Lightning strikes during flight
No danger despite high voltage
In commercial aviation adverse weather conditions mostly happen underneath cruising altitudes. But even on lower flight levels they are rarely a big issue because every modern commercial airliner is equipped with weather radar. These systems help the flight crew to identify storm fronts in the flight path of the airplane and give hints to circle around the bad weather. But sometimes an airplane might have to fly right through adverse weather regions because there is not enough time to avoid the detected hailstorms or thunderstorms. The latter are not uncommon in this case to scare the passengers with a bright flash of light and loud bang, mostly followed directly by an announcement from the cockpit: "Lightning strike–. These incidents are very rare in modern air travel, even frequent flyers only have a very low chance to witness a lightning strike on one of their flights. A commercial aircraft however is struck several times during its whole service life.
In such a case, the airplane acts like a lightning rod. Its metal structure provides the lowest resistance for the electrical discharge on its way between the clouds and the ground. It is not uncommon that the airplane is thereby struck by a complete series of discharges, mostly between three and five, in exceptional cases up to 25. Since an airplane in flight has no form of grounding, the lightning first enters the structure and leaves it again a split second later. The principle behind this occurrence is known by most people from physics lessons in school. The airframe acts as a so called "Faraday cage–. Like an automobile body the aluminium structure, when struck by a lightning, passes the electric energy around the interior and keeps the passengers safe. The crucial technical equipment is thereby also kept safe from the high voltage and the aircraft can, in most cases, proceed normally with its flight. But to play it safe every lightning strike is documented by the cockpit crew and the aircraft is treated with a special inspection routine on its next check. Lightning strike inspections like these are regularly carried out at Lufthansa Technik.
A special lightning strike inspection procedure is manifested in every aircraft's maintenance manual and is usually divided in two stages. Stage one comprises in-depth visual inspections of the entry and emergence points the electrical discharge went through. Scorch marks of the high voltage are quite hard to locate on the painted surface of the aircraft because their diameter is often smaller in size than a pinhead. However, the highly qualified personnel of Lufthansa Technik is very quick to detect the traces of a lightning strike, because from their decades-long experience they know exactly where to take a look at the different aircraft types. The myth that a lightning always strikes at the highest elevation is in the air as false as on the ground. Not the high tailfin is the favourite point of impact, at most aircraft types the lightning enters the airframe at the edge of the cockpit windows or the leading-edge of the wings. The favourite exit points for lightning turned out to be the winglets or the tips of the aircraft's control surfaces. Scorch marks caused by electrical discharge are also often found at the border of rivets and the trailing edge of the wings, where so called "electrostatic dischargers– are mounted. These thin sticks, which normally return the natural electrostatic charge the airplane collects through atmospheric friction, provide one of the easiest exits for lightning energy. Sometimes these parts get even completely burned away by a lightning strike, which sounds dramatic but has no negative effect on the safety of the aircraft.
Lightning strike inspection: a routine business for Lufthansa Technik
Inspecting an aircraft struck by lightning can take up to several hours depending on the aircraft size and visibility. When the personnel of Lufthansa Technik has located the entrance and exit marks of the lightning strike, stage two of the inspection procedure comes into effect. Parts with suspicious scorch marks are now looked at more closely, from the outside as well as from the inside. Burned off static dischargers are replaced with new ones. Rivets affected by lightning damage are examined with methods of non-destructive testing and replaced if necessary. Lightning damages in the airframe, the wings or the empennage normally never occur if these structures are made of aluminium. The use of composite materials like carbon-fibre reinforced plastics (CFRP) however, makes these parts more sensible to lightning strikes. The high temperatures generated by the electrical discharge can boil and melt the resins used in composite materials and hence weaken the structure. Aircraft types currently in development like the Boeing 787 or Airbus A350, who will feature a nearly all-composite fuselage, have therefore to be protected by special mesh of glass or metallic fibres to derive the electric energy from the airframe structure. The radome in the aircrafts nose is harder to protect, because a metallic mesh would cause interference in the weather radar underneath it. To avoid false readings in this systems the radome is protected with narrow metallic "lightning stripes– which are electrically connected to the fuselage structure to ensure a safe conduction. Moving parts like landing gear doors or high lift devices are also cable-connected to the airframe structure for this reason.
The structure of a commercial airplane is hence well protected against lightning strikes, even if the surface is wetted by rain. Much more problematic are water ingressions inside the airframe structure, which can lead to problems with aluminium as well as composite structures. The high energy of a lightning strike can evaporate the water within a split second which causes a sudden expansion that can in some cases damage the structure. But since commercial aircraft are regularly checked for water ingressions because of corrosion protection issues, such a case is highly unlikely. Another component in need of protection is the avionics equipment, comprising all electronic devices of the aircraft. Although its main parts are, like the passengers and the crew, protected by the airframe's Faraday cage, the avionics equipment can as well be affected by a lightning strike, for example through necessary external sources like antennas. All electronic devices on board are therefore protected by an overvoltage arrester and designed as redundant, so that in case of a malfunction a backup system will quickly take over. The protective systems successfully proved to be working: The personnel of Lufthansa Technik have, despite decades of experience, never discovered any damage to avionics components caused by a lightning strike. The safety of a modern commercial airliner is hence normally never be affected by such incidents. But to play it safe and to further ensure the operational safety, Lufthansa Technik always carries out the described inspections prior to the next flight after a lightning strike.