Sudden pitch-up attitude in altitude hold mode

[FRØ]

  On 3/8/2013 at 9:43 AM, M252 said:

I have exactly same problem with a sudden unmotivated pitch up in my M20C/J with the KFC200 Bendix King Autopilot. 
I am trying to debug the problem - but no solution yet.

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[carusoam]

Welcome aboard Fro.

 

Something to figure out with the AP being turned on and off...

There are two controls we use in flight for attitude...

When hand flying, we are very aware of the status of both...

When the AP is flying... the pilot is less aware of what the AP has done... until it gets abruptly turned off... 

Take a look at the trim position when the AP is engaged...   see where it is when the AP is dis-engaged...

Or consider how the trim is used by the pilot compared to the AP...

Expect that the trim is set one way, while the AP is operating the elevator as needed... when the AP gets turned off, the plane tries to adjust to its trimmed position...

For an extreme case... test on a flight simulator... MSFS... demonstrates this wacky behavior really well...

When reviewing this thread.... there is much input for attitude control challenges for APs...

• Clean and lubricate controls is actually important...
• Trim Servo check...
• Attitude data from the AI...
• altitude info...
• Sources of static system errors... including Alt air...
• Some APs use their own tube to get Alt air... make sure it is still connected... some rubber hoses have gone bad...

PP thoughts only, not a mechanic or AP expert...

Best regards,

-a-

[skykrawler]

I think the cause of this pitch excursion when disconnecting the AP is the pitch servo torque sensor.   The servo produces  torque signals that return to the AP computer.  When the computer sees the torque high for an extended time it commands trim up or down to eliminate the torque reported by the pitch servo.   If the pitch servo torque signal indicating nose down elevator is missing or out of adjustment the AP will not attempt to trim and thus the stand-off of the pitch servo.  Disconnecting the AP releases the servo, the elevator jumps to neutral and the airplane pitches up.

One way to tell is from level flight trim for and establish a climb.   Engage the autopilot and altitude hold.  Monitor the trim wheel with you hand - it should trim nose does as the AP tries to level off.    If not, that is an indication.   In alt hold, you can pull the yoke with a few pound of force and the AP should try to trim nose down - monitor the wheel with your hand.

In the mean time, always check the trim after level off using the CWS button - hold the yoke firmly.  Keeps the passengers calm.