airliners are struck routinely, and no commercial jet has been lost to a lightning strike in more than fifty years.