The Air Accidents Investigation Branch (AAIB) has released its final report into the 17 January 2008 crash-landing of a Boeing 777 at London's Heathrow, confirming its earlier interim conclusion that ice in the fuel feed system caused the incident.
BA038 (G-YMMM), flying in after a routine flight from Beijing, suffered reduced thrust in both engines while coming in to land and fell short of the runway.
A 2008 AAIB interim report stated:
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.
(The "unusually cold environment" during flight BA038 was "a region of particularly cold air, with ambient temperatures as low as -76°C, in the area between the Urals and Eastern Scandinavia".)
The AAIB's second interim report noted:
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.
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 has now offered four "probable causal factors that led to the fuel flow restrictions":
1) 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.
2) 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 localised fuel temperatures were in an area described as the ‘sticky range’.
3) 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.
4) Certification requirements, with which the aircraft and engine fuel systems had to comply, did not take account of this phenomenon as the risk was unrecognised at that time.
Among the AAIB's raft of safety recommendations (here, pdf) is that the "Federal Aviation Administration and the European Aviation Safety Agency, in conjunction with Boeing and Rolls‑Royce, introduce interim measures for the Boeing 777, powered by Trent 800 engines, to reduce the risk of ice formed from water in aviation turbine fuel causing a restriction in the fuel feed system".
The branch further recommends 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".