Originally Posted by
Rob21
IMHO, at the present level of automation, I don't see enough reasons for an autopilot disconnection. If the solution for UAS is so simple (pitch & power), why the autopilot "refused" to hold the same pitch & power when they lost airspeed indications?
Two answers from me, and I'll gladly stand corrected by the 'bus' experts.
- Most of the gains (and some of the logic) in the
basic control loops (even in attitude hold) are functions of IAS (or, more usually, Mach).
For a designer it's not easy or obvious to pick a 'default' value to drop back to, when those air data go belly-up (NCD).... otherwise, if you think about it, he wouldn't have introduced those variable gains in the first place.
- The solution for UAS is not simply "maintain pitch and power", and neither are the optimum pitch & power the same values just after take-off or at FL350.
What's more, the actual pitch & power at the moment of the UAS first occurring are not necessarily the ones you want to maintain (depending on the moment in the flight where it occurs).
Finally, the
autothrottle did leave the power at the last setting.
Whether it would have been preferable for the
autopilot to have reverted to a basic attitude hold is IMHO very much an open question, especially in turbulence.
It amazes me that a computer "logic" can not perform a partial autopilot disconnect, holding only attitude and power..... The computer still had enough data to keep the a/c flying, but it wasn't "programed" to do so...
I tried to explain above
why it wasn't "programmed" for that.
Question to all : has anybody already worked out from the report, what the AP (long and lat) and AT modes were at the moment of the disconnect?
Rob's remark above about 'partial disconnect' reminded me of some systems I've seen, where partial input data loss would disengage the affected 'higher' mode, but without disengaging the AP itself, which would revert to ATT HLD and/or HDG HLD.