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Ice in fuel feed caused Heathrow 777 crash
Further AAIB tests confirm early findings
The Air Accidents Investigation Branch (AAIB) has presented further evidence to confirm its findings last September that "ice within the fuel feed system" caused the 17 January 2008 crash-landing of a Boeing 777 at Heathrow.
BA038 (G-YMMM), following an uneventful flight from Beijing, suffered reduced thrust in both engines while coming into land and fell short of the runway. The AAIB reported last year:
The investigation has shown that the fuel flow to both engines was restricted; most probably due to ice within the fuel feed system. The ice is likely to have formed from water that occurred naturally in the fuel whilst the aircraft operated for a long period, with low fuel flows, in an unusually cold environment*; although, G-YMMM was operated within the certified operational envelope at all times.
Its second interim report concludes: "Based on the available data, testing, and the analysis contained in the AAIB initial interim report, the investigation has established, that with a relatively low fuel flow, ice would start to form on the inside of the fuel feed pipes that pass through the main fuel tank whilst the centre tank was supplying fuel to the engines."
The AAIB continues:
It is considered that, in the later stages of the approach, the engine accelerations, and perhaps a combination of other factors such as turbulence, aircraft pitch changes and an increase in the strut temperature, could have contributed to a sudden release of soft ice in the fuel feed system for both engines. This ice would have travelled through the fuel feed pipes, where it could have formed a restriction on the face of the FOHE [Fuel Oil Heat Exchanger] sufficient to cause the subsequent engine rollbacks.
The AAIB does, however, offer this caveat:
Whilst this is considered to be the most likely cause of the engine roll backs on G-YMMM, and is consistent with data from the incident to N862DA, it has not been possible, due to limitations in the available recorded data, to totally eliminate the possibility that a fuel restriction, from ice, formed elsewhere in the fuel system which, in addition to an FOHE restriction, contributed to the engine roll backs on G-YMMM. It should be noted that extensive testing and data analysis has not identified any features elsewhere in the aircraft fuel system which would have caused a large enough concentration of ice to accumulate and cause a restriction.
(N862DA is the Delta Airlines Boeing 777-200ER which on 27 November last year suffered "uncommanded rollback" on a Rolls-Royce Trent 895 engine. There are more details on that incident here.)
In reponse to the AAIB's findings, Boeing and Rolls-Royce have accepted the following: "It is recommended that Boeing and Rolls‑Royce jointly review the aircraft and engine fuel system design for the Boeing 777, powered by Rolls‑Royce Trent 800 engines, to develop changes which prevent ice from causing a restriction to the fuel flow at the fuel oil heat exchanger." ®
*The AAIB noted that during flight BA038 "there was a region of particularly cold air, with ambient temperatures as low as -76°C, in the area between the Urals and Eastern Scandinavia".