Another article
http://www.theregister.co.uk/2008/09/04/heathrow_777_verdict/
Ice in fuel caused Heathrow 777 crash
Nasty chill provoked reduced fuel flow
By Lester Haines
Published Thursday 4th September 2008 16:37 GMT
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".
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°C, in the area between the Urals and Eastern Scandinavia".
