BA038 (B777) Thread

[777fly]

I am not sure that there are any software implications in this incident. The AAIB report makes it clear that the engine software and systems responded correctly but there was a problem with the fuel supply, not the demand. This is supported by the evidence of HP pump cavitation.

There is actually one instance in which Boeing suggest that a fuel supply problem can cause engine thrust deterioration or flame out. This is when an engine has to revert to suction feed at altitude, following dual wing tank pump failure. (No mention is made of the low altitude situation.) The background to this advisory is that dissolved air in the fuel is released at altitude and can accumulate in the suction feed pipe. The implication being that low atmospheric pressure in the tank, plus air in the suction line will disrupt or cut off the fuel flow under suction conditions.

Note from the fuel synoptic posted by Jet11 that the suction pipe is connected directly to the engine fuel supply manifold, isolated only by a pressure bypass valve. Also that the tank pump nominal output pressure is 14psi, but can be much lower. For argument's sake, I will assume that the suction bypass valve opens when there is a pressure differential around 5 psi and it closed under normal conditions.

In Bejjing, BA038 takes on fuel with an unusually high dissolved air content. This is due to a combination of production/storage/transportation/pumping and weather variables. The a/c flies high, long and cold. The air is released from the fuel and the suction pipe is completely filled with (relatively) dense air. This air remains in the pipe during the descent and once below 6000ft is inceasingly pressurised by denser ambient air and rising temperatures. At 1000ft atmospheric pressure reaches 14psi, equalling the fuel manifold pressure. Fuel hydrostatic static pressure and adiabetic heating add another 6psi, causing the suction bypass valve to open against tank pump pressure.This releases pressurised air into the fuel supply manifold and disrupts the fuel supply. This happens in both tanks, but not simultaneously. The opening of the valve could be assisted by a momentary drop in fuel manifold pressure during the initial power demand.

Just another theory. My assumptions about the bypass valve are probably wrong. Also, why is this not a regular occurence? Maybe the fuel and meteorological conditions were unique to BA38. Maybe we have always been operating close to this scenario.........