PC Wing Leveler help needed

[MarquezJC]

Looking for some help with the PC system in my  66’ F model with the PC leveler and Accutrak II AP.  Everything works flawlessly when following a course with the AP engaged.  But if only using the PC system, I get a constant left bank tendency.  I can cancel it out mostly with roll trim but it’s pretty much maxed out with right trim to correct the left bank. 
im reading through past posts but haven’t found really where to start with this specific issue. 
Anyone have any advice on where to start looking at this?  Confusing as I thought it would affect both systems. Thanks. 

[outermarker]

Does it bank with more than you in the airplane? When you taxi, does the yoke move opposite in a turn as if to correct for the turn?

[MarquezJC]

It banks the same whether I’m by myself or with someone else.  Haven’t tried to check during taxi.  The PC button is usually velcroed down until I need it.  

[takair]

Check it on the ground. If it turns then you likely have a leak. Could be the servos, lines or trim lines. Need to start leak checking. Servo would be the first place to start and most likely leak. 

[McMooney]

you have a leak,  find the leaky seal/boot/hose 

[Hank]

My C developed a constant right bank. While fearing it was an aileron servo boot, which are very tight and difficult to reach, it was actually a torn boot on a rudder servo.

[Echo]

13 hours ago, takair said:

Check it on the ground. If it turns then you likely have a leak. Could be the servos, lines or trim lines. Need to start leak checking. Servo would be the first place to start and most likely leak. 

Can you explain this comment in detail please?  My understanding was that in a right turn on the ground the yoke SHOULD turn left and vice versa.  Are you saying this is not correct?

[Hank]

2 minutes ago, Echo said:

Can you explain this comment in detail please?  My understanding was that in a right turn on the ground the yoke SHOULD turn left and vice versa.  Are you saying this is not correct?

That is how it should work when taxiing.

[Kevin Westbrook]

Marquez this is Kevin Westbrook I worked at Brittain Ind 40 +years please call me I can help you with the PC could be leak or rate gyro is off center! 539-292-0474

[Kevin Westbrook]

Marquez sorry Kevin again I think it's rate gyro because if servo leak it would be there when tracking all so?

[Igor_U]

1 hour ago, Kevin Westbrook said:

Marquez sorry Kevin again I think it's rate gyro because if servo leak it would be there when tracking all so?

Kevin,

Interesting. I am familiar with the leak but not the gyro issue. Would you please elaborate what is typical reason and solution? OH of the gyro, or adjustment?

Thank you.

[jeremyc209]

Anyone have a picture of what the tail section vacuum lines are "supposed" to look like with the PC (rear turn coordinator 1965) + Accutrak installed? Mine came disabled with two aileron servos gone and the lines in the tail cone are all different types. I'm about ready to reinstall the servos and replace all the lines but I'm curious what baseline I should be shooting for.

What do you guys use for vacuum tee's back there? Clamps? 

[takair]

10 hours ago, Echo said:

Can you explain this comment in detail please?  My understanding was that in a right turn on the ground the yoke SHOULD turn left and vice versa.  Are you saying this is not correct?

Sorry. Incomplete response. You are correct regarding ground function when taxing. What I meant is that if sitting on the ground with the engine running, but not moving, if the yoke goes hard over it could be a leak. I’ve knocked off a trim hose and it will also cause a hard over. You can compensate for that with the heading bug, but it will be offset. Kevin is probably right regarding gyro, but still worth leak checks. 

[Cfidave]

I have a similar problem on my 1966 M20C. It has a constant left bank turn, even with the trim knob all the way to the right.

Ground testing works as it should, opposite the turn. Trim knob works the way it should to the left, but is very slow and weak to the right.

I am in the process of checking for leaks, but it is going to be tough.

 

 

[outermarker]

During a taxi, as you make a turn, the yoke should move in the opposite direction (it's trying to correct). If the yoke doesn't move, or if the yoke moves in the same direction, you have a leak. The rudder servos are the easiest to check. The aileron servos are a bear to get a wrench on... unless you do what Don Maxwell does, which is to drill a small hole in the aileron where the servo attaches, just large enough to get the socket through to secure the servo. 

I took out all the servos and inspected for pinholes or bad tape. This is easy to do. put a hose on the end of the servo. Put the servo in water and apply a slight positive pressure. This could be done by blowing into the tube. This will expand the rubber boot to expose all leaks. Fixing pinholes or small tears are covered in a Brittian guide. Realize that when the servo is put back into service, it is operating under vacuum, meaning that the rubber will be against the metal can, which can hide small leaks. That is why I tested with positive pressure under water. Using the proper tape is important too. 

[jeremyc209]

I used a little ~4" socket wrench with a rubber/square handle and it only took a few minutes to deal with the aileron servos.

You can also pull the aileron lines at the turn coordinator (no remote gyro in the '66?) and test vacuum there for leaks.

