Basic AP Pitch question

[Ka8 Flyer]

Hi folks,

I have a question regarding the pitch channel of modern, complex autopilots.
In FLCH / VNAV / Managed Climb / Open Climb with the MCP/FCU Alt selected being higher than the current altitude, the autopilot will try to climb at a selected Air-/mach speed.
What happens if
a) the target speed is too high and the aircraft's performance is not enough to reach that speed and climb at the same time. Will the AP "settle for less" and maintain a minimal VS and accept any speed? Or will it even consider descending (hope not) to reach that speed?

b) assuming one turns off the autothrottle/autothrust and pulls the throttle/thrust levers back to idle during a climb (very stupid, I know). Will the aircraft pitch up gradually and stall or in this case start descending? I guess Airbus will in the worst case switch to alpha protection and lower the nose by its own but does the AP have a similar function? And what about Boeing and other non FBW aircraft (let's not ignore the 777 ;) )?

[EMIT]

Remember the following relation between Autopilot and Autothrust:

A/P and A/T always WORK TOGETHER:

When changing altitude, A/T supplies a given amount of thrust, A/P controls pitch to maintain desired speed.

When maintaining altitude, A/P controls pitch to maintain altitude, A/T controls thrust to maintain desired speed.
(Acquisition of altitude, in this regard, is conceptually the same as maintaining altitude).

Vertical speed mode is a strange mode in this respect: A/P controls pitch to maintain desired rate of climb or descent, A/T controls thrust to maintain desired speed.
HOWEVER, if the rate of altitude change is chosen unwisely, speed control will suffer.
If the chosen rate of climb is too high for the amount of available (CLB) thrust, speed will drop below desired value, eventually all the way to the various low speed protection that may be built into the A/P A/T system.
If the chosen rate of descent is too great for the minimum amount of (idle) thrust, speed will increase above desired value, eventually all the way into the various overspeed warnings and/or protections that may be built into the A/P A/T system.
A 'gotcha' of this mode is the decrease of climb thrust with altitude: a rate of climb that can be maintained at 10.000 feet, may be too high a rate at 25.000 ft, so pilot attention is required when using V/S mode.

If you look at scenario's of A/P engaged, but thrust controlled manually and even (deliberately) mishandled, yes, than things may get out of control. If you want to figure out 'how much out of control?', then reason along the rules I have stated above.

By the way, (Boeing) VNAV or (Airbus) CLB work along the lines described above for altitude change, the only thing is that the FMC / FMGS will automatically change desired speed, according to the programmed rules (e.g. 250 kts below 10.000 ft).
In VNAV PATH descent, respectively managed DEScent, the A/P in principle controls pitch to follow the planned PATH, while A/T controls thrust to maintain speed. The difference with V/S mode lies in the fact that the guidance system will have set limits within which the speed is allowed to vary - if the speed gets too high, even at idle thrust, the pitch mode will revert to SPEED mode, controlling pitch to keep speed at a safe value. Of course, in a descent, thrust should never be insufficient to maintain speed.

Hope this makes matters clear.
Happy gliding. Do you really still fly a Ka-8? (beautiful aircraft it still is).

[Denti]

In the 737 you have to be careful during descent as the autopilot will quite happily fly you into the clacker (and overspeed protection if installed, its just an option) in VNAV descent if you put in wrong values (descent wind for example) or if you mishandle the thrust, as thrust is only in HLD. Auto-reversion to speed mode will only occur if you have that particular overspeed option on your plane and then only closely before reaching MMO/VMO.

And i do agree about the Ka-8, was my first single seat airplane :)

[Capt. Inop]

Auto-reversion to speed mode will only occur if you have that particular overspeed option on your plane

I guess they made that a customer option in the 10.7 update.

[Ka8 Flyer]

Hi folks,

thanks for the replies! Well, when I signed up here, that's when I last flew the Ka8. But she did leave an impression!

Jumping a few lightyears ahead with regard to technology, if I understand you correctly, the AP will not disengage (let's leave the FBW Airbus aside) if you leave the design performance parameters - instead it will happily stall the aircraft and trim the aircraft nose up if you are foolish enough to pull back the thrust levers in FLCH (probably fighting to maintain level flight at the onset of the stall). Interestingly, it is my understanding that the classic 737 had more or less an undocumented feature where in V/S mode you could not stall the aircraft. Strangely, this feature did not make it to the B757/767/744 later on (maybe the upgraded FCC's had this option?) so climbing in V/S mode was discouraged.

Will the alpha floor protection on the FBW airbus disengage the AP?

[Denti]

The classic 737 has on some models (depending on the options ordered and the MCP version as well as the flight control computer version) the feature that it will revert from VS into Level Change when the speed drops a certain amount below the MCP selected speed. That can be very annoying indeed if that limit is small and the reversion happens at only 5 to 10 kts below the selected speed.

However, that is not installed on all models, we had a quite mixed fleet over the years with around 15 different models (all 733 except one 735) and behaviour can be different between any of them. Quite interesting to see and try out what they do or dont do (wtf is it doing now?). On some of them you can indeed stall the airplane using VS, on others you can't. Best to be carefull using VS.