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Lightning – Generally Not a Hazard Unless it Penetrates a Fuel Tank

Wed, May 18, 2011 — David Evans

Articles

What does an electric current bolt of 20,000 amps hitting a jetliner look like? A photograph of a lighting strike on an Emirates A380 illustrates the dramatic appearance.

A video taken by Chris Dawson near London’s Heathrow Airport shows lightning striking the giant, double-deck plane, wrapping around it and continuing through the air to the ground.

The Emirates A380 on approach to Heathrow in bad weather

“I saw a storm coming and I thought there could be lightning,” said Dawson of the 23 April event. “I wasn’t expecting it to hit the plane but I just got lucky.” He lives directly below a flight path to Heathrow.

An airplane’s metal hull forms a Faraday cage that protects it from lightning, carrying the lightning’s electric charge through the surface of the hull and expelling it at an output point without harming the aircraft, its electronics, or the occupants inside.

The Emirates A380 landed without incident.

A clearer photo of an Emirates 550-passenger A380 super-jumbo jet

Boeing’s new B787 is built primarily of composites, which can be penetrated by lightning and cause damage to systems and persons inside. To replicate the Faraday cage benefit of the more traditional aluminum structure, the B787 will feature small metal pieces embedded in the composite material to carry away the electrical charge. In addition, the B787’s fuel tanks will be filled with an inert gas, displacing any oxygen in the tanks’ void spaces. If lightning penetrates the fuel tanks, it is hoped that it will not trigger a fuel vapor explosion because of the presence of nitrogen-enriched air in the tank. This nitrogen-enriched air in the tanks will lower the percent of oxygen to the point where it is not sufficient to sustain an explosion in the event of lightning penetration.

Boeing's new all-composite B787, which will feature added protections against lightning

Without an effort to replicate the Faraday cage structure, lightning can severely damage an aircraft’s structure. A lightning strike in 1999 on a glider – which is built primarily of composites – blew the wing off and forced the two pilots to bail out and parachute to the ground. (See Aviation Safety Journal, January 2010, “Lightning Strike Standards Now Difficult to Discern”)

A Lufthansa Technik document explains the problem:

“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 airframe acts as a so-called ‘Faraday cage’ …

“Lightning damage in the airframe, the wings or the empennage normally never occur if these structures are made of aluminum. The use of composite materials … however, makes these parts more sensitive 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, which will feature nearly all-composite fuselages, have therefore to be protected by special mesh or glass or metallic fibers to direct the electric energy [away] from the airplane structure.”

In 1996, TWA Flight 800 blew up from an electrical spark inside its center wing tank. The equivalent – albeit at considerably less power – of a lightning strike. The momentary spark in the dark confines of the TWA’s B747-100 center fuel tank ignited flammable vapors inside and destroyed the airplane.

The U.S. National Transportation Safety Board (NTSB), which investigated the TWA 800 tragedy, urged the Federal Aviation Administration (FAA) to require fuel tank inerting on all existing and new transport-category aircraft. Inerting entails filling the void space in the fuel tank with a non-explosive gas, such as nitrogen, to suppress any tendency of fuel-air vapors to explode.

The A380 was certificated without inerting, despite the NTSB’s recommendation. All existing airliners must retrofit inerting systems by 2018, according to a final rule published by the FAA in the Federal Register on 21 July 2008. In other words, the last airplanes will be retrofitted fully 22 years after TWA 800 exploded. (See Aviation Safety Journal, August 2008, “Significant Regulatory & Related Activity”)

The rule affects airplanes with heated center wing tanks only. By this is meant fuel tanks with heat sources such as air conditioning packs adjacent to the tank. The new A380 does not feature a “heated” center wing tank and is therefore exempt from the rule.

The NTSB urged that all fuel tanks (wing, body, empennage, auxiliary) be inerted, irrespective of whether they are heated or not. It should be pointed out that 8 of 17 fuel tank explosions in the jet age have involved unheated wing tanks. The A380 exemption from the inerting requirement ignores this deadly history.

Passenger jets built prior to 1972 are excused from the new requirement on the grounds that they do not have significant remaining useful life in passenger service. If some of these aircraft are converted to all-cargo use, they do not require retrofit of an inerting system. So much for the FAA’s “one level of safety” theology; cargo pilots will be operating the same aircraft as their passenger-carrying brethren without the same safety system.

The FAA’s inerting mandate does not feature any quality control requirements for the tank heating and cooling calculation, although they are obviously of crucial importance.


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