PC system

[Luxuryairtravel]

Great article! I learned a lot, thanks Kerry McIntyre!

Are You Positive?

PC Systems

ShopTalk - June 2018

by Kerry McIntyre

 

This month’s ShopTalk will take the reader deep into the mysterious world of PC (positive control) aircraft systems and break down the components and how the system works. This will lead us to the troubleshooting process of getting this system to work properly. 

Most of our pre-1977 Mooneys originally came with a PC system as a wing leveler, a poor man’s autopilot. This system, manufactured by Brittain Industries, is powered by vacuum pressure that passes through the turn coordinator to a pilot valve located behind the pilot’s instrument panel along with four rubber‑cupped vacuum servo cans and a thumb button (cutoff valve or switch) in the pilot’s control wheel yoke. For a schematic overview, see Figure 1. In 1977, Mooney replaced the pneumatic thumb button with an electrical switch and a remote solenoid valve (see Figure 2). 

From an engineering viewpoint, the PC system is elegantly simple. The muscle to move the control surface (rudder or aileron) is the BI-706 which Brittain refers to as a servo assembly. Technically, this is not a servo but rather an actuator as there is no feedback mechanism. Still, we’ll take Brittain’s lead and call it a servo. The BI-706 consists of a 4.5-inch diameter can with a rubberized cup fitted to the open end (see Figure 3). As pressure is reduced in the can through the vacuum tube, the rubber cup membrane is pushed into the can by atmospheric pressure. A circular plate, affixed to the membrane, attaches to and pulls a cable (rudder) or a chain (aileron) to move the respective control surface.

Each aileron has one servo attached to the control arm in each wing. In the tail cone near and below the battery box are two servos attached to the rudder push-pull tube These two servos work against each other depending on how vacuum pressure is routed to them via the turn coordinator and the co-located roll-trim valve. All four servos are interchangeable. Some planes also came with a servo on the retractable step. This servo has the same design as the others but has a longer stroke and a larger diameter - more force. Therefore it is not interchangeable.

A thumb button (cutoff valve) is installed on the left side of the pilot control wheel. This thumb button when depressed allows vacuum to bleed off thereby releasing the control servos’ pull on the controls. It has no effect on the step servo. If this button is removed from the yoke it allows for hand flying with no vacuum forces on any kind hindering the pilot's input. The turn coordinator is dual powered, both electric and vacuum operated, so an electrical failure does not affect its operation or that of the PC system.

Vacuum pressure is ported off of the vacuum regulator and directed to the turn coordinator. One line will go through the pilot valve and the other will go directly to the turn coordinator. There are 2 air filters that filter the air entering the turn coordinator. These are mounted behind the pilot’s instrument panel. We will change these filters every 1000 hrs, but back in the 70s when people smoked in their plane they were changed every 500 hours.

When a PC-equipped aircraft comes in for an annual we look for clues that the PC system is malfunctioning: Is the roll-trim knob turned full stop either direction or is the yoke buttonremoved? When we see this it is time to ask the pilot: how well the system works, what does it do in flight?

Once we have determined the PC system needs help (and most do), we pull the top cowl and place the plane on jacks with the wheels free from the ground. Then disconnect the vacuum pump from the firewall and connect the vacuum regulator to an electric standby vacuum pump powered by a remote battery.

With the electric vacuum pump running we can determine the response of the PC system to the roll-trim knob on the turn coordinator. Typically the system biases to one side or the other and will not pull on the controls evenly, left and right.

There are a number of things that can go wrong as these systems age. Most common is one or more rubber cups on the four servos will be cracked/ torn and leaking. This causes an imbalance between the servos allowing a good servo to overpower a torn or leaking servo. If the step will not retract or retracts partially this is often a servo cup leaking also. Once in a while, we see the tubing that provides vacuum pressure pulled off the servo because the fitting on the line broke.

It is very rare to find the turn coordinator control or pilot valve causing problems in the system, but they can fail as these systems age. There are two o-rings in the pilot,s cutoff thumb button that can deteriorate and weaken operation by constantly bleeding off vacuum pressure. The rubber tubing from the pilot valve to the thumb button can crack and leak vacuum pressure where it goes into the control wheel shaft behind the instrument panel.

