KFC 225 Altitude hold instability in turns

[FlyingScot]

Hi all - happy Oshkosh week to all. I sure wish I could be there . 

I have a KFC 225 AP driven by a Garmin 275. I kept my encoding altimeter, though, when I had the panel updated, and I believe the auto pilot is still getting its altitude information there. 

Recently, I noticed the AP is having a bit of trouble holding altitude in turns. Captures fine, and in straight and level or climbs/descents, it works great. Rock stable. However when I command it to turn, either in NAV mode or HDG mode, it starts to oscillate slowly around the hold altitude. Typically loses a bit of altitude then over shoots in the climb, then descends more vigorously. Never uncontrolled, but unnerving. 

Anyone have a similar experience, or a suggestion? Could just be the pitch trim servo misbehaving?

Thanks in advance!  – Bob

 

[Jake@BevanAviation]

If the AI was replaced with a GI275 there is a alignment procedure that needs to be done.  If the alignment is not done the system could be over responsive in corrections resulting from gyro gain out of alignment spec.  Any shop that has the diagnostic cable and flight line/ install manual should be able to check the alignment.  There is a rack extender to gain access to the adjustment pots on the side of unit.  If the AI gains are within spec, check the pitch bridle cable tension and the pitch servo start up voltage.

[carusoam]

@FlyingScot message came in for you from Jake above…

Best regards,

-a-

[FlyingScot]

13 minutes ago, Jake@BevanAviation said:

If the AI was replaced with a GI275 there is a alignment procedure that needs to be done.  If the alignment is not done the system could be over responsive in corrections resulting from gyro gain out of alignment spec.  Any shop that has the diagnostic cable and flight line/ install manual should be able to check the alignment.  There is a rack extender to gain access to the adjustment pots on the side of unit.  If the AI gains are within spec, check the pitch bridle cable tension and the pitch servo start up voltage.

Thank you - when the 275 install was done the calibration you recommend was done - and the AP has been working great since then. The altitude hold issue has recently emerged. Does this point to the pitch bridle cable tension or the pitch servo start up voltage - and if so is this common that they should change?

thank you for the assist - Bob

[FlyingScot]

3 minutes ago, carusoam said:

@FlyingScot message came in for you from Jake above…

Best regards,

-a-

Thanks!

[Jake@BevanAviation]

@FlyingScot I would start checking the bridles and start up voltage if it was me.  They can also check the strain gauge output in the diagnostic menu as it is known to stick over time and cause weird oscillation issues.  If you are having any type of out of trim conditions on disconnect I would look really hard at the strain gauge in the pitch servo.  Years ago when we could service the servos in the field before the dealer agreement changed, the most common issue I saw was servo start-up and strain gauge failures.

[FlyingScot]

27 minutes ago, Jake@BevanAviation said:

@FlyingScot I would start checking the bridles and start up voltage if it was me.  They can also check the strain gauge output in the diagnostic menu as it is known to stick over time and cause weird oscillation issues.  If you are having any type of out of trim conditions on disconnect I would look really hard at the strain gauge in the pitch servo.  Years ago when we could service the servos in the field before the dealer agreement changed, the most common issue I saw was servo start-up and strain gauge failures.

Great help - thank you. When I get an answer, I will post - I appreciate the detailed info very much - Bob