The fuel cells could be used as auxiliary power units to produce electricity when an aircraft is on the ground, for example, allowing its engines to be turned off.

ECONOMIST: Electric planes

In a second category A case, which was reported recently by the Air Accidents Investigation Branch, a departing BAe 146 aircraft almost collided on the ground with an arriving two-seater Piper aircraft at Birmingham airport.

BBC: Jet in near miss mid-air collision

An engine failed during tests on the ground in South Carolina last July and inspectors found a similar problem on another aircraft in September.

CNN: Airline: Dreamliner fuel leak due to an open valve