BA Boeing 777 G-YMMM crash at Heathrow - cause remains a mystery
It is a natural reaction to believe that when a massive airplane falls out of the sky, intact with all systems, crew, passengers and complete records that it would be easy to resolve the cause of failure . Modern aircraft are immensely complex machines, so when they fail investigation can prove to be problematic.
The Air Accidents Investigation Board have published a 3rd Special Bulletin on the crash of the BA Boeing 777 G-YMMM on the 17th January 2008 at Heathrow Airport and demonstrates that whilst many causes can now be ruled out, the fundamental causes remain unknown.
The cause of failure was soon determined - On approach the autothrottles demanded an increase in thrust which occurred intitally and then the right and then the left engines reduced thrust due to a reduced fuel flow - which is refelected in all subsequent recorded engine parameters. The engine control system detected the reduced fuel flow and commanded the fuel metering valve to open fully. The fuel metering valve responded and opened fully - but there was no appreciable change in the fuel flow to either engine.
Extensive examination of the aircraft and detailed analysis of the recorded data have revealed no evidence of an aircraft or engine control system malfunction.
There is no evidence of :
1. Air turbulence ( wake vortex encounter) from preceding aircraft
2. An engine bird strike
3. Core engine icing
4. Anomalous behaviour of any of the aircraft or engine systems that suggests electromagnetic interference - which leaves Gordon Brown's security sweeps off the hook.
The remaining fuel has been tested extensively; it is of good quality, in many respects it exceeds the appropriate specification, and shows no evidence of contamination or excessive water - despite all the fervid press talk of dodgy Chinese fuel supplies.
Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages.
The spar valves and the aircraft fuel boost pumps were serviceable and operated correctly during the flight.
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.
Tracing the flight route and copnditions shows that there was a region of very cold
air, with ambient temperatures as low as -76ºC, in the area between the Urals and Eastern Scandinavia. Described by the Met Office 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.
Faced with this it has not ben possible to determine a cause or to suggest any operational changes either by the AAIB, Boeing or Rolls-Royce.
Meanwhiel a data analysis team, working with QINETIQ, are reviewing and analysing the recorded data from a large sample of flights on similar aircraft.
No individual parameter from the flight of G-YMMM has been identified to be outside previous operating experience. The analysis is concentrating on identifying abnormal combinations of parameters.
1 comment:
I love a good mystery. This passage from the report to date seems to be the key
"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."
If one Googles
Cavitation damage
a list of related technical reports such as this appear:
http://www.insidersecretstohydraulics.com/newsletters/issue03.html
The author says cavitation can cause erosion; meaning loss of mass, a bit at a time. "Damage to the outlet ports" suggests that the bits lost by the pump impellers were big/sharp/rough enough to damage an orifice merely by passing through it - on their way to the fuel nozzles feeding the combustion chambers of the engines themselves.
This raises a couple of questions
a. Is there an inline filter between the damaged pump(s) and the nozzles, to intercept the products of erosion?
b. If not, could said p.o.e. accumulate in the nozzles, clog them up, and reduce their delivery capacity?
Earlier in the report the following appears
"The engine control system detected the reduced fuel flow and commanded the fuel metering valve to open fully. The fuel metering valve responded and opened fully - but there was no appreciable change in the fuel flow to either engine.
By the time the system issued its command to the metering valve, the high-pressure pumps had been cavitating for a finite (recorded?) number of minutes and the damage was done. Be that as it may, I don't understand how the following denial of 'control system malfunction' can be justified.
"Extensive examination of the aircraft and detailed analysis of the recorded data have revealed no evidence of an aircraft or engine control system malfunction."
If the high pressure pumps demanded more fuel, and nobody/no thing told the metering valves, then that would seem to be a 'system malfunction'.
It's not surprising that the fuel appears not to have 'frozen' because there'd be tons of it. But if the relatively low-mass fuel metering valve(s) are outside the fuel tanks then they, and/or the fuel lines did get too cold. It seems very likely that the metering valves became frozen in the partially open position required for cruising and the (computer) system responds to (unmet) HP pump demand instead of always issuing commands to the metering valves when the throttles are opened.
All just guesswork, of course.
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