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Autopilot Altitude Tolerance


c1tice

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I have a KFC-200 on my 1981 M20K I purchased last September. It is my first plane with an autopilot. (Previously had a Tri-Pacer, Cherokee and 172) I have searched the web and I am curious of the expectation of altitude hold. Should I expect a 200 foot deviation? Flying yesterday in a lot of turbulence the AP would stray +-200 feet or better. Once it pitched down about 20 degrees and lost 500 feet. I was mildly surprised ATC never inquired as I was in IFR flight plan. I would engage CWS and reset the ALT button or just turn the AP off then back on, it would hold for a while then porpoise again.  I have appointment with shop this week but I am curious about this behavior.  The AP was just rebuilt 2 months ago to the tune of $3750. I shot the LNAV approach (VFR) same flight at my destination and it held the glidepath perfectly. 

3 other things I would like to hear experiences on-

1. The red trim light and sounder go off intermittently. In a 3 hour flight it lit/sounded maybe 12 times. I know this indicates a trim issue but a bit more detail would be helpful.

2. Twice I have had the trim lock up. The autopilot was engaged then disconnected with the trim switch. The wheel was stuck. Neither the switch nor the wheel would move. I thought I was going to break the wheel. I pulled the circuit breaker then reset it. I landed safely then on the ground the trim worked fine. I do not know what if anything I did corrected the problem. 

3. During start up with the master switch on and the avionics master off the son alert sounds, loudly. It is quite annoying. When the avionics master is turned on the son alert stops sounding. It is so loud you cannot hear the stall warning horn during preflight unless you turn on the avionics master to silence the son alert. Just curious if anyone else has this issue. 

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I have a K with KFC200 and the altitude hold keeps the selected altitude within 50 feet or better. From what you describe, I suspect you have a problem with your trim system. The jack screws are known to get stiff with age if not serviced, and eventually the electric trim motor will not be able to cut it. You should not hear the trim warning sonalert in flight unless the trim has failed, and with the electric trim off, you should be able to manually trim with very little effort on the trim wheel, so something is definitely wrong.

I would start with the maintenance log and see when it was last serviced.

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I should add the @carusoam caveat that I am not an AME / A&P or avionics technician, so take my comments as worthless until confirmed by one of those. There are several really good ones on this forum so expect to hear from one soon.

The Carusoam Caveat. Did I just coin a new phrase?

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If its only an issue in Turbulence I wouldn't worry. I woud never use it turbulent air, the pilot can do a far better job than the AP. If it must be used in turblent air, then it should be used only in Pitch mode rather than altitude hold since the AP will stress the wings and tail far more than necessary. 

But mine will hold altitude within 20' in reasonably smooth air. 

Trim lock ups are a serious problem with an easy fix - see SB20-325 and SIM20-88A in the tech pubs section on www.Mooney.com

The parts for the SI are not expensive nor is the labor to install it, this issue totalled a K model during landing not that long ago discussed here on MS too.

On your #3, make sure the elevator trim rocker CB is always on before turning on master and avionics and that should fix the sonalert going on.

Edited by kortopates
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Great questions C1!

1) You have identified that your AP is experiencing some challenges...

2) You have documented things that aren’t working correctly...

3) You have initiated a great problem solving step...

 

Now...

If the trim isn’t working the way you expect... 

The KFC probably isn’t smart enough to know how to adjust to that either...

 

Fortunately...

The trim system is mostly visible, where cleaning and regreasing can work wonders... common challenge around here...

While on the ground, The trim system can be tested from full up to full down, while cleaning and regreasing... if there is any stiffness at any point in its full travel... they can be cleaned up.

 

That’s probably the lowest cost, quickest fix, most obvious thing to look into for a PP...

After that, checking servos starts to get more challenging and more costly.... See how many hours are on each servo since their last maintenance... 

We have a really good AP guy around here as well...   well worth the money once you get past the clean and Lubricate phase....

 

Overall... Alt hold is exactly that... it holds altitude better than the regulations require for IFR flight.... under ordinary weather conditions... which includes a fair amount of turbulence...

IOW... an old BK Alt hold... can hold altitude better than I can...

+1 for the various carusoam caveats...

  • Not a mechanic
  • Not a cfi
  • Not an avionics technician

+2 for the guys around here that are CFIs, mechanics, and avionics technicians...

:)
 

Best regards,

-a-

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The KFC 200 in my 1978 M20J had erratic altitude holding and I found water in the autopilot pressure sensor. I shook it out and dried it and it would hold within 10 feet afterward in smooth air.

One day in the summer at 11,500' over the Arizona desert in moderate thermal turbulence it was only holding within +/- 100'. I turned it off and hand flew and, working really hard, the best I could do was +/- 50'. I turned it back on.

With the power off on the ground, the trim should manually move smoothly from stop to stop. If it is hard to turn, the problem should be investigated. 

The trim locking issue that Paul noted is caused when the autopilot trim drives the trim system hard to either up or down stop which can cause a jam on older systems that have not been modified with stepped stop nuts.

The trim servo has a clutch that you should be able to overpower. You can test that by turning on the autopilot and manually moving the trim wheel. There should be more than the normal amount of resistance, but you should be able to move it without undue effort. This should cause the autopilot to sense uncommanded trim in motion and turn itself off and you should feel the clutch release and the trim wheel rotate more freely.

Skip

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In smooth air alt hold should be within 10-20' from target.  Depending on the severity of the turbulence it will increase but it should never go into continuous oscillations.  Continuous oscillations are typically related to servo or trim issues, sometimes they can be related to the attitude source as well but are more prone to oscillate in smooth air.  I checked the flight line book and the maintenance manual and neither give a reference for tolerance only the specs for the output error of 40mvDC/foot from the transducer.

For the trim issues, the slip clutches are designed so they can be overpowered in a runaway condition so you can regain control of the aircraft and pull the breaker/remove power from the trim system.  As others have mentioned, even with the system engaged you should never feel like you are going to break something in the attempt to override the trim.  Any additional load or friction in the trim system has the potential to put the trim solenoid in a bind.  With the solenoid in a bind even removing power from the servo will not allow it to properly release. 

There is even a test in the maintenance manual for the servo that requires you to put a load on the capstan at a preset torque limit, run the servo so the clutch will slip and then remove power from the solenoid.  There should be a instant release with the solenoid and smooth operation by hand. If the solenoid does not instantly release, you adjust the alignment of the solenoid. I would suggest doing an thorough trim test/inspection of the system and replace/lubricate any parts that need attention.  NOTE: do not spray any lubricant on the servo capstan or the slip clutch assembly.

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Mine is dead on. Oh, there might be a 10 or 20 foot deviation, but so small I don't notice it. I have had it disconnect once or twice in turbulence, but that is once or twice in about 1,200 hours. I am getting minor oscillations in cruise, probably time to have it looked at. I would guess you have a lube problem.

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Recently learned that your servos need to be pulled and cleaned every few years... 

I have a K with a KFC-200 as well and the altitude is usually right on.  But I had some pitch oscillation after the avionics upgrade and before tweaking the avionics the avionics guy first pulled the pitch servo, which was in bad shape.  Luckily it was salvageable, but he said in some cases they just get caked with carbon dust and other contaminates and they wear themselves to pieces. 

So before making all sorts of tweaks to your controller or the avionics.  You might check with your avionics or mechanic and see what they think about your servos.  And apparently getting motors and other parts is becoming VERY tight for our KFC-200 (and similar).

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Thanks for all the replies. It is going in the shop today and I will report back what they find.

Kortopates  I can't find SIM20-88A, I found the other document and am bringing it to the shop. 

 

 

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