SkyMaster 337 trim Erratic

[lewblok 8]

I like this airplane, but when turning on the AP the trim just goes crazy.  Flying at 12,000 on an IFR flight in Wyoming, it suddenly starts to descend and the trim tab just goes crazy at 2,000 ft/m.  I had been flying for 2 hours and suddenly descends and try to adjust the trim. will not respond and trim wheel is rolled full down to the bottom.  Turn off AP and the the trim or yoke will not respond to my commands and it goes up and down 2000 up and 2000 down.  The pitch wheel on the AP is at bottom with nose up and still is acting crazy.  I have had this problem with this aircraft ever since I bought the software 2 years ago.  I also recently did an update on the 337 from Caranado.  Do I need to re-install the software or is there a fix?  Appreciate any help or directions to go to fix this.  Thanks

Lewis Block

[Bert Pieke 7,584]

I have never seen this, but before reinstalling the airplane...

Is it possible that you are losing power... try checking Airspeed first.. then Pitot heat for icing and Mixture setting...

If you are losing airspeed,  the autopilot may try to hold altitude while the airplane is losing speed and stalling with full trim up..

Are you are saying you have had this problem for two years?  And you have not figured out what is causing this to happen in all that time....

[hs118 707]

Agree with Bert. I have a lot of hours in this plane, a good bit of it over 10000' altitude. Never had a problem with the "old school" AP (besides it being "old school").

Could it be that you are not leaning the mixture out? And, thus, losing power?

 

Edited March 2, 2020 by yurei

[lewblok 8]

Thanks for the replies.  The mixture is set in the green and adjusted periodically.  Airspeed holding constant with no problem.  Just did not assume icing conditions on pitot tube in the flight sim?  I am a pilot and flew a Skylane 182.  This AP on the Carenado works fine most of the time.  Yesterday on my flight from Jackson Hole, Wy to KDEN I am cruising above the cloud layer on an IFR flight.  AP on HDG set ALT st at 17,000 and ATC will not let me lower my altitude as I have been flying for well over an hour and now past the higher terrain.  All of the sudden, the plane starts descending and I disconnect the AP and try to gain control adjust the trim and not getting response to my inputs.  So it makes me think that the AP is going bonkers and somehow messing up the control of the aircraft.  That is why I was asking if I need to uninstall and re-install the Carenado 337 to see if it would fix the glitch.  Never have a problem when flying other aircraft.  Just wondering.  Thanks for any help.

 

Lewis Block

[Bert Pieke 7,584]

By all means try to reinstall and see if that fixes it.. but I am pretty sure that the autopilot does not suddenly malfunction.. there is something going on in the background..

First of all, you may have to lean the mixture more than you would in a regular aircraft.. that is a Flightsim "feature"..

Or icing.. suggest you turn on Pitot Heat as a precaution..

Lastly.. suggest keeping an eye on trim settings as this happens... shades of the 737MAX..  runaway trim... 

Edited March 2, 2020 by Bert Pieke

[FDEdev 955]

What doesn't make any sense is that he's unable to trim and there's no response to yoke inputs once the AP disengages. 

[lewblok 8]

Thanks Bert.  When I turn on the AP, the trim wheel marker goes all the way to the bottom of the trim slot.  It just acts weird.  Had no idea icing would show itself on flight sim.  Will make that part of routine.  FDEev this action makes me think I have a glitch that starts interfering with the control inputs.  Will uninstall and re-load.  Appreciate the feedback.  Have a good day!

 

Lewis

[Bert Pieke 7,584]

3 hours ago, FDEdev said:

What doesn't make any sense is that he's unable to trim and there's no response to yoke inputs once the AP disengages. 

Once the trim is all the way at the extreme, you cannot recover with yoke input alone.

You have to first fix the runaway trim..

What causes the trim to run away, we have not yet identified in this instance..

[Bert Pieke 7,584]

1 hour ago, lewblok said:

Thanks Bert.  When I turn on the AP, the trim wheel marker goes all the way to the bottom of the trim slot.  It just acts weird.  Had no idea icing would show itself on flight sim.  Will make that part of routine.  FDEev this action makes me think I have a glitch that starts interfering with the control inputs.  Will uninstall and re-load.  Appreciate the feedback.  Have a good day!

 

Lewis

Could it be weight distribution in the airplane? 

[lewblok 8]

Bert, no problem with CG as only pilot and passenger.  I was going to delete and re-install but for the life of me cannot remember where I purchased the download so I have no serial number to enter to install.  So next time I make a flight with the 337 I will try to get a recording.  Boy I sure do like this aircraft.  Sure do thank you’ll for helping out.  
 

 

Lewis

[FDEdev 955]

7 hours ago, Bert Pieke said:

Once the trim is all the way at the extreme, you cannot recover with yoke input alone.

You have to first fix the runaway trim..

What causes the trim to run away, we have not yet identified in this instance..

Don't have the 337 installed so I can't check, but without a moveable horizontal stabilizer a 'runaway' trim doesn't make the aircraft unrecoverable/unflyable IRL.

You have still a lot of elevator power available since the full deflection angle and the stabilizer angle remain unchanged.

Some FIs let their pupils fly a complete traffic pattern from takeoff until landing with a fully mistrimmed aircraft. 

Due to the FSX/P3D unrealistic trim simulation, the elevator trim is usually too powerful.

Furthermore lewblok mentioned that with the AP off, neither the yoke nor trim is working. 

@lewblok, You reports aren't very clear. First you wrote that you are cruising with the AP engaged and suddenly the AP starts trimming nose down and in the above post you wrote that the AP starts trimming as soon as you engage the AP. These are two different issues.

 

Edited March 3, 2020 by FDEdev

[lewblok 8]

When I turn on the AP, the trim wheel marker rolls all the way to the bottom but the plane does not do a quick nose up.  I was wondering if the pitch wheel down on the console on the AP has a neutral position or if it should be neutralized someway before turning on AP.  Will try that and play with it.  I just did a flight from KAMA to KDAL and everything worked great.  When I downloaded the 337, I chose another 337 rather than the one I normally fly.  Trying everything to try to get to the problem.  Will get back after some more flights.  Thanks a Bunch.

[Bert Pieke 7,584]

Interesting.. I just did a flight test with my C337 and although I can stall the airplane at 12,000 feet by not leaning the engines, even with the trim all the way up, I can still disconnect the AP and fly the airplane.. so I have not experienced the loss of control that you described earlier..

Had not flown the C337 in a while... but neat little airplane

 

[FDEdev 955]

2 hours ago, lewblok said:

When I turn on the AP, the trim wheel marker rolls all the way to the bottom but the plane does not do a quick nose up

I was wondering if the pitch wheel down on the console on the AP has a neutral position or if it should be neutralized someway before turning on AP.  

That's very strange. Have you trimmed the 337 for level flight before turning on the AP? What's your IAS when engaging the AP?

The trim wheel does have a neutral position. When you load the aircraft it is at neutral but this has nothing to do with the AP.  The only important point is that you should only engage the AP when the aircraft is in trim and flies hands off with your current configuration/power setting.

[ThomasAH 61]

I guess it is not the problem here, but just to make sure (and it generally is a good idea):

The same bug affects the C337, so in short: Turn on carb heat using the H key (the real aircraft does not have or need carb heat, so there is no corresponding cockpit switch).

Another things to check, though the issue here seems different:

- Verify your axis assignments in the sim and (if you're using it) in FSUIPC or similar addons.

- Do you use any special cockpit hardware? (like an external AP panel)