Positive control help.

[Paul donahue]

I have a new to me, 1967 m20c. It has a PC system that I doesn't work. At least I don't think it's doing anything. Although when I fill one tank and the other is empty (fuel leak), it's not that noticable on the imbalance. 

   Anyway, what kind of disconnect switches were used. I have an electric yoke switch but it's wired to my ptt. Where are the servos/actuators located? Is there a methodical leak detection technique?

[Hank]

Aileron servos are behind the outermost inspection panel. Tail servos are in the avionics bay, stick your head in and look to the right. 

If the yoke button was removed, other parts may also be missing. Look at the top of the avionics bay for its brain, too.

[Marauder]

There should be a yoke mounted switch for temporarily disabling the PC while you’re maneuvering. Many were converted to have a switch on the panel that allowed you to disengage the PC system completely. It just dumps the vacuum.

It’s been a while since mine was removed but the servos are mounted on riveted panels in the last third of the wings. Look for red and green tubing.

Typical failure mode is one of the black rubber servos failed or the tubing is cracked. The turn coordinator on mine used to be the “leveler” to correct turning tendencies in flight.

Brittain Industries that made the autopilot may still be in business. The owner died but his wife I believe took it over. They were I thought based in the Oklahoma area.


Sent from my iPhone using Tapatalk Pro

[Shadrach]

The system is easily tested during taxi. If working, the yoke should bank in the opposite direction of ground turns.

[Kevin Westbrook]

Paul I worked at Brittain plz call me my name Kevin Westbrook 539-292- 0474

[Mooneymite]

3 hours ago, Paul donahue said:

I have a new to me, 1967 m20c. It has a PC system that I doesn't work. At least I don't think it's doing anything. Although when I fill one tank and the other is empty (fuel leak), it's not that noticable on the imbalance. 

   Anyway, what kind of disconnect switches were used. I have an electric yoke switch but it's wired to my ptt. Where are the servos/actuators located? Is there a methodical leak detection technique?

Paul, have you scoured the aircraft logbooks?  Have you found any entries that point to repairs, or removal of parts?

When it comes to the PC, realize that not all PC's are the same.  Brittain made changes over the years.  Advice for one version may not be good advice on another version even though the various versions' basics are the same.

[Bob E]

Hi Paul,

In your panel photos, that little metal knob to the lower left of your turn coordinator is actually the wing-leveler adjustment (roll trim) knob that was originally above the clock on the yoke.  I'm not sure why someone would want to reposition it from the yoke to the panel; that's very odd.  Check to see whether the knob is even functional: look under the panel and see if there are little pneumatic tubes connected to it.  Maybe there are -- the little felt-tip pen mark on the turn coordinator bezel appears to be intended as the center position for the wing-leveler: align the dot on the knob with the felt-tip mark. If so, maybe the PC system can be repaired. If there are no tubes going into the knob, you should just accept that your PC system is disabled.

The yoke-mounted temporary disconnect switch Marauder refers to in his post above is that little silver button at the top of the left-hand side of the pilot yoke. Back when I had the PC in my M20C, I'd hold that button down while maneuvering so that the PC system wouldn't try to level the wings at the same time. As you can see from the photo below, taken in 1986 when I bought my M20C, I had a rubber band over the PC disengage switch to keep it disengaged all the time; also a common thing back then.

Many Mooney owners have disabled their PC wing levelers. It's ancient technology and expensive to maintain. I removed mine entirely when I ditched the vacuum system. As long as your aircraft is properly rigged, you won't miss it. (Recommendation: spend aircraft budget money on upgraded radios rather than fixing the PC system.)

Regarding whatever that black thing is above your clock:  Is that the PTT button you were referring to? If so, your next upgrade should be a PTT switch velcro'd to the yoke next to where your hand is. You shouldn't have to move your hand off the yoke every time you talk on the radio.  See the photo -- I still use the same PTT switch to this day. 

