Positive Control Stuck "On"

[BenP]

Hey all! I have a head scratcher for the Mooney brain trust. My Positive Control system was recently repaired. Now it is stuck "on" and the cutoff button doesn't disable the system anymore.

Short version: 1967 M20F with original PC system. The turn coordinator was previously INOP, and we just had it replaced with a used PC-compatible turn coordinator. Now the PC works great, but the cutoff button on the yoke does not disable the system. 

Long version: My wife and I bought this 1967 M20F in the spring of 2023. The PC system had been INOP, but the previous owner successfully repaired it by replacing all of the vacuum tubes shortly before we purchased it. The turn coordinator and the PC system functioned normally during a quick hop I made with the seller/previous owner at that time. The aircraft needed a new engine plus an annual when we bought it, and consequently it was stuck in a shop for several months after we bought it. When I finally started flying it in February 2024 for the insurance checkout, the PC system *including the cutoff button* was working but the turn coordinator had become INOP during its time in the shop. During this time the plane "thought" it was wings level all the time so the PC system worked but not correctly. We deferred the issue for a few months. Last month, we had some miscellaneous avionics work done. Garmin 430W install, avionics master switch install, intercom re-wire, ADF delete, DME delete, COM2/NAV2 delete, etc. We also had the INOP turn coordinator replaced with a used turn coordinator that is PC compatible with the roll trim knob.

Now, the PC system works really well except for one problem: The cutoff button does not disable the PC system. It doesn't matter how hard you press it; it doesn't shut it off. If you want to do any turns, you have to fight the PC system the whole time. 

My question is: Why would simply replacing the turn coordinator (and doing nothing else) cause the cutoff button to stop working? How do we fix it? It's going in for annual in a couple of weeks, and we're hoping to take care of it then. A real autopilot won't be feasible with our budget for at least a couple of more years, so we'd like to keep the PC system functioning as long as we can. Any tips or insight would be greatly appreciated. Thank you all in advance!

[N201MKTurbo]

There is a valve behind the panel that the tubing from the button goes to. You can trace it from the back of the yoke shaft. You can test the button by removing that tube and sucking on it. There should be no airflow through the tube until you push the button then there should be airflow. You can test the valve by disconnecting the tube and the PC should be INOP. It's possible they bypassed the disconnect valve when they connected the new turn coordinator.

[Igor_U]

What N201MKTurbo said is true. The issue described may be a result of a simple kink in a vacuum tube from the yoke button to the Pilot valve resulting in blockage. Check the simple stuff first.

[AJ88V]

Sounds like good guidance above.  Suspect this won't help (now), but does the system disengage if you completely remove the button? (pull it out of the yoke?)

When you get the Brittain wing leveler system working properly, you should be able to pull the button out half way to disconnect the system.  This is what I do before takeoff, and then push the button to its default 'on' position when climb out is completed and course direction is set.  Said another way, if you pull the button out half way, you're hand flying and don't need to press the button through turning maneuvers.  Others install a disconnect 'switch' in the panel, but pulling the button out half way should do the same thing.  

I usually leave the button in its default-on position during taxi as a way to observe the leveler system is working properly as you turn the airplane.  Just pull it out before takeoff....

[Echo]

Ths system is super simple.  Do you have a disconnect on the yoke or the panel?  As you know the disconnect is just a dump of vacuum resulting in defeat of the pc system.  Look under panel for a kink in the pilot cutoff tube.  Do you have a integral pc system in turn coord. or in tail?  Regardless there is a black tube coming out of yoke or panel disconnect.  There is a kink in this tube defeating the disconnect.  Good luck.  Having a functioning PC is awesome.  Enjoy.

[BenP]

Quick update: I traced the PC button hose from the yoke shaft to a small box that eventually leads to the turn coordinator. There are no obvious kinks or damage. I was unable to take 201KTurbo's advice to test the switch, as the hose is frozen/fused to the aforementioned box and it doesn't seem like I can disconnect it from that end without breaking something. 

It would be much easier to access it from above. I have the original windshield with the avionics bay panels on top, but I just had them weather-stripped and I am reluctant to pull them off just for this. 

It goes in for annual next week and I'm doing an owner-assisted, so I'll give it another look with my A&P at that time. 

On an unrelated note, I found a loose hose clamp leading to the attitude indicator. The attitude indicator is working normally. 

[Slick Nick]

What's that fuel filter thing used for?

[N201MKTurbo]

27 minutes ago, Slick Nick said:

What's that fuel filter thing used for?

It is used as an air filter.

