Quote:Machinbird
Ninety-nine point nine nine nine percent of your flight time or more has been in Normal Law. Just because your regular autotrim has been so sweet doesn't mean she does not have an ugly sister.
Originally Posted by Dani
Maybe, I still don't see where autotrim was to blame in AF447's case. It did autotrim what the PF ordered. Autotrim has no artificial intelligence to know what's in a pilot's mind. It stupidly does what it has been told: When you pull for a long time it trims this position to zero force. That's the definition of trim. What's wrong about this function on AF447? If only he would have pushed the stick as long as he pulled it, autotrim would have ordered THS back, this I'm pretty sure.
Hi Dani, the reason I called Alt2 autotrim an ugly sister is that it deceptively behaves exactly like in Normal law, but has no limits other than physical ones. Nothing more.
When a pilot is mentally confused about the practical meaning of Alternate law, as the PF of AF447 appears to have been, then it is more dangerous if the trim does not stop when commanded a particular direction until it reaches the physical limits.
Think about how often you have seen your trim approaching the physical limits in normal operation (assuming you feel the need to monitor it). Almost never, and definitely not in cruise no matter where your cg is.
I agree that the PF pulled the nose up into the stall using the elevator, and the THS only slightly helped the stall entry since it was perhaps about 1 degree (above cruise setting)during its run for the upper limits.
I have seen evidence in another accident report (DC-8) where the crew brought their roll control inputs to a higher priority than pitch control although they were testing the stall warning when they got into trouble and should have known that the source of their roll control problems was excessive AOA. When the shiny side starts to point down, you airline guys tend to give that a real high priority, don't you?
I believe that the PF for AF447 did do something similar. He over-controlled the roll and trusted Mother Airbus to handle the AOA while he sorted things out, forgetting that he was in a different Law where Mother Airbus had passed all responsibilities to him. He almost got away with it, except that when he finally began to get the roll and then pitch under control, the stall warning sounded and a fresh round of wing flailing began as the TOGA thrust (and excessive aft stick) sent the aircraft out of the envelope. The heavy buffet that must have resulted was another thing that must have increased crew disorientation.
PF's eventual adoption of full aft stick was an attempt to damp the bobbing of the nose of the now stalled aircraft (In my estimation.). By this point, he was in full panic mode because the controls were not responding properly. Logical thought was an impossibility.
The THS was quietly running nose up while the crew tried to make sense of this strange land and had the effect of trapping the aircraft in a deep stall. If it had not run nose up, it is not certain that a recovery would have been effected during one of the attempts by the crew of nose down stick, but by being trimmed so far nose up, it became a virtual barrier to recognition of how to escape the stall.
Originally Posted by AZR
So autotrim only slightly helped putting the aircraft into a stall, but then effectively became a barrier to stall escape.
All those reasons are tiny holes in the cheese, and of course are less important (IMO) than a crew performing the right action when confronted to a stall (warning). Nonetheless, clogging those tiny holes may perhaps save the day, one time. Then if there is no crucial disadvantage to do so, why not implement it?
AZR, looks like we are in agreement.
MM43, this should answer your question. Earlier speculation regarding possible disabling of pitch trim when all airspeed inputs are invalid is another additional possibility, but is not at all proven/demonstrated.