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 HeathrowAirport.

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.

‘The problem is when the fuel is subjected to very low temperatures for long periods. The average temperature at 35,000 feet is -50oC. Thus, any aircraft flying high for long distances is susceptible to this icing risk. Most aircraft do not operate like this and if those that do take recommended procedural actions then there should be little or no additional risk.’

Despite this view, the NTSB’s report said that interim safety measures put in place to prevent similar instances had been ‘insufficient’ and there was a ‘high probability’ of another incident occurring.

Ling added: ‘There should be no need to worry. The safety record of these aircraft is excellent – there have been no fatalities and changes to operating procedures would reduce any risk of this occurring again.’

Last month, Rolls-Royce said that they had begun a redesign of the engine, which it expects to test and install within the next 12 months. The NTSB has recommended that after the redesign is completed the new system be installed on all B-777 aircraft within six months.

Chairman of the NTSB, Mark Rosenker, said: ‘We are encouraged to see that Rolls-Royce is already working on a redesign, and we are confident that with the FAA and EASA European Aviation Safety Agency (EASA) overseeing the process, this flight-safety issue – even one as complex as this – will be successfully and expeditiously resolved.’

There are currently 228 B-777 aircrafts with Rolls-Royce engines in service worldwide, and 15 of these are owned by British Airways.

A spokesperson for British Airways said: ‘We will continue to work closely with the relevant authorities and comply with requirements issued to all operators of Boeing 777s powered by Rolls-Royce Trent 800 engines.

‘We have already fully implemented the FAA airworthiness directives from last September and have complied with all advice to operators following the Delta event. As a precaution, we also stopped operating several back-to-back flights with the same aircraft where cold temperatures are expected.’

Ellie Zolfagharifard