BTW it looks like you have an iPad mini.  You can rig up a mount so it will fit right on your yoke!  (See other photo below.)

 

[Paul donahue]

I found the on/off valve. It's a small brass valve near my knee. Sort of hard to see. It looks like it is definitely leaking at least there. Previous owner said it just stopped working. Not disabled. That was a full size iPad, the mini is on the way. I've got a yoke mount for it. The adjustment knob is part of the turn coordinator.

[Bob E]

23 minutes ago, Paul donahue said:

The adjustment knob is part of the turn coordinator.

????   Since when do turn coordinators have adjustment knobs?

That knob on your panel sure looks like the PC roll trim knob.  Yours is on the left, mine is on the right:

 

[Hank]

1 hour ago, Bob E said:

????   Since when do turn coordinators have adjustment knobs?

That knob on your panel sure looks like the PC roll trim knob.  Yours is on the left, mine is on the right:

My 1970 C has the PC roll trim knob on the turn coordinator. Above my yoke clock is a post light to see the clock at night. 

[Bob E]

7 minutes ago, Hank said:

My 1970 C has the PC roll trim knob on the turn coordinator. Above my yoke clock is a post light to see the clock at night. 

Wow!  I have been officially educated.  Did the factory put the roll trim knob there?  Never heard of that before.  The light over the clock makes sense since the overhead lights don't light up the yoke. And when you think about it having the roll trim adjustment right near the turn coordinator makes sense too.

Anyway, thanks for this info -- it's really interesting.  (In the sense that the position of a roll trim knob is interesting only to M20 owners )

[Guest]

1 hour ago, Bob E said:

????   Since when do turn coordinators have adjustment knobs?

That knob on your panel sure looks like the PC roll trim knob.  Yours is on the left, mine is on the right:

 

Early systems with the PC gyro either in the tail section or between the instrument panel and firewall had a roll trim knob in the pilot’s yoke.  Later systems incorporated the roll trim in the panel turn coordinator.

Clarence

[Hank]

15 minutes ago, Bob E said:

Wow!  I have been officially educated.  Did the factory put the roll trim knob there?  Never heard of that before.

I never heard of putting the roll knob on the clock, so we're even.

Since it's in my Owners Manual, I'm assuming it's from the factory. 

[David Lloyd]

Like M20Doc said, the 1965 models had a remote gyro in the aft radio bay to control the PC system.  There was a roll trim knob on the pilot yoke over the clock. The momentary disconnect button was on the left side of the pilot yoke.  That disconnect button de-energized a piloted valve behind the panel that provided vacuum to the PC system. The 2" turn co-ordinator in the panel is electric driven and has no connection to the PC.

I always get a kick out of the manual referring to the piloted valve as a pilot valve.  Us pilots get confused. In pneumatics, piloted refers to the valve being operated by air pressure or vacuum rather than electrically.

The 1966 models have a standard size, electric driven turn and bank that controls the PC.  Roll trim knob was one the pilot yoke over the clock. The momentary disconnect button was on the left side of the pilot yoke.  That disconnect button de-energized a piloted valve behind the panel that provided vacuum to the PC system. 

The 1967 and later models have a standard size turn and bank that is both electrically and vacuum driven that controls the PC.  The roll trim knob is part of the T&B.  There is a piloted valve behind the panel.  At some point rather than the disconnect button directly controlling the vacuum to the piloted valve, an electronic valve was used.

Point is, there are several variations.  Each is shown in the parts manual.  Get an idea on how it is laid out and where all the parts are before trying to troubleshoot.  Fairly good pictures of the 1965, 66 and 67 are in the parts manual available in the downloads section.  Mooney parts M20C 1965 to 1967.pdf. I think is in the engine section, 2nd page.  The 1967 pictures look almost like the 1976 except for the piloted/electrical valve.

