Originally Posted by bobcat4
Well… We’re talking about A/T in Flight Level Change (FLCH), right? I imagine an approach and landing is not a flight level change (although technically it is; a flight level change to runway altitude).
Assuming FLCH is not a common/normal mode during approach and landing, I repeat: Why would anyone want A/T HOLD when throttle leavers reaches idle in FLCH mode? This is obviously an intended design.
Hmm, yes I think I see what you mean. Why does the FLCH let the airplane stall or get close to stall even at altitude. In other words, what are the protections, if any, vs. A/T off?
Several posters a few posts back explained the FLCH mode in more detail.
I did some digging(aka google) and I found these explanations on another forum. They are quite interesting!
Source:
From the 777 FCOM, in the flight controls section (Pitch Envelope Protection & Stall Protection):
"The autothrottle can support stall protection if armed and not activated. If speed decreases to near stick shaker activation, the autothrottle automatically activates in the appropriate mode (SPD or THR REF) and advances thrust to maintain minimum maneuvering speed"
I spoke to a friend of mine who flies the 300 variant, and he told me something I also validated in the FCOM: the EEC enters an "approach idle" mode when certain criteria is met... one of these being using flaps 25 or 30 (which was the case). This prevents "under spooling" the engines in case of a GA or if an engine goes out.
Just for the sake of completeness: the 777 FLCH trap is described as follows.
When flying with both A/P and A/T engaged, and the AFDS working in a speed-on-elevator mode such as FLCH, for a descent the A/T will sit at idle and then HOLD as you expect. If you then disengage the A/P but leave the A/T engaged, the AFDS will keep the A/T in the speed-on-elevator mode -- i.e., it does not activate the throttle servo if your speed drops under the target MCP speed. It simply expects its other half (the now disengaged A/P or the real pilot) to fix the speed by pitch.
It is quite possible there is another protection far lower down the speed tape, just above stall speed, but either that didn't work or it wasn't in time. I can imagine that if you pull up hard enough to get the PAPIs back to two red, two white, you bleed off speed so rapidly that by the time the A/T comes back up, you face spool-up times longer than time to impact.
Also remember that although Asiana 214 got pretty close to stall (103kts!) it never did actually stall. I believe speed at time of impact was 122kts as engines were starting to kick in.