[Igor_U]

11 hours ago, BenP said:

Quick update: I traced the PC button hose from the yoke shaft to a small box that eventually leads to the turn coordinator. There are no obvious kinks or damage. I was unable to take 201KTurbo's advice to test the switch, as the hose is frozen/fused to the aforementioned box and it doesn't seem like I can disconnect it from that end without breaking something. 

 

The box is called pilot valve. I think your kink is at the other end, in the yoke where the PVC tube connects to the metal tube.

My bet is your clock (if you have it in the yoke) is pressing on the tube and restricting the airflow. Good luck.

 

[Hank]

I rarely use the thumb button, as I find the additional resistance a good reminder to bank gently in the pattern. Don't usually make a whole lot of turns unless I'm looking at something, and then use the thumb button. 

[BenP]

5 hours ago, Igor_U said:

The box is called pilot valve. I think your kink is at the other end, in the yoke where the PVC tube connects to the metal tube.

My bet is your clock (if you have it in the yoke) is pressing on the tube and restricting the airflow. Good luck.

 

Good call. The yoke didn't have a clock when we bought it. It had a mount for an old yoke-mounted GPS. We had the yoke mount replaced with a blank plate since we're running a tablet on a RAM mount in that spot now. If they weren't careful, they may have kinked it when they screwed the new blank plate on. I will double check that and report back.  Thank you!

I do agree with your reasoning that the problem is most likely between the forward end of the yoke shaft and the button itself. That makes the most sense to me given the symptom and the recent history of the plane. 

[N201MKTurbo]

You could pull the button and see if it disables the PC.

[Kevin Westbrook]

Ben p call me Kevin Westbrook I worked at Brittain I worked on lots of pilot valve I 539-292 -0474

[Kevin Westbrook]

Ben p call me Kevin Westbrook I worked at Brittain I worked on lots of pilot valve I 539-292 -0474 Ben p Kevin again there are two hose going to gyro valve hose and rotor hose they are REVERSED so u are turning off vac to rotor not valve!!!!!!!!

 

[Pasturepilot]

@BenP see above! 

[BenP]

On 10/3/2024 at 6:42 PM, Igor_U said:

The box is called pilot valve. I think your kink is at the other end, in the yoke where the PVC tube connects to the metal tube.

My bet is your clock (if you have it in the yoke) is pressing on the tube and restricting the airflow. Good luck.

 

We pull the blank panel off, and the hose was not kinked or otherwise compromised inside the old chronograph space. I think the problem is further back in the system. 

[BenP]

18 hours ago, Kevin Westbrook said:

Ben p call me Kevin Westbrook I worked at Brittain I worked on lots of pilot valve I 539-292 -0474 Ben p Kevin again there are two hose going to gyro valve hose and rotor hose they are REVERSED so u are turning off vac to rotor not valve!!!!!!!!

 

I'll try to give you a call next, Kevin. 

That being said, if the hoses are reversed: Would pressing the PC button dump the whole vacuum system? If so, we do not see a change in suction when the button is pressed. 

[Igor_U]

On 10/5/2024 at 7:26 AM, BenP said:

We pull the blank panel off, and the hose was not kinked or otherwise compromised inside the old chronograph space. I think the problem is further back in the system. 

 

Did you try to remove red hose from the metal bibb and see if you disconnect the wing leveler? Or suck on it to see if it's not plugged as Rich suggested? that would effectively remove the vacuum button out of the picture and possibility of a plugged tube to the yoke and hose.

If the hoses are reversed you can compare to the schematics in SM or Parts catalog (you can get them from Downloads) and see if they connect it wrong. the best is to remove the seats and go under the panel with the flashlight. Beware, it's a explosion in spaghetti factory down there. 

I find interesting that the hose is red at the control wheel end and white at the pilot valve end (your first photo). 

[Echo]

FYI what mine looks like.  I have left right in yoke and cut off now on panel.

 

[BenP]

On 10/4/2024 at 3:10 PM, Kevin Westbrook said:

Ben p call me Kevin Westbrook I worked at Brittain I worked on lots of pilot valve I 539-292 -0474 Ben p Kevin again there are two hose going to gyro valve hose and rotor hose they are REVERSED so u are turning off vac to rotor not valve!!!!!!!!

 

Thank you Kevin! You were exactly right. 

All - My A&P spoke to Mr. Westbrook. The hoses were re-installed incorrectly by the avionics shop. My A&P re-installed it according to the schematic and it works normally now. The PC system passed a ground run test. After it's out of annual we'll verify with a flight test, but we should be good. 

If anyone else has an issue with the PC being stuck "on" and you rule out a problem with the cutoff switch, check the hose routing. Thank you very much again to Kevin Westbrook. I owe you one.