KFC 200 "Wandering" off and back on course

[Ksaunders]

When using my KFC 200 Auto-Pilot recently, I notice when in GPS mode (Not while in Heading Mode) that my bird is drifting to the left then back to the right (center) over and over again.  It's enough to rock you to sleep.  Very Annoying.  I can definitely feel the aircraft slowly swaying back and forth.  By monitoring my Attitude Indicator it is confirming that the auto-pilot is wandering just off the "center" indicator to the left then the plane is correcting and wandering back to the right then centering back up again on my AI.  It may be a coincidental but it was happening most frequently when both my wing tanks were completely full.  It also seemed that as I burned off fuel and turned the auto-pilot off and re-engaged - the wandering issue decreased.

 

Not happening when I switch her over to Heading Mode and fly with my bug.          

 

I'm "Wandering" if anyone else has experienced this -  (?) - any ideas, suggestions, thoughts?   

 

 

[Bill Dunn]

What kind of GPSS? And GPS? It could be something as simple as a gyro gain adjustment or a GPSS gain adjustment.

I have a KFC 200 in the shop I'm working on right now.

[Piloto]

Sounds like a connection resistance into the A/P NAV input. If the CDI or HSI shows the deviation but the A/P fails to correct at small indicated deviations it is likely due to not getting enough deviation voltage. Try cleaning with contact cleaner the A/P connectors.

 

José 

[Ksaunders]

"DAC GPS Steering"

Garmin GNS 530 WAAS

King KCS 55A - HSI

 

Not sure if that answers your question. 

 

What would be involved with a gyro gain adjustment or a GPSS gain adjustment?  This stuff is pretty foreign to me. 

[Piloto]

If the A/P holds the course in VOR/ILS mode (GPSS not enabled) then your A/P is good.

 

José

[philiplane]

Connectors rarely cause the problems described. A known KFC connector snafu occurs when a windshield leak makes its way onto the connector on the back of the ADI or HSI. Corrosion in the contacts can cause communications failure with the nav source (GPS or NAV) but this makes it completely inoperative, It also has nothing to do with what mode (GPS, ILS, or GPSS) the autopilot is in. The most likely cause is worn or tarnished servos, requiring high start-up voltage at the roll servo. Servos are designed to start moving with as little as 1.5 volts. As they age, the brushes develop a coating, and the copper on the rotor tarnishes. Now the autopilot computer has to feed increasing voltage to the servo, to make it start moving. Of course, once it gets 5-6 volts, it wakes up and moves quickly. Too quickly. So it oversteers, and then has to be corrected back. This manifests itself as a roll oscillation on a course, especially on a localizer. The plane saws back and forth rather than making gentle corrections. The cure is a repair of the roll servo, followed by a flight adjustment of the heading centering adjustment.

 

Secondary causes are loose attaching screws from the servo motor to the mount, and extremely loose bridle cables.

[Bob - S50]

How does the ball look on your turn and slip?  Is it possible that your plane shows you need a little right rudder while flying (step on the ball)?  Could it be that a slightly out of trim rudder is slowly pulling you to the left and that your autopilot does not make a correction until far enough off course that you notice the changes?  If you think that might be the problem, try holding just a bit of right rudder while on autopilot and see if the problem goes away.

 

In the DC9, if I noticed the autopilot flying in a bank to maintain a heading or course, I would apply rudder trim until it flew wings level.  I would then adjust aileron trim until the yoke was level.

 

In the Mooney, we can't adjust either of those in flight.  However, if I'm not mistaken, there is a small tab on the rudder that your A&P can literally bend a little to make the adjustment.  You then go out and fly it to see if it was bent the correct amount. Of course, the setting you end up with will only be correct for one airspeed so be sure to pick the cruise speed you use the most.

 

Just a thought.

 

Bob