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Is it the pitch servo?

Tommy

By Tommy
in General Mooney Talk

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Tommy Mentor

Tommy

Posted
Posted

Ok I did the test on my KFC 200 (1981 M20J) today on the ground. I also put two go pro camera to film the servos. 

 

The test method:

Pull the yoke to neutral, engage the autopilot with the alt mode off, use the pitch modifier to command UP, the FD bars show fly up and the yoke should move out, CWS, neutralize yoke, release CWS, pitch modifier down, FD down, yoke moves in, CWS, neutralize yoke, release CWS, ALT mode on, push yoke slowly ~3 second delay for trim to slowly start moving, pull yoke slowly, ~3 second delay for trim to move the other way. 

Observations

1) when test button is pressed - the trim warning doesn't flash at all.
2) the manual electric trim is working well - full range and reasonably fast (90seconds -clip 1). It disengage the A/P

3) The FD computer is commanding correctly
4) The pitch servo doesn't seem to be doing anything at all (the cable wheel doesn't seem to rotate). No matter what I do, it just sits stationary (see clip 2). The yoke doesn't move in or out when I engage FD/A/P (No alt mode) and then command pitch down / up using rocker switch. 
5) when I engage FD/Alt/AP and push in the yoke, the trim wheel starts to move up in 3 seconds as it should but nothing happens when I pull the yoke back. The trim warning then flashes + sounds.

My friend said he can hear the pitch servo clicks but no movement. I have cleaned the computer terminals and lub all the ball bearing. The cable tension seems ok. 

So is it conclusive that it is the pitch servo that's playing up? Before this I was getting frequently "trim warning" - can you get "trim" warning with a pitch servo problem or does it limit to trim servo only?

Thanks, guys!

Av8 Enthusiast

Av8

Posted
Posted

I had what sounds like possible similar issue - trim warning light, however my electric trim was not working at all.  Despite suspicions of problems in the tail -   issue was a loose wire in AP disconnect switch on the yoke - this was replaced and then all back in good/normal order.  

N201MKTurbo Grand Master

N201MKTurbo

Posted
Posted

I'm not familiar with the KFC200, but the Century system has a disconnect relay between the computer and the servo. That relay has caused me a lot of grief. The servo has two signals, one to engage the clutch. It is an on/off voltage. It makes a good click when it engages. The other is a bidirectional analog signal from the amplifier that drives the motor. They both go through the relay. When you engage the autopilot the clutch should engage and the analog signal should go positive and negative in relation to how far the computer needs to move the yoke. This can all be tested with a volt meter.

Just trying to keep you from an expensive overhaul if you don't need it.

Piloto Grand Master

Piloto

Posted
Posted

Impressive video. What was the noise in the background? I had a similar problem and it turn out the CWS switch wire chaffing in the yoke. It was chaffing only in-flight but not on the ground. A 10 cents ty-wrap fixed the problem.

 

José

Tommy Mentor

Tommy

Posted
  • Author
Posted

That's the engine running. And the clicks are from the cabin. The problem is pretty predicable and repeatable - so I doubt it's the chaffing of CWS that's doing it. The CWS is working perfectly as far as I can tell.

Two things that I don't quite understand were 1) why the auto trim works only in one direction but I was told that there is a sensor in the pitch servo that might be defective. 2) why the trim didn't flash when test was pressed - is it actually an indication of a fault in the trim system?

But I think it's quite convincing that the pitch servo is definitely not working at all...

MB65E Grand Master

MB65E

Posted
Posted

Might try re racking all the connections. Spray them down with DEOXIT (radio shack). I've had the AP disconnect Switch on the yoke jive me tons of trouble in the past...

Hopefully it comes online!

-Matt

  • 2 weeks later...
Tommy Mentor

Tommy

Posted
  • Author
Posted

So I emailed AP Central. Now they told me the pitch servo is KS177 (PN 065-0050-04)!! But my is a KS270! Unless someone stole and replaced my servo without me knowning?!

 

Can anyone confirm the P/N of the pitch servo of a M20J KFC200 system for me please?

 

Big Thanks!

Cruiser Grand Master

Cruiser

Posted
Posted

I believe the KFC200 uses

 

KS 272 Pitch Trim servo

KS 270 Pitch Servo

KS 271 Roll Servo

 

I would recheck with AP Central, I think they got it wrong. Please note there is a Pitch Trim and a Pitch servo. They are different.

 

The KS 177 probably for the KFC 150

Tommy Mentor

Tommy

Posted
  • Author
Posted

Yes, I went back to my pre-purchase inspeciton photo and it is clearly a KS270. PN/065-0027-02 Apparently there are 4 versions of this KS270. What's your guys P/N? Is it end in 02?

KSMooniac Grand Master

KSMooniac

Posted
Posted

 

Yes, I went back to my pre-purchase inspeciton photo and it is clearly a KS270. PN/065-0027-02 Apparently there are 4 versions of this KS270. What's your guys P/N? Is it end in 02?

 

 

That is the same that came out of my '81.  S/N 7977.  In my notes I recorded a replacement part number of 065-0059-02, but I don't recall the difference.  There are also KS 270A versions as well in the newer planes, but again I do not know the difference.  I believe the A version is backwards-compatible.

Bill Dunn Enthusiast

Bill Dunn

Posted
Posted

KS270 is the correct servo and there are different models of it (-00,01,02, etc.) I don't know who would do it in aussie land but someone should be able to rebuild it. they still make some of the parts.

Tommy Mentor

Tommy

Posted
  • Author
Posted

What's the difference between 01, 02, etc etc model?

Also can the torque be adjusted on individual unit? On my bracking plate there is a sticker that has the torque value of 23.

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