Autopilot roll oscillations

[marooneypilot]

On a recent flight, my autopilot has started performing small roll oscillations. Here's what I know:

• KFC 200
• Yoke rotates about 5 degrees either direction, on about a 1-2 second period
• Happens in HDG mode and wings-level, haven't tried NAV or APCH
• Altitude mode and pitch-hold don't affect it
• Tried cycling the circuit breaker
• Happened on multiple flights

I've read that it's possible this is a problem with the tachometer feedback from the servo to the AP computer. Does that sound likely? Anyone else experienced this or have other hypotheses?

David

[201er]

Rocks you to sleep doesn't it?

[marooneypilot]

Yeah, right up til the point your passengers start to turn green

[jasona900]

1 hour ago, djm181 said:

On a recent flight, my autopilot has started performing small roll oscillations. Here's what I know:

• KFC 200
• Yoke rotates about 5 degrees either direction, on about a 1-2 second period
• Happens in HDG mode and wings-level, haven't tried NAV or APCH
• Altitude mode and pitch-hold don't affect it
• Tried cycling the circuit breaker
• Happened on multiple flights

I've read that it's possible this is a problem with the tachometer feedback from the servo to the AP computer. Does that sound likely? Anyone else experienced this or have other hypotheses?

David

Over the weekend, on a local flight, I was having an issue with my autopilot (Stec 55x) holding altitude.  It would slowly climb 20' and then decend 20'.  About 5-10 seconds per oscillation.  No issues with the roll.  Oddly enough, on the same flight I kept getting "Loss of Integrity" warnings on my GNS-480... when checking the status page, it would loose all satellites, and then regain them seconds, sometimes minutes later.  My iPad connected to a Stratus had no issues with GPS reception.

I normally turn my cellphone to airplane mode when I fly, but forgot to on this flight.  I wonder (hope) if there was some interference coming off my cellphone.    I also read elsewhere that a person had loss of integrity issues with his -480 and found the cause related to a bad rectifier in his alternator.   If it is the latter, might be worth checking for more clues with your other electrical devices and ensure they are operating normal to rule out any electrical supply issues.  That is what I will check on my next flight, as I suspect that my autopilot and GPS issues are related.

[thinwing]

2 hours ago, djm181 said:

On a recent flight, my autopilot has started performing small roll oscillations. Here's what I know:

• KFC 200
• Yoke rotates about 5 degrees either direction, on about a 1-2 second period
• Happens in HDG mode and wings-level, haven't tried NAV or APCH
• Altitude mode and pitch-hold don't affect it
• Tried cycling the circuit breaker
• Happened on multiple flights

I've read that it's possible this is a problem with the tachometer feedback from the servo to the AP computer. Does that sound likely? Anyone else experienced this or have other hypotheses?

David

Yes those were my exact symptoms on a pitch servo...bad tach sensor...it looks like a little moter 

[Cardinal767]

I had the same issue. I disconnected the connector at the servo in the wing and slid it on and off several times to make sure it had good continuity. Done, problem solved. When the pitch started doing the same thing a few months later, same fix. Only issue I have now is since I fixed the pitch trim servo. It trims one direction, a few seconds later, it trims the other way. It’s nonstop, not bad, but I worry about burning the servo motor out. Bob at Aerolab keeps replying that he will get back with me about the problem, but it’s been two months now, and counting.

Bob 

Edited April 11, 2018 by Cardinal767

[carusoam]

Quick review on servos...

These are a type of electric motor with a sensor system to know how far it has rotated... the controller gets confused when it loses proper sensing... could be age related, could be dirt related, could be both...

Generally, people report losing altitude hold related servos most often.

Do you know which servo is your roll servo?

Getting it pulled and OH’d isn’t too hard to do.  Getting it tested in the field, probably not so simple...

Autopilots Central is often a go to kinda place for this... might be worth a call with all that you know...

PP thoughts only, I haven’t serviced any aviation servos yet...

Best regards,

-a-

[marooneypilot]

So I tried the fix that @Cardinal767 suggested. PSA: The King servo is located in the starboard wing, one inspection panel outboard of the aileron pushrod. (I discovered this after looking all over the port-wing based on something else I'd read).

Everything in there is strapped down and connected so tight that I can't imagine there was any poor connectivity, but I gave it all a wiggle anyway. I haven't had a chance to fly since trying Cardinal's trick, but will report my results here when I do.

However I did have about a minute of perfect autopilot flight last weekend, so at least I know the problem is intermittent.

David

[M016576]

While you’re in there, might as well hit the terminals on the connectors with some contact cleaner.m, rather than just unplug/replug.

[flumag]

 

Hi David,

I had the same.

For me it was looking like that (see video).

Longer story short only a servo overhaul was fixing it. There was quite some experimenting around that had only cost me time and money.  I sent the servos finally to Redbird for the overhaul. This was not cheap but I haven't had any issues again since. (knock on wood)

Hendrik

Edited May 10, 2018 by flumag

[0TreeLemur]

I thought KFC was fried chicken?

[Hank]

2 hours ago, Fred_2O said:

I thought KFC was fried chicken?

Hey, you're right! Fred found the problem, chickens don't fly very well. 

Remove KFC (regardless if original recipe or extra krispy), and install Peking Duck (or is it "Beijing" Duck now?). Ducks are good flyers, maneuverable and fast, so you may pick up some extra speed, too.  

[Awful_Charlie]

Mine wasn't as rapid as Hendrik's, but equally as annoying. Martin at Straubing found the problem quickly (IIRC one or two hours of time including removal, fix, re-install & test flight) which was two resistors out of tolerance (so less than a dollar of parts)

[Piloto]

Roll oscillation is also caused by a loose clutch on the roll servo. On the ground engage the autopilot and rotate the heading bug to center the yoke. Try to rotate the yoke by hand. You should feel substantial resistance against rotating the yoke. If there is no substantial resistance the clutch on the roll servo needs to be readjusted or cleaned with contact cleaner. On most servos the clutch is exposed and subject to be contaminated with lubricant.

José

[jetdriven]

The clutch is frequently contaminated by corrosion-x.  Amazingly people still dont know you MUST bag and seal off the servo clutches when treating an airplane.

[Navionpaul]

Hi David, Did you find out what was causing the roll oscillations with your KFC200 2 years ago? I have that problem now with my Navion.  Thanks, Paul

[Bob Weber]

2 hours ago, Navionpaul said:

Hi David, Did you find out what was causing the roll oscillations with your KFC200 2 years ago? I have that problem now with my Navion.  Thanks, Paul

Paul I am very familiar with both the autopilot as well as the airframe, give a call if you would like 616 822 1999. 8am-8pm Michigan time.

Autopilot Consultant

webairconsulting.com

 

Edited April 9, 2020 by Bob Weber

[KB4]

Another MSer had pitch oscillations on his GFC500, lubricating controls from firewall to tail solved his his "pitch" oscillation.  May be worth a shot since all it takes is some Triflow, LPS3 and time, don't forget to get up and personal with the rudder pedals there are a few lube points hidden behind that center console thingy.  I wadded up some paper towel stuffed behind the heim and shot it with triflow then let sit for a minute to soak up drips, pull out paper towel--no mess, my controls never felt better.