C172 climbs out of control in autopilot

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kbergphd
Airman
Posts: 19
Joined: 17 Aug 2018, 14:42

C172 climbs out of control in autopilot

Post by kbergphd »

I setup a flight plan from KSBP to KSNA at 9500 feet. I was climbing at 700 FPM and got to 7800 ft (autopilot on) and it pitched up sending the plane into a stall. This has happened before and can not figure out what is causing this. The Trim had gone to 100% nose up. I put it in the shop and it indicated "Elevator" in red which I clicked on and it went away and it still is happening, with no indication of any problem ("In excellent shape"). Any idea what might be wrong?

Also, I have posted a couple of messages before, and when I went to "View your posts" they were not there?

Thanks, Kent.

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Oracle427
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Joined: 02 Sep 2013, 19:30
Location: 3N6
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Re: C172 climbs out of control in autopilot

Post by Oracle427 »

You can't maintain a 700FPM climb at that altitude. The autopilot is trying to do what you have commanded and that is to stall the aircraft. You will need to reduce the climb rate as you climb higher so you don't exceed the capabilities of the aircraft. At that altitude, you'd probably barely maintain 500 FPM depending on loading.

700 FPM is reasonable at sea level. Naturally aspirated engines loss power as they climb rather quickly. You need to adjust mixture to maintain power and you need to adjust pitch to adjust for reduced excess thrust from the loss of power.

Generally speaking you lose 3% for every 1,000 feet of altitude you go up. Temperature plays a big part as the hotter it is the less dense the air is. This means the engine will produce less power as it will be the equivalent of being at a higher altitude. On a summer day on the east coast, high temps can add nearly 3,000 feet to your density altitude.
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

kbergphd
Airman
Posts: 19
Joined: 17 Aug 2018, 14:42

Re: C172 climbs out of control in autopilot

Post by kbergphd »

Thanks, Master Chief.

I am going back to Ground School. :oops: What a stupid mistake!

Kent.

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Oracle427
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Posts: 3916
Joined: 02 Sep 2013, 19:30
Location: 3N6
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Re: C172 climbs out of control in autopilot

Post by Oracle427 »

Never stop learning.

I was shooting some real world non precision VOR approaches just two nights ago.

This point is driven home at each level off. When using an AP, the airspeed comes off very fast at each level off and you need to mash that throttle in to avoid a stall.

One moment you are descending at 80 knots, the next the airspeed is winding down and it will probably go down to 65 and counting within 10 seconds if you don't catch it.
Flight Simmer since 1983. PP ASEL IR Tailwheel
N28021 1979 Super Viking 17-30A

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