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For most commercial flight director, autopilot and auto-throttle systems (e.g. the A330-300, B737-800), the flight director/autopilot flies the altitude hold (ALT) or vertical speed (V/S) mode by closing the flight path/altitude loop with pitch control, while the auto-throttle maintains airspeed. It's only in flight level change (FLC/CLB/DES) or go-around (GA) mode that the autopilot controls airspeed via pitch control and the auto-throttle controls a thrust rating.

Closing the loop around airspeed via throttle and closing the loop around flight path via pitch control work fine when the aircraft is on the front-side of the thrust curve. However, this control strategy leads to instability when flying on the back-side. While this scenario is not likely to be encountered in normal flight operations, the best climb speed for maximum weight would be pretty close to the boundary (with some margin built in by the manufacturers).

How do the automatic flight controls handle flying on the back-side? Or do manufacturers ensure that the floor speed/AOA for system engagement always have margin to the minimum drag?

JZYL
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  • You can easily find yourself on the back side if you are cruising at or near service ceiling with not a lot of margin above Min Drag speed, and unwittingly fly through some smooth descending mountain wave, where the only indication is a very gentle pitch up as ALT mode corrects to hold the altitude, with speed also gently bleeding off. Don't catch it in time and you have no choice but to descend. Very intriguing question. The only autothrottle I've flown was an STC add on to a corporate CRJ, and it had a speed mode independent of a/p ALT hold, so it would react w a thrust increase on its own. – John K Apr 20 '20 at 00:55
  • Fly-by-wire comes most of the time with protections that try to avoid abnormal flight conditions. Yet, autopilots are designed to disconnect and give control back to pilot when not able to handle a situation. – Manu H Apr 20 '20 at 07:22
  • @ManuH That's part of my question: does it disconnect? Some A/T (like the A220, A3X0 varieties) have floor features that push thrust when AOA/airspeed below critical, but that's provided the A/T is engaged. As far as I know, A/P does not automatically disengage unless you have unusual attitudes or stick shaker. – JZYL Apr 20 '20 at 11:37
  • @JZYL my understanding is that protections are not part of Autopilot. They are here whatever the input source (pilot and/or autopilot) but in some cases autopilot disengages but there is no link with flight envelop protection. Note that the AF447 autopilot disengages when loosing reliable dynamic pressure reading (and also lose part of its flight envelop protection at the same time). So I'll guess autopilot will disengage in all situation it is not designed for. – Manu H Apr 20 '20 at 12:24
  • Just to double-check what you mean, would this accident be related to your question? Here the AP commanded more pitch with the thrust idle, and the AP was disconnected manually after the stickshaker according to the FDR plot (also this graph). –  Apr 20 '20 at 15:09
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    @ymb1 This accident is quite different from my question. This accident is a result of unannunciated sensor degradation and crew's failure to monitor airspeed. My question assumes all systems are functional. – JZYL Apr 20 '20 at 17:07

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