C172 Classic Auto Pilot

I’ve had a few bad experiences with the AP on the steam gauge version. I suspect it has come from turning it on while on the ground but a few times it’s sent the airplane into a massive dive or stall just from turning it on.

The last time I gut to cruise altitude and speed. I hit the AP button and before anything else I could do I went into a steep dive. I deactivated and then was fighting a 100% nose down trim setting.

I had pulled back all the way on the elevators but then had a to work through the slow motion trim adjustment.

Then another time I was in altitude hold mode, I dialed in a new altitude and just pushed the Up button without going into VS mode. The Cessna thought at that point it could climb like a fighter jet and stalled.

I disengaged and was then left fighting a super high trim pitch that again I had to adjust in slow motion mode.

I briefly tried in the C172S and it didn’t seem like the AP was trying to kill me nearly as much.

I guess I need to watch some more videos, but is the real AP system like that?

I’ll try to answer a few things, but someone will come along and tell you there is a mod for those systems that works much better than the stock ones in the default 172. I’m sure that’s very true, I just don’t run mods on the version I use to test things.
But to answer a few things. No…the real ones didn’t/don’t work as badly as that. The FAA (or other countries’ equivalent) would have yanked the use of those systems if they did the stuff you’re seeing!
Here are a few things I have tested. If you are in ALT hold and just hit the “UP” button, it will initially try to climb at a way too high vertical speed. The way around it is to set up the new desired altitude, then set the VS and it will begin to climb to the desired altitude.
Also, you should not be engaging the AP while on the ground. You can set up the parameters you want (desired ALT, desired VS, nav mode, etc), but don’t actually engage the AP until after airborne. In real life, I suspect the manual for that system specifies a minimum altitude before the AP should be engaged (for example, the G1000 generally specifies a minimum of 1000’ AGL before engaging AP).
The AP shouldn’t put you in a major dive either, so I know that’s wrong, but I haven’t actually tested what’s causing that issue in the sim yet (sounds like the unit is initiating an incorrect default VS to some previously set/default altitude, but that’s untested by me so far).
Hope that helps a bit. You might look at the mod. You can search for that topic, and see if it sounds like something you want to try. If nothing else, those threads may have more info on the problems with the default system.


Thanks for the feedback. While on the ground I had turned the AP on, set parameters, and then turned it back off.

My set altitude was higher, so the dive was quite a shock!

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I’d have to look again, but I think when you turn the AP off, it resets some of the variables, specifically I think it resets your preset altidute to something else (field elevation??). But I’m out for the evening, so I’ll see if I can recheck soon.

Ok, the default system is truly and mightily messed up! :grinning:
You might want to just sit on the ground and watch what happens with the AP as you turn it on/off while on the ground.
I sat on the ground at KSAR (just a plain airport I test short stuff out of) which is about 540MSL. I set my heading bug to the runway heading, and set in a desired altitude of 3500’ MSL, with a VS of 500 FPM and verified trim was set right at takeoff. As soon as I turned the AP on, the trim started moving up on the indicator (nose down). By the time I got everything set, the trim had set itself to full nose down. After I turned the AP off, the trim stayed at full nose down…not a good thing! LOL
I could manually move the trim to T/O, and then when I was in the air I turned the AP back on, and watched the trim once again go full nose down.
There were several other really odd things that happened too, but basically that AP is just bad. I’d consider looking at the thread for the mod, and see if you’d rather use that (which by all accounts is much better).
Even the default C-172 G1000 version AP works much better than the steam gauge version of that plane.
So without being able to share a cockpit (hopefully someday!), it appears all or most of the issues are caused by the default AP, and not something you’re doing!

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I really appreciate it! Moments like that sometimes make me wonder if I have what it takes! The first time it stalled the airplane on me I was in clouds over the ocean. I lost sense of the horizon (in VR).

I used it a little more today. It tried to dive/stall on me a few times but I just went back to Alt hold right away. Otherwise I made it!

Sometimes clicking the buttons doesn’t update the display either. It’s kind of random with the inputs.

I updated the GNS530. I’ll look to do the same with the AP.

It’s a little disappointing since it’s a premium package airplane.

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I watch those air disaster shows and a few times I’ve heard pilots commenting the airplane “feels heavy”. Yet they still tried taking off.

As soon as I realized this thing feels heavy I managed to abort.

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Do you have a link to that thread? I can’t find anything related to a community mod to the AP system.

I’m sorry, I had in my mind the GNS 530/430 mod, rather than the KAP 140 system in that plane.
I’m not sure there is a mod for the KAP 140 (but I haven’t searched extensively).
I’ll look some more when I can, but my recommendation for now is, don’t turn on the AP while on the ground, make sure you have all the parameters set (desired altitude/VS/heading or NAV mode) before you engage the AP once airborne. Most of the issues you are having seem related to the AP driving the trim to extremes when it shouldn’t engage at all (i.e. when on the ground).
I’m sure the system shouldn’t drive the trim while on the ground like it’s doing, but not sure anyone has a fix for that.
I’ll do some more research later and get back!
PS. Might vote on this thread. https://forums.flightsimulator.com/t/vote-to-fix-of-the-kap-140-autopilot-of-the-cessna-172-steam-gauges/319245

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