Mr. L - Yes that of course makes more sense. Have scanned through the manuals and can't really find any good explanation to this phenomenon
Suspect that the engineers didn't really realize what a curious, kneed-to-know-every-little-detail, kind of people we are
On the other hand, how could one make the right decision if put in LEM's situation, without knowing its function and more importantly what to expect when it's not functioning? Both AP's still engaged, FLARE armed and everything seems fine. One could always see this as a non-normal operation of the AP system and do a G/A. Or, one could say that if this was a G/A situation shouldn’t the logic of the AP disconnect or give a warning?
When it comes to the steady red AP light below 800 feet RA and during app, it will come on if the Stab is out of trim. And in this case it was more in trim than had the normal 400 feet RA nose-up trim taken place.
Found one little note on the subject though that refer to the automatic G/A.
"The pitch mode cannot be changed from TOGA until sufficient nose-down trim has been input to allow single channel AP operation. This nose-down trim is automatically added by the AP to reset the trim input made by the AP at 400 feet RA and at 50 feet RA during the approach."
The AP is actually trimming the airplane out of the single channel operational range, which makes even less sense as to my previous post about preparing for G/A. On the other hand, flying around with a nose heavy airplane close to the ground seems to be risky business should the AP suddenly disconnect.
So the question remains why the AP trim the airplane in this way so close to the ground. Doesn't seem to be any obvious logic to it. I mean doesn’t this trim go against the fail-passive definition? During fail-passive operations should the autopilot disconnect the airplane should be in trim which it clearly isn't below 400 feet RA?
The fact is that anything more nose heavy than in trim at this particular stage would in my opinion be an accident waiting to happen. So maybe that’s the answer? We heard that on the 757 there is a distinction between LAND2 and LAND3 (fail-passive and fail-operational). Maybe this is a safety precaution when getting close to the ground? Scenario; AP disconnects just before minima and you have insufficient visual cues. Time for a GA, right!? Well in this case the nose or the control column pressure will tell you exactly where to go: UP not down. The natural response will be to TOGA and to get that GA thrust in there… and then you got some trimming to do but at least then there will be no doubt about where you’re going... or supposed to be going.
During a dual channel app you are expected to let the AP do the landing, if a non-normal scenario presents it self the airplane will in a pretty obvious way suggest what it thinks you should do. During a single channel app this is not the case since you are expected to disconnect at the appropriate time.