Ice in fuel caused Heathrow 777 crash

Nasty chill provoked reduced fuel flow
By Lester Haines $B"*(B More by this author
Published Thursday 4th September 2008 16:37 GMT
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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 auto-throttles 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". (R)
Bootnote

*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$B!k(BC, in the area between the Urals and
Eastern Scandinavia".
http://www.theregister.co.uk/2008/09...w_777_verdict/