Fuel flow restriction slowly building up
Hello everybody.
I'm new to this forum. French my native language. No modern jet experience. Some airmanship (I hope). Be clement we me ...
I think that we are perhaps misleaded by the assumption that the problem occured "almost simultneously" to both engines, when they failed to spool up.
General scenario :
More likely, the flow fuel restriction problem was there well before the demand for higher engine output occured on short final. Maybe the fuel flow started to be restricted (cold related - any kind fuel of contamination, make you choice : ice, wax, bioslush ...) much earlier in the descent. The restriction built up slowly, somewhere between the LP pump in the wing fuel tank and the engine HP fuel pump. In a very cold spot? In a bend? Idlle fuel flow and very low temps at TOD might be a factor. Everything almost symetric (same cause, same effects). The restriction went unnoticed as long as only minimum fuel flow was required. Then ... when more fuel was required, i-e when the valves downstream the HP pump opened, the engines initialy accellerated, burning all the available fuel in the line, until the fuel flow became regulated by the fuel restriction upstream.
So : almost simultaneous evidence, but a relatively long standing cause.
Ok, this is only a theory, I'm not fluent with the 777 fuel system (I did learn a lot reading this thread in full). But this theory does free us from the double "sudden" failure mode : the failure were not sudden, the problem was a "progressing" one, going unnoticed for a while.
My two cents ...