PMDG 747 Autopilot Problem

[Guest TobiasReinecke]

Good afternoon!Please excuse my english but I am from Germany.I have a problem with the PMDG 747 autopilot. During climb with 3000 fett/min and a speed of 300 knots the nose and the vertical speed suddenly goes up until the plane stalls.Maybe the autopilot tries to steer against this movement but is not able to do this. The pitch trim did not change since takeoff so I can not ascertain any reason!Thank you,Tobias

[realpilotsimpilot 11]

Tobias, What are your autopilot and autothrottle settings? In other words, what modes are engaged (VNAV, V/S, etc. and THR, FLCH, SPD, etc.,)? 300K and 3000 ft/min seems kind of steep (especially if its loaded), so that might be part of the problem.

[martybuzz 2]

Tobias, What are your autopilot and autothrottle settings? In other words, what modes are engaged (VNAV, V/S, etc. and THR, FLCH, SPD, etc.,)? 300K and 3000 ft/min seems kind of steep (especially if its loaded), so that might be part of the problem.

It sounds like you're climbing in Speed/Vertical Speed mode. What will happen is the autothrottle will try and maintain speed, but because you have V/S set the speed will decay as you get higher.Use straight VNAV, but if you haven't got a flight plan set and you're just going for a fly, use Fletch mode (FLCH)So, once you have rotated and are climbing out, set HDG and FLCH and dial in your speed. Your FMA on the PFD in relation to Vertical and Lateral navigation will then read "HDG" "FLCH SPD"Buzz

[Cras 6]

It sounds like you're climbing in Speed/Vertical Speed mode. What will happen is the autothrottle will try and maintain speed, but because you have V/S set the speed will decay as you get higher.

This is what I thought. When are you turning on Autopilot? I find that I do not turn it on until at least 1500 AGSL, that way I can make sure the plane has established a positive rate and is on a proper climb angle, then I let VNAV take over.Trying to climb at 300 kts and 3000ft/min right out of take off is highly irregular and I would advise against trying to reach that kind of speed below 10000 MSL