Flight risk

The US National Transportation Safety Board (NTSB) has issued revised safety recommendations for Boeing 777 aircraft equipped with Rolls-Royce RB211 Trent 800 engines.

The recommendations follow two incidents last year involving uncommanded loss of thrust, also known as rollback, caused by fuel icing. The first incident occurred on 17 January 2008, when a Boeing 777 experienced a dual-engine rollback on final approach and crashed near a runway at London's Heathrow Airport.

The second, involving a Delta Air Lines Boeing 777 aircraft, experienced a single-engine rollback during a flight from Shanghai to Atlanta in November 2008.

Reports published on both incidents concluded that, while there were not absolutely certain, it was possible that the build-up of ice on the fuel delivery pipes became dislodged and clogged a diffuser plate at the entrance of the exchange.

Speaking to The Engineer about the findings, John Ling, head of transport at the Institute of Mechanical Engineers (IMechE), said: ‘The problem appears to lay with the fuel system and not the engine itself. Aviation fuel contains around 70 parts per million of water, which is not a problem for combustion (it can actually help produce more power) and aircraft are generally fitted with systems to stop water accumulating in pockets.