Yesterday, the UK Air Accident Investigations Branch (AAIB) released an interim report on the British Airways Boeing 777 that crashed at London Heathrow last January. The report indicates that the AAIB suspects that the plane's fuel flow became restricted somewhere between the engines and the fuel tanks, causing the plane's engines to become starved of fuel.
The report says, "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."
The report also says that the focus of the investigation "continues to be the fuel system of both the aircraft and the engines, in order to understand why neither engine responded to the demanded increase in power when all of the engine control functions operated normally."
One area of investigation is whether an area of very cold air through which the aircraft flew was a culprit, although the indications are that it should not have been a concern.
"During the flight there was a region of particularly cold air, with ambient temperatures as low as -76ºC, in the area between the Urals and Eastern Scandinavia. The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’. The lowest total air temperature recorded during the flight was ‑45ºC, and the minimum recorded fuel temperature was -34ºC. The specified fuel freezing temperature for Jet A-1 is not above ‑47ºC; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft complied with the Jet A-1 specification and had a measured fuel freezing temperature of -57ºC. The aircraft was operated within its certified flight envelope throughout the flight."
Discovering the reason(s) for the crash has proven much harder than anyone expected, given that the plane was much more intact than in most crash investigations. As one expert was quoted in a story in the Washington Post:
" 'This is a great mystery, and I never expected this accident to be this difficult to solve, given the state-of-art tools on the plane and the fact that the aircraft was largely intact,' said Bill Voss, president of the Flight Safety Foundation, an organization that advocates for improving aviation safety. 'This has potentially broad implications that go beyond this one airplane, depending on what they find.' "
I'll let you know what the final report says.
Comments (1)
Myself and other retired technical flight crew have been of the opinion since shortly after the occurence, that the probable cause was the min fuel tank temp being exceeded.
From my own experience, between Alaska and LHR, TOAT would drop to around -65C at this temp the min fuel would be hovering around -35/-36C, in general flight manual advises not letting fuel temp drop within 5 deg of min freeze point.
From FM advice to Flight planners, as a guide ind temp is higher than TOAT by 25C therefore when using -40C FP fuel, temps of -65C could be critical.
Points to note: difference between tank temp and rate of decrease, period of time that OAT is likely to be encountered.
WARNING! SUBSEQUENT INCREASE IN IOAT WILL RESULT IN LITTLE OR NO INCREASE IN TANK TEMP FOR SOME CONSIDERABLE TIME.
Note:- when uploading -50C fuel after -40C the fuel freezepoint should be taken as -42C.
also we must take into account that the volume of fuel on final approach is at its lowest. very therefore conductivity will be higher, I have never encountered -76C in 26 years of flying so it was unusual. I am very dubious of the contents of this report some of the figures and findings don't add up.
Posted by Jeff Gittins | September 17, 2008 2:36 PM
Posted on September 17, 2008 14:36