 

[McMooney]

7 hours ago, Kevin Westbrook said:

Paul I worked at Brittain plz call me my name Kevin Westbrook 539-292- 0474

Hey Kevin, what need i do to get my system to follow heading instead of the magnetic doohicky

[0TreeLemur]

47 minutes ago, McMooney said:

Hey Kevin, what need i do to get my system to follow heading instead of the magnetic doohicky

You need to install an Accutrak.   When connected to the output of a CDI it follows a magenta line all day or a VOR radial until you get out of range.   They are awesome.

[McMooney]

3 minutes ago, 0TreeLemur said:

You need to install an Accutrak.   When connected to the output of a CDI it follows a magenta line all day or a VOR radial until you get out of range.   They are awesome.

already have accutrack, i want to switch from the magnetic deal to follow a dg

[0TreeLemur]

7 hours ago, Bob E said:

Many Mooney owners have disabled their PC wing levelers. It's ancient technology and expensive to maintain.

I respectfully disagree.  It is elegant technology, and when properly maintained it significantly reduces pilot workload compared to having no autopilot.  This is especially true for flying in IMC.  Add the Accutrak and it is a two axis autopilot.   Add altitude hold and it is a 3-axis autopilot with rate-of-climb control.  The PC system is actually pretty easy to troubleshoot.  It's just plumbing.    Working vacuum-can actuators are available from many sellers.

[0TreeLemur]

1 minute ago, McMooney said:

already have accutrack, i want to switch from the magnetic deal to follow a dg

You need an Accuflite DG coupler and an autopilot capable DG.   Just added one to our C this year.   It's awesome.

[Mooneymite]

26 minutes ago, 0TreeLemur said:

I respectfully disagree.  It is elegant technology, and when properly maintained it significantly reduces pilot workload compared to having no autopilot.  This is especially true for flying in IMC.  Add the Accutrak and it is a two axis autopilot.   Add altitude hold and it is a 3-axis autopilot with rate-of-climb control.  The PC system is actually pretty easy to troubleshoot.  It's just plumbing.    Working vacuum-can actuators are available from many sellers.

I agree!  Over the 23 years of ownership, my expenditures on my PC have been limited to electrical tape and some Permatex.  Works good, lasts long time.

[Hank]

2 hours ago, Mooneymite said:

I agree!  Over the 23 years of ownership, my expenditures on my PC have been limited to electrical tape and some Permatex.  Works good, lasts long time.

The only hard part is getting the aileron servos out of the wing and back in. Helps to hands like a 10-year-old-girl.

[DCarlton]

Guys.  If you want to know how this system works, call Kevin as suggested above.  He overhauled my Brittain system in Tulsa with Jerry Walters years ago and helped me recently troubleshooting my DG to DG adapter interface over the phone.  I've flown my M20F for over 17 years with the Brittain system.  It hasn't been expensive to maintain or overhaul.  In fact, it's been relatively cheap.  It was a great design without one line of software.  I have the variant that tracks the DG heading bug and plan to fly it as long as I can. 

[takair]

12 hours ago, McMooney said:

already have accutrack, i want to switch from the magnetic deal to follow a dg

Which basic system do you have?  My guess is a B5 or B6.  I have a B5 for sale that has a DG adaptor so that it uses DG instead of the magnetic deal.  I suspect it would work for the B6 too.  I’m sure Kevin can answer those questions.  I have a picture and ad in the classifieds if you are interested….

[McMooney]

57 minutes ago, takair said:

 

Which basic system do you have?  My guess is a B5 or B6.  I have a B5 for sale that has a DG adaptor so that it uses DG instead of the magnetic deal.  I suspect it would work for the B6 too.  I’m sure Kevin can answer those questions.  I have a picture and ad in the classifieds if you are interested….

I believe it's a b5 or b6, so just have to find a dg coupler and i'm golden

Edited November 20, 2021 by McMooney

[takair]

1 hour ago, McMooney said:

I believe it's a b5 or b6, so just have to find a dg coupler and i'm golden

I have have this combo listed in classified for $350…..