a) The basic rule is that the A/C must never do anything that is un-commanded. Pitch-up and pitch-down included.
b) The reversion is a reaction to what is effectively a failure of the DES mode (unable to compute), and that - just like on any other aircraft - must be fail-passive. I.e. ok-to-fail-but-need-to-stay.
In the very likely case when the DES is following a V-NAV geometric segment (60% of the time, my guess), or not flying exactly at the target speed (almost all of the other time), the reversion from DES to OPDES would either
- (1) set IDLE THR and dive the aircraft for the speed
- (2-i) increase pitch and the descent rate to accelerate towards the IAS target
- (2-ii) reduce pitch and the descent rate to slow down towards the IAS target.
Either of those would be a violation of the basic FBW design principles ("a" above) or certification predicaments ("b" above).
Or in simple layman's view: DES is V-NAV path (geometrical trajectory) mode. Once you invalidate that by pulling HDG, V/S is the next closest thing. OP DES is most definitely not.
[Dislclaimer: I enjoyed HDG+V-NAV on the 737.]
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I think that vilas had actually something else as his main point, this was just a side-quest.
It is that designers of the aircraft and of the procedures work in tight coordination to give us a carefully assembled product that is safe, proofed against human error as much as possible. And go seriously wide and deep to achieve that, embracing all the experience the fleet has accumulated in 25 years worldwide. Those who disregard manufacturer's advice given through OEM's FCOM SOPs are brave, bold and playing with fire. It can be done, but to do it right the resources needed are enormous and result never better than the original. More likeable? Sure, but not better.
Think of this one: The aircraft is born and released to the world with the following legal and binding documents
- Noise Certificate
- W&B Manual
- AFM
- ECAM
- QRH
- MMEL
Not the FCOM. FCOM is just a suggestion for operating it inside the limits set in the above certification package. If you re-create the FCOM, do you also re-create the MEL OPS-PROC section? If not, do the MMEL procedures still fit against the SOP's that you have? Similar for OEB's. Just an example.
It is human to think to know better, applies to chief-pilots and CAAs (!) as well. Some steer away from the desire, some fall for it, only few make it through. Hat's off to them, though the whole endeavour is of a questionable motivation.
my 3pc
Last edited by FlightDetent; 7th Sep 2018 at 12:22.