When troubleshooting this system on jacks, first look at the rubber items: servo cups, hoses, and the thumb button. If you question the pilot valve, then bypass it and run straight vacuum directly to the system to see if it works. If you question the thumb button then disconnect it from the pilot valve and plug that line to see how that affects the system operation.

A year ago, an M20E was in the shop for some new radios and instrument panels. Once the installation was complete, the PC system was checked for proper operation. Low and behold, it did not work at all! After repairing a broken line at the right aileron servo it still did not work. The next thing was to bypass the thumb button by capping off the line to the pilot valve, and guess what? Part of the system operated, just not completely. When the pilot valve was bypassed, the system did a hard right turn - now we are getting somewhere.

Now it is time to find out which servo or servos were causing the problem. We already know the pilot valve is FUBAR, so by bypassing it, we can troubleshoot the rest of the system. Most commonly the rudder servo directly under the battery box will be rotten and sure enough, it was. This PC system needed a pilot valve and a servo replacement.

In the 70s and 80s, one could buy just the rubber cups for the servos, but the FAA now requires repair from only an approved repair facility and so a rebuilt servo from Brittian Industries was purchased for $175. The pilot valve was also exchanged for another. When the system was tested, guess what?…. It still did not work correctly. 

At this point we found the tubing going to the thumb button cracked under a clamp that secures it to the control yoke shaft just as it enters the shaft. The thumb button was also missing one of the o-rings that seals the vacuum pressure. Both o-rings were replaced.

A new test was run on the system and Voila!, it worked perfectly. When the roll-trim knob was placed in the middle position all the controls were in neutral and a roll to the right was just as fast and complete as a roll to the left. Our 48-year-old PC system was finally up and running correctly.

One thing I realized as during this troubleshooting process was that no one knew anything about these systems but were all fascinated to understand how it worked. This came as a bit of a surprise to me. See, when I got into GA as a young A&P in the 70s, I learned about these systems from all those old guys that I worked with. Well, it seems those old guys are long gone and this knowledge that they passed on to me is not being passed on to the next generation very well. It also seems that I’ve become one of those guys. It must be the aviation circle of life (Hakuna Matata). That’s why I write the MAPA ShopTalk column and have the drive to continue to write it.

I remember talking to Bill Wheat years ago about all his knowledge and passing it on in a written form. Bill started with Mooney (in the late 50s) as a test pilot, then an engineer and he knew darn near everything about the Mooney aircraft systems. However, he was not interested in writing anything down and that is a shame. Bill is long gone (along with many of those other old guys) and I am grateful he shared some of his stories about the development of the M20 line of aircraft.

Since that M20E was repaired, Brittian Industries has ceased operation and is for sale due to the death of their chief engineer. With any luck, someone will buy the company and continue to offer parts so we Mooney aficionados can keep this fleet of aging aircraft operational.

If you have any questions about this article or another Shop Talk article, feel free to email me or call me at my aircraft repair shop: 307-789-6866. You can also read other ShopTalk articles on this website. Until the next ShopTalk, enjoy flying your Mooney.

[carusoam]

For more from Kerry M.

http://www.knr-inc.com/shoptalk-articles

awesome Mooney Maintenance writer...

Best regards,

-a-

[HRM]

Great article on the PC system. I was doing some training with a CFI/I once and after we had flown around for at least 45 minutes or so, he commented: "You have EXCEPTIONAL rudder skills!" I almost turned around to see who he was talking to as I rarely, if ever, use the rudder. Then I realized he didn't know I had Brittain's homunculus helping me. The system is so gentle and unobtrusive that I am always somewhat surprised when it does something obvious. Sad about the company, but good to know there are techs out there like Kerry McIntyre willing to work on it. 

[Bob R]

Hey Everyone,

My PC system was working until a couple of weeks ago. It now wants to make left turns which means the issue is on the right side, somewhere. Since all of those rubber hoses are 50+ years old, they are cracking. Therefore, I am replacing them. While I was crammed under the panel. I decided to check the suction running to the right and left side by just using my finger and turning the roll trim know, I will test with a gauge this weekend. Certainly more suction running left than right. Oddly, when I switched the left and right lines on the back of the turn coordinator, the control still turned left.

