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DoomBoom29

Plane drops out of sky when I disable Autopilot

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This happened the last 2 flights which were pretty long flights.  I'm not sure if this is a bug or icing?  I'm well above the clouds in freezing temps and have pitot heats on but no inertia or airframe on since I'm out of the clouds.  There is no visible Ice on the airframe. 

After a couple of hours of flight the autopilot starts wandering off the NAV path a bit as if NAV has disabled itself.  When I disable autopilot to get back on track the plane banks hard right and enters an unrecoverable spin and dives into the ground. 

I bought the plane last week so I assume its the most up to date.  Running X plane 11.3 beta.  Had no problems on shorter flights til now. 

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I use to have this problem on my first flights but I put it down to manoeuvring and airspeed.  Problem stopped when I kept speed under control.  Never knew the true cause but problem went away once I dropped power/airspeed especially during decent. 

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Check airframe/control surfaces in the maintenance manager. The TBM autopilot uses the trim tabs when flying. So if your control surfaces are not working properly, the autopilot will keep trimming until it can’t trim anymore. Hence the gradual deviation off the flight path/heading. When you disengage autopilot, it goes back to using control surfaces for control. And because they’re so far out of whack, you’ll go into an unrecoverable state. 

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This could also happen if you forget to switch on automatic  fuel tank switching. If the imbalance gets to big, you cańt control the airplane anymore...

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I used to notice this when the AutoPilot used to set extreme levels of trim, usually in pitch . Now before switching autopilot off I cast an eye on the trim indicators on the MFD

Edited by fireone

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On 1/8/2019 at 7:18 PM, wsando1 said:

I use to have this problem on my first flights but I put it down to manoeuvring and airspeed.  Problem stopped when I kept speed under control.  Never knew the true cause but problem went away once I dropped power/airspeed especially during decent. 

I've been doing this the last few flight and it hasn't happened again.  Before I was just keeping the throttle up and speeding down on decent.  I mean, I'm not over-speeding so I should be able to fly around 240kts on decent right?  It was way north and well below freezing though.  So maybe frozen pitot tube at those speeds (I had the pitot heats on)  I don't know? 

 

On 1/8/2019 at 10:13 PM, Goran_M said:

Check airframe/control surfaces in the maintenance manager. The TBM autopilot uses the trim tabs when flying. So if your control surfaces are not working properly, the autopilot will keep trimming until it can’t trim anymore. Hence the gradual deviation off the flight path/heading. When you disengage autopilot, it goes back to using control surfaces for control. And because they’re so far out of whack, you’ll go into an unrecoverable state. 

I had just repaired the Air frame prior to this flight because I added reverse thrust while standing still and the tail hit the ground breaking my tail trims and body.  It was quite fun after I took off because I didn't know and got cabin pressure warnings above 10K the flight just before.  Emergency landed and fixed it and I always check Maintenance components before starting up

 

13 hours ago, fireone said:

I used to notice this when the AutoPilot used to set extreme levels of trim, usually in pitch . Now before switching autopilot off I cast an eye on the trim indicators on the MFD

I've started paying attention to this now and it hasn't happened since.  Very likely as there was some extreme wind and I never re-set aileron and elevator trim before disabling autopilot.  Still doesn't explain wandering off Nav Autopilot though.

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Actually I just realized that I had global mean time to failure at 1000 hours set in x plane for all system failures.  I realized this because I had an engine seizure for absolutely no reason last flight and engine seizure showed in the failures tab.  I know MBTF failures for all systems settings can be pretty common even at 1000 hours.  This could totally be the reason. 

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