Originally Posted by
Takwis
I think he means "stick free stability", as in you could let go of the stick, and the plane would be stable. I think.
and:
That's a pretty clear confirmation of what we are talking about.
From Lemme article
From AC25-7D:
Probability of artificial stall warning and stall identification systems operating inadvertently: The probability of inadvertent operation of artificial stall warning systems, during critical phases of flight, should not be greater than 10-5 per flight hour. To ensure that inadvertent operation of the stall identification system does not jeopardize safe flight, and to maintain crew confidence in the system, it should be shown that:
- No single failure will result in inadvertent operation of the stall identification system; and
- The probability of inadvertent operation from all causes is improbable (not greater than 10-5 per flight hour).
A single failure of an AoA vane causes both MCAS to trigger and Stall Warning to trigger "inadvertently", but it does not cause Speed Trim to trigger (it is based on airspeed). Speed Trim is still single-thread on airspeed malfunction, and from hardware output.
Boeing has previously denied MCAS to be a part of stall identification.
The JATR report was justifiably concerned on the need to protect from a single-point malfunction.
From the JATR Report:
Recommendation R3.7: The FAA should review how compliance was shown for the stall identification system on the B737 MAX with respect to inadvertent operation due to single failures.
Finding F3.7-A: The JATR team considers that system features on the B737 MAX might constitute a stall identification system. This system is vulnerable to inadvertent actuation due to a single failure, which would not meet the accepted guidance contained within AC 25-7C, Chapter 8, Section 228.
Bold added by me.
Am I speculating too much if I say that Boeing denies that MCAS is a stall identification system because otherwise it would have not been approved? A stall identification system cannot be triggered by a single failure, which is the case of MCAS?
Furthermore still from Lemme article
From AC25-7D:
A means to quickly deactivate the stall identification system should be provided and be available to both pilots.
It should be effective at all times and should be capable of preventing the system from making any input to the longitudinal control system.
It should also be capable of canceling any input that has already been applied, from either normal operation or from a failure condition.
MCAS does not fulfill none of the above....