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Cessna 550 - disengage autopilot - disaster :-)

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Cessna 550 - disengage autopilot - disaster :-)

Postby fredsie » Sun Nov 18, 2018 4:57 pm

I've been fairly successful (by my standards!) in setting the autopilot on this plane to get it to do what I want it to. Nothing clever - just to fly straight and level for long flights. I set an altitude hold. plus a heading locked onto the heading bug. and control power manually.

As I say, this all seems fine. But I don't want to use the autopilot for approach/landing, I want to fly this manually. The problem is I can't find a way to disengage the autopilot without totally losing control of the plane. On disengage, the plane enters a steep roll and dramatic pitching. Sometimes I can recover from this but I then find that ailerons or rudder - or both - are no longer centred on my joystick, making it impossible to continue the flight.

Is this expected behaviour? Is there any way I can achieve this successfully?

Thanks
Fred
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Re: Cessna 550 - disengage autopilot - disaster :-)

Postby stuart » Sun Nov 18, 2018 5:13 pm

Hi Fred,

It's possible that the autopilot is having to use significant elevator control and the plane is flying significantly "out of trim". So when you switch off the autopilot, the ailerons/elevators immediately got to 0 (or whatever position you're joystick is in).

I'd suggest using the HUD to see how the autopilot is changing the ailerons, elevators and trim settings. That might give you some insight.

-Stuart
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Re: Cessna 550 - disengage autopilot - disaster :-)

Postby wkitty42 » Sun Nov 18, 2018 8:20 pm

if you have the autopilot on, maybe make sure you hit the 5 on the number pad to recenter the controls before you disable the AP... it sounds like the controls are off a little bit when you turn it off...

eg: in the UFO, with the AP on, if you try to turn left (4) or right (6), the control is immediately recentered... if you try to increase (2) or decrease (8) altitude, the controls are not recentered but the craft does compensate for them... if you turn off the AP at this point, the controls are not centered and the trims are way out... thus i learned to hit the keypad 5 before turning off the AP just to make sure my controls are centered...
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Re: Cessna 550 - disengage autopilot - disaster :-)

Postby fredsie » Sun Nov 18, 2018 9:20 pm

Thanks both.

Yes, your diagnosis is probably correct. it's really a limitation of simulators compared with real planes I guess, in which the autopilot movement of the control surfaces is reflected in movement of the yoke and pedals. I didn't know there was a way to get the sim's control positions to reset to null. Is this 5 with Num lock on? I normally have that off to use the "shift view" function of the keypad.
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Re: Cessna 550 - disengage autopilot - disaster :-)

Postby fredsie » Mon Nov 19, 2018 9:02 am

Yep, tested this through an external view. The control surfaces are centred with 5 whether num lock is on or not.

Thanks both!
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Re: Cessna 550 - disengage autopilot - disaster :-)

Postby fredsie » Sat Jan 05, 2019 11:54 am

Thought for the record I'd add what I've learned with more experience.

I'd been setting heading and altitude as explained previously in the AP dialogue box. What I hadn't mentioned, or noticed before, is that by default the Flight Director is set to ON, and I'd been ignoring that. I now make my settings butat the same time turn FD off. Since then, no problems.

I guess that previously, when I disengaged bearing and alt with FD still on, the plane switched to some random existing FD settings and caused my original problem.
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Re: Cessna 550 - disengage autopilot - disaster :-)

Postby legoboyvdlp » Sat Jan 05, 2019 11:57 am

The FD should not be controlling the plane - it's simply a guide that shows up on the primary flight displays to show the crew what commands to input on the controls to fly what they input on the MCP / FCU. That sounds like a bug in the autopilot?
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Re: Cessna 550 - disengage autopilot - disaster :-)

Postby fredsie » Mon Jan 07, 2019 4:41 pm

Oh right, probably is a bug then as keeping it off resolves my original problem.
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