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Sunday, February 21, 2010

Final Report on BA 777 Crash at Heathrow

By Ramon Lopez/Editor, AT’s Daily Brief

The UK Air Accidents Investigation Branch’s (AAIB) final report on the Jan 2008 crash of a twin-engine British Airways Boeing 777 while landing at London Heathrow Airport concludes that an “unrecognized icing problem at the time” caused the accident. The AAIB’s final report remains faithful to preliminary reports on the accident involving Rolls-Royce RB211 Trent 800-powered Boeing 777s.

On January 17, 2008 a BA B-777-236ER (G-YMMM) experienced a dual engine rollback on final approach and crashed short of the runway at Heathrow Airport.  British Airways Flight 38 was arriving from Beijing.  UK accident investigators noted that during the flight the Boeing 777 passed through a region of particularly cold air (with ambient temperatures as low as –76 degrees C) between the Urals and Eastern Scandinavia.

The AAIB said ice in fuel lines “most probably” caused the Trent 800-equipped B-777 to lose all power just prior to landing on Runway 27L at London Heathrow. The crash heavily damaged the new jetliner, but there was no loss of life.

A second event occurred on November 28, 2008 when a Delta Air Lines B-777-200ER (N862DA) experienced a single engine rollback during cruise flight over Montana while en route from Shanghai to Atlanta.

Testing in support of the UK accident investigation led Boeing to develop procedures to help prevent ice accumulation, and to recover thrust in cases of ice blockage.  As more information from the Delta rollback event was developed, Boeing modified the procedures, which became the basis of an airworthiness directive (AD) issued by the FAA. The AD requires revision of the aircraft flight manual to include in-flight procedures for pilots to follow in certain cold weather conditions and requires fuel circulation procedures on the ground when certain conditions exist.

A total of 18 safety recommendations have been made following the earlier AAIB reports into the BA incident. Boeing and Rolls-Royce have taken steps to prevent the ice phenomenon from re-occurring. The AAIB has made nine further safety recommendations, including some that address plane crashworthiness.

While on approach to London Heathrow at 720 feet agl, the right engine of G-YMMM ceased responding to autothrottle commands for increased power and instead the power reduced to 1.03 Engine Pressure Ratio (EPR). Seven seconds later the left engine power reduced to 1.02 EPR. This reduction led to a loss of airspeed and the aircraft touching down some 330 m short of the paved surface of Runway 27L.

An investigation determined that the reduction in thrust was due to restricted fuel flow to both engines. It was determined that this restriction occurred on the right engine at its Fuel Oil Heat Exchanger (FOHE). For the left engine, the investigation concluded that the restriction most likely occurred at its FOHE. However, due to limitations in available recorded data, it was not possible totally to eliminate the possibility of a restriction elsewhere in the fuel system, although the testing and data mining activity carried out for this investigation suggested that this was very unlikely. Further, the likelihood of a separate restriction mechanism occurring within seven seconds of that for the right engine was determined to be very low.
 
The investigation identified the following probable causal factors that led to the fuel flow restrictions:   
 
Accreted ice from within the fuel system released, causing a restriction to the engine fuel flow at the face of the FOHE, on both of the engines.

Ice had formed within the fuel system, from water that occurred naturally in the fuel, whilst the aircraft operated with low fuel flows over a long period and the localized fuel temperatures were in an area described as the ‘sticky range’. 

The FOHE, although compliant with the applicable certification requirements, was shown to be susceptible to restriction when presented with soft ice in a high concentration, with a fuel temperature that is below -10°C and a fuel flow above flight idle.

Certification requirements, with which the aircraft and engine fuel systems had to comply, did not take account of this phenomenon as the risk was unrecognized at that time.

Ramon Lopez also serves as editor-in-chief of Air Safety Week; he has been covering air safety for more than three decades (rlopez@accessintel.com).

www.aviationtoday.com/ramon_lopez_bio.html


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