Asiana flight crash at San Francisco

[suninmyeyes]

The 777 can catch you out with with what is known as the "FLCH trap."

When you are above the glide slope and need to get down in a hurry Flight Level Change (FLCH) is a useful mode to use. Normally you transfer to another mode like glideslope or vertical speed, or you switch off the flight directors.

However in this situation the glideslope was off the air so the ILS would not have ben selected or armed. If the flight directors were left on and the plane was descending at a high rate in FLCH the autothrottle would have been inhibited and would not have put on power so the thrust levers would have stayed at idle.

If the Asiana was a bit high (quite normal for SFO) then regained the visual glideslope, the rate of descent would have decreased and the speed would have started slowly reducing but with the thrust levers staying at idle the 777 would now be in the same situation as the Turkish 737 at AMS, ie speed decreasing below Vref and not being noticed.

The 777 has autothrottle wake up, ie when the aircraft approaches a stall the power comes on automatically to almost full power. This gives pilots great confidence however autothrottle wake up is inhibited in FLCH.

So 777 pilots will be looking at this scenario and wondering if Asiana were in FLCH with flight directors on, too high, stabilised late and did not notice they were still in FLCH and that the autothrottle was not keeping the speed to Vref plus 5 untl too late.

Just a theory but I think it far more likely than engine failure, radalt failure or autothrottle failure and I suspect when the events are unravelled this will be what has happend.