Originally Posted by
CONF iture
Now, if it's normal by conception to autotrim under stall warning, it is aberration.
This is a serious question, I promise I'm not being facetious here... Why do you consider it an aberration?
Would you not use manual trim in a conventional aircraft if elevator authority was not enough to help you escape the stall, or indeed if the elevators jammed? Autotrim is simply a way of providing trim function through the sidestick, and in the Alternate Laws the pilot has full trim authority through the sidestick. Alternate 2 removes some or all of the soft protections (soft meaning that sidestick input can override them, so in this case there would have been negligible difference had Alternate 1 latched, because the PF did not let go of the stick throughout the sequence).
The very reason for the existence of Alternate 2 is because with certain sets of failures, the FCU is programmed to recognise that it cannot manage the flight, and so full authority is given to the pilot. This is entirely in keeping with the Airbus engineering philosophy, which acknowledges that the last line of defence is the pilot, but could potentially cause problems in an operational sense if the airline concerned has used the existence of the FCU protections in approximately 99.9 (rec) of flight time to under-train pilots in manual handling.
In terms of systems architecture, Stall Warning is part of the notification subsystem and is not connected to the FCU protections in any way. The purpose of the notification subsystem is to tell the pilot, who in Alternate Law has full authority, if the aircraft is in a non-normal/dangerous state. The design relies on the pilot to heed that warning and take appropriate corrective action, as is the case in any other airliner. The notification and protection systems are complementary rather than co-operative or consequential - if the protection systems are functioning then the severe notification logic (e.g. overspeed warning, stall warning) should sound either briefly while the protections compensate, or not at all.
One of the fundamental misunderstood perceptions of the Airbus (and I suspect Boeing) FBW systems is that they are seen as a monolithic system in which everything is tied together - they are not. FMS/AP is distinct from FCU is distinct from reporting systems - they interoperate, but they do not form a cohesive whole from an engineering standpoint - nor should they lest they risk creating single points of failure. The protections are part of the FCU architecture, and the reporting systems are part of the avionics.
The design of the FCU states that absent specific parameters (in this case ADR data), then the pilot should have full authority - the safe limits will be notified via the reporting subsystem to which the pilot is expected to respond in a timely and correct manner, just as has always been the case before the days of FBW and the FCU system.
I don't see anything inherently wrong in retaining autotrim in Alternate Law, but the ramifications of retaining it must be trained for and understood.