Originally Posted by
lomapaseo
Thanks. It would seem that kind of system must be fast enough to protect if it goes too far. I guess so far in the grainy photos of this event there doesn't seem to be a big enough gap viewable to set off a large effect on the FADEC. However as always I'm never sure about the pedigree of in-serevice older planes in their upgrade history. Of course I'm still wondering about the degree and timing of any loss of thrust in this event since they ran out of options on the last approach
I don't know about the CFM56-5, but the GE engines I worked, the FADEC had a "major frame" of between 200ms and 240ms - i.e. it cycled through all the engine control functions every 200ms. So less than a quarter second reaction time (of course it takes time for the engine to react - which is what killed Lauda - the FADEC pulled back fuel flow when the T/R deployed, but by the time the engine spooled down from Max Climb they'd already lost control of the aircraft).
Many pages back, someone posted a photo of the CFM56-5 gearbox installation. Looking at that, I postulated it's quite likely that when they did the gear-up landing, the gearbox hit the tarmac hard enough to fracture the gearbox housing - and losing nearly all the engine oil. After that it was only a matter of time. Obviously the impact didn't
destroy the gearbox, since that would have failed the fuel pump and the engines would have stopped almost instantly.