Regardless, it’s probably a hose issue but, I think I can rule out out the “servos”, maybe. It’s possible it’s the the vacuum diverted, for lack of a better term, on the back of the TC as well. My question, does anyone know what makes up the parts in the diverter? I’d love to find a schematic and have emailed Brittain who has answered me before.

Anyone have any insight.

Thanks, Bob

[McMooney]

mine was a hole in the servo, i'd check there first.

btw, electrical tape is your friend

[carusoam]

Word of the day, credits to Harley!
 

Homunculus...  essentially a tiny human from which babies were assumed to be from...

-a-

[Hank]

14 hours ago, carusoam said:

Word of the day, credits to Harley!
Homunculus...  essentially a tiny human from which babies were assumed to be from...

And here I thought homonculi were tiny human-like assistants to witches and other magic-using people . . . . But what do I know? My D&D days are far behind me. 

[Bob R]

My apologies to the person who was actually correct that if the control wheel turns left the problem is on the left I thought it was on the right. I found the issue. The good news is that my servos are good. Now I just got to find some red poly tubing.

Bob

[carusoam]

16 hours ago, Bob R said:

My apologies to the person who was actually correct that if the control wheel turns left the problem is on the left I thought it was on the right. I found the issue. The good news is that my servos are good. Now I just got to find some red poly tubing.

Bob

The Brittain guy left her contact info...

Check if she has the the exact proper tubing...

Look for Cecilia... @  CSmith

-a-

 

[Robert67]

Great article. Can anyone tell me what kind of vacuum pump to use to test the system when the aircraft is on jacks.  

Thanks

[carusoam]

Welcome aboard Robert!

Depending on what you are testing... there is a brake Bleeder vacuum pump tool that works for some things...

I am only a PP so I probably didn’t describe it very well...

Use that idea in the search tool... you will probably find what you want to know...

Or wait a bit... somebody better in the know will be by soon... 

Best regards,

-a-

[Bob R]

On 1/21/2020 at 5:04 AM, Yetti said:

As much as I like machining stuff.   I really would give these fine folks a call to see if they stock the item

https://www.bostongear.com/-/media/Files/Literature/Brand/boston-gear/catalogs/p-1930-bg-sections/p-1930-bg_engineering-info-worms-and-worm-gears.ashx

 

 

On 1/21/2020 at 3:41 PM, N201MKTurbo said:

BTW,

Are you talking about violating regulations or getting sued? the two have little to do with each other.

 

On 2/26/2020 at 7:04 AM, RDuplechin said:

 

I happen to have a 2 stage but I do notI think it really matters. Zach is it a put out you know 10 inches of vacuum the regulars do the rest. I fixed the line that was broken temporarily which stopped the left-hand turns. In testing the vacuum with a gauge left and right vaccu  lines at the turn coordinator pulled about 7.5 inches. It did not matter which way I turned the roll trim. I am definitely not a Mooney pc expert, but I would think that's a vacuum should change. However, it stayed constant. Have any of you seen this? I'm thinking it's an issue with the vacuum diverter valve, not sure of it's real name, not diverting vacuum. The pilot controlled release button works just fine.

Bob

 

[Robert67]

Thanks for all the help.  Where can you purchase a portable electric vacuum pump that pulls 10 inches?  I want to run the PC system while bypassing the aircrafts vacuum pump.  Thanks Robert

[carusoam]

Robert,

You have most likely entered into STC territory...

Connecting things that are portable to the plane is one thing...

But, connecting them to the flight controls probably takes on a whole new level of requirements...

technically...

You might start with known electric pumps that are installed in Mooneys to begin with...

There are a few that become available every now and then with people going all electric... they use the same type of vac pump that is mounted on the engine... and designed to pull the same amount of vac required by the ship’s instrument system...

Finding a 12V Electrolux is probably going the wrong direction... 

PP thoughts only, not a mechanic...

Best regards,

-a-

 

 

[Robert67]

Thanks.  For troubleshooting the PC system, thePC service manual states “If desired, an external vacuum pump may be utilized if the vacