The Air Accidents Investigation Branch (AAIB) has concluded that the 17 January crash-landing of a Boeing 777 at Heathrow was probably caused by "ice within the fuel feed system" which restricted flow to the engines.
BA038 (G-YMMM), after a routine flight from Beijing, suffered reduced thrust in both engines while coming into land and fell short of the runway. The AAIB explained earlier this year:
The first officer took control for the landing at a height of approximately 780 ft, in accordance with the briefed procedure, and shortly afterwards the autothrottles commanded an increase in thrust from both engines. The engines initially responded but, at a height of about 720 ft, the thrust of the right engine reduced. Some seven seconds later, the thrust reduced on the left engine to a similar level. The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust. The engines failed to respond to further demands for increased thrust from the autothrottles, and subsequent movement of the thrust levers fully forward by the flight crew.
Following examination of the aircraft, the AAIB was able to report: "The high pressure (HP) fuel pumps from both engines have unusual and fresh cavitation damage to the outlet ports consistent with operation at low inlet pressure.
"The evidence to date indicates that both engines had low fuel pressure at the inlet to the HP pump. Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected."
Quite what caused this restricted flow proved a bit of a poser, but the AAIB now confirms (pdf):
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 AAIB, while describing the incident as "the first known occurrence of this nature in any large modern transport aircraft", stresses: "All aviation fuel contains water which cannot be completely removed, either by sumping or other means. Therefore, if the fuel temperature drops below the freezing point of the water, it will form ice. The majority of flights have bulk fuel temperatures below the freezing."
Among its recommendations attached to the latest report, the AAIB urges "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". ®
*During its investigation, 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".