[DCarlton]

6 hours ago, jeremyc209 said:

Mine was tested for leaks by removing the vacuum lines at the turn coordinator.  Once the side with the leak was isolated (red or green line), the leak was further isolated to the rudder or aileron by pulling a vacuum in the tail at the rudder servo.  My leak turned out to be a rudder servo.  You may be able to find the leek without removing anything.  An inexpensive vacuum pump with a gage from harbor freight worked fine if you get a good seal when you attach it to the hose.  

[Cfidave]

Thanks guys, very helpful. My turn coordinator is remote mounted under the avionics bay cover. It appears the tubes are all connected to the bottom of the TC.

 I will have a look today.

 

[eman1200]

On 5/13/2025 at 4:12 PM, Kevin Westbrook said:

Marquez this is Kevin Westbrook I worked at Brittain Ind 40 +years please call me I can help you with the PC could be leak or rate gyro is off center! 539-292-0474

 

Kevin where are you located?

[Zippy_Bird]

Hi MarquezJC,

I recently completed fixing an issue just like this in my 66 M20E and the key was good troubleshooting.  Here are some instructions on how I built a cheap DIY testing tool and how I did my testing to find a leaking servo.

All you need is an analog vacuum gauge from amazon (ideally with +/- 10inHg range), tubing from your local hardware store, and a few push-on hose fittings.  I used 3/16 inner diameter plastic tubing from Lowes and corresponding 3/16 fittings.  You'll need a "T" fitting, one male-to-male fitting, and whatever fitting is need to attach to your vacuum gauge.  Make up a vacuum hose test kit such that with the "T" fitting you have a connection for your plane (10ft hose), a connection to your vacuum gauge (1ft hose), and a hose leg for your mouth (1ft hose).  Make sure to "Test" it holds a vacuum by plugging the open line with your finger and drawing vacuum with your mouth.

With your plane in a cool shaded place, crawl under the pilot side panel and look upward for the thick red and green lines running into your PC gyro.  These are the port and starboard vacuum lines that go out to four servos, two per line (per side).  Using plyers you can slide the Corbin clamps away and disconnect these colored lines by hand.  Advise disconnecting, testing, and reconnecting one at a time to avoid a reconnection mistake.  With your test kit, connect to the red or green line and use your mouth to start drawing vacuum.  Note, the yolk will roll to one side.  Watch the vacuum gauge as you draw as much vacuum at you can, then "stopper" the line with your tongue.  NOW, watch the vacuum gauge closely.  In a properly working system it should HOLD the vacuum steady.  In my case, the green side held indefinitely but the red (port-side) vacuum equalized immediately after applying vacuum... less that 3 seconds.  It was an aggressive leak.

Once you test both sides (red and green) you will know a lot!  Based on your description of the inflight behavior, I'll hazard a guess you'll detect a leak on your green line (starboard right side).  I believe you roll left because the red line (port left side) has NO leaks while your green line (starboard right side) IS leaking.  Additionally, even with all the right side trim (extra vacuum on the green line) it still does not overcome the leak.  Now, I further guess you do not see this with the Accutrak II AP engaged because that system adds a servo control valve in-line that routes additional vacuum to either side for rolling the plane on course.  Again this is just a guess, but I believe this valve is able to "overcome" a leak and is constantly doing so in flight.

Assuming you find a leak on the red or green line, you'll want to track down the origin.  While the lines themselves can leak it is not common.  The servos are the most likely candidates.  There are two servos connected to each line/side.  There is one servo in each wing (red line goes the left wing servo, green line goes to the right wing servo) and two in the tail.  The tail ones are mounted in opposition to each other connected to the rudder pushrod.  I advise starting in the tail because it is easy to remove the side panel for testing.  You can test your servos by disconnecting the inbound vacuum line and connecting your test kit directly to each.  Alternatively, if you have a long hose run like I do, you can stay connected to the vacuum line in the cockpit running the hose out out the pilot window, and walk around freely with your test gauge in hand.  In my case, the leak was so bad I could hear the bad servo leaking.  Once you find the leak source you'll know what to repair.

I know this is a lot.  But, if you want to fix and maintain your PC system this Tool is worth building and keeping handy.  Once I built mine, I found my bad servo in 10mins.

Please let us know what you find out after doing a vacuum test, especially if you find that both sides hold vacuum 100%.  That would indicate something else... a gyro issue.

Good Luck,

Zach

 

 

Edited Monday at 04:16 PM by Zippy_Bird

[Cfidave]

Zach, thanks for the info. Any chance you could upload a few pictures of the test rig. 

[DCarlton]

I used a hand vacuum pump from Harbor Freight that worked well.  It has a vacuum gage good for 6" and it has a release valve.  Made it easy to pump up the correct vacuum if you are in an awkward position.  You just gotta get a good seal between their tube and yours but the kit comes with various fittings; a little electrical tape helps.  If you have a small leak, you'll do a lot of pumping and nothing will happen.  The pump I used was similar to this (see link).  Worked great and you can pull 5-6" and hold it.  Brittain had a spec for how long to hold a vacuum during a test but it'll likely either leak bad or it won't.  

https://www.harborfreight.com/mityvac-vacuum-pump-39522.html

[Cfidave]

Actually I saw that and wondered how it would work. Good to know it would work out for pulling a vacuum, thanks much.