phylosocopter, #1295
Re “code or standards around the state of display and alarms in case of AP”
There are standards in CS 25 etc and ACs, however many are interpretive and do not cover grandfather rights per se.
For auto flight, my old 146 had a trim light (amber, no chime) which basically indicated that the AP auto trim function was having difficulty I keeping up - why there may be small deviations, or in preparedness for possible disconnection (red / audio) at a limiting condition, parameter excursion and/or trim value - but not an extreme limit.
The flap trim compensation system was independent of the AP, and if this failed - not available, there was an annunciation of loss of function; manual trim required.
The 737 trim issue relates to use of the trim system by an ‘external’ add-on computation when the AP is not engaged. Thus, add-on systems (STS, MCAS) are conceptually independent, but the use of computation within the AP ‘box’ probably enabled a grandfather certification without additional annunciation.
Also, as some of previous threads have discussed, how might failure of the add-on systems be detected - single / dual sensing; or even a separate trim runaway. Significant problems for situation awareness.
STS uses air-data thus is under the ‘umbrella’ of Airspeed Disagree. MCAS is not, but some engineering views suggest that it was mistakenly considered that was because of it incorporation with the AP; again grandfather thinking - certification did not check the detail as should have been done for a new system.