KFC 150 Alt Hold INOP

[tomn]

The altitude hold on my KFC 150 went out yesterday.  It had been acting up a little the day before with some intermittent dropping out. Everything else on the AP works fine and the ALT light comes on when I press the button.  I can't get any movement on the trim wheel pushing the UP/DOWN rocker switch.  Powering the unit down and recycling makes no difference.  

 

Anyone have any suggestions or ideas of what the issue may be?

 

Tom

[John Pleisse]

Jammed trim servo or bad trim servo. Even if your altitude-can was bad, you'd be able to move your trim wheel. Careful....I'd get it fixed before you fly.

[carusoam]

Have you tried to clean the switch gear?

I believe that may be something to look into.

That's from an old fuzzy memory...

Best regards,

-a-

[BorealOne]

The diagnostics on the unit should make the AP inoperative in this situation. The fact that it isn't going offline would worry me. APs aren't to be trusted when they get wonky - get it fixed before you fly with it in IMC.

[tomn]

Guys, Sorry I did not make it clear - the up/down rocker switch on the AP would not move the trim but the yoke trim switch will.   I was able to manually trim the plane and fly no problem -  just a little aggravating through 3+ hours of flying.  Are there two motors here - one servo for the AP and one for the electric trim?  

 

Tom

[carusoam]

Trim for an M20 is probably different than for the F33...

The M20 is moving the whole tail via a jack screw and a giant hinge. Making the M20 unique in this way...

There are three motors that handle the control function of the similar KAP 150...

Ailerons, rudder, and elevator.

Elevator can be trimmed manually...

Elevator can be trimmed using the yoke mounted switch...

Climb angle can be held by another yoke mounted switch...

Climb rate can be set by pushing buttons on the AP itself. 100fpm per push (two modes ALT on and off)

Altitude can be held by pushing the ALT button

The system test when turned on beeps as it tests five? functions and can give a hint of what is not functioning properly...

Is it possible that when you push the alt button, it disables the ability to trim? It is busy holding altitude and does not allow interference...?

This is from old memories, the detail may not be correct...

Best regards,

-a-

[thinwing]

if like my 225...there are two servos horizontal attitude and one for roll...I believe you have a bad pitch servo ( could be rpm sensor or the global moter)these are about 1200 to 1800 rebuilt exchange from midwest...I had an intermittent roll servo that liked to roll me inverted while in imc...self test frequently wont reveal a bad servo..

[Bolter]

I think that with altitude hold on this model, the up-down trim is an altitude "nudge" function.  Moving the setpoint a small amount up or down.  The AP will continue to trim the plane as it wants it to be trimmed, once at the new setpoint.  If you turn off altitude hold, then the up-down becomes a regular trim adjustment and effectively allows setting a descent/climb rate.

 

-dan

[Piloto]

Very interesting Tomn. I have exactly the same problem with my Century 31, but only inflight. On the ground the A/P rocker switch will push/pull the elevator and trim wheel. But when inflight the A/P rocker switch does nothing. Check it on the ground and let us know. I sent the computer and pitch servo to Autopilot Central. They told me they could not find the problem (at least they were honest), but that there was a service bulleting that may fix it. Had the SB done and servo for $900. Got the unit back and still doing the same. I still looking into it.

 

José   

[tomn]

Thanks for the replies. It ended up being the pitch servo motor. It has shorted out and is not serviceable. We have one on the way and hopefully it will be back ready to go in a day or so. The KC 192 did not pick up the bad motor during the test mode and everything would come up fine - just would not auto trim or hold ALT even though the light would come and appear to be engaged. The elevator trim uses a separate motor on mine allowing me to trim in flight manually.

I bet the new AP's that are out now for experimentals fare better than these units. Maybe one day we'll actually have the option to install better safer hardware in our planes for a fraction of the cost.

Still miss my E by the way....

Tom

[Tommy]

Hi Tomn.

Resurrecting this topic as it seems like my 1981 J has exactly the same problem. Can I just ask:

Where is the separate motor for manual eletric trim?

How much time did it take to replace the trim servo?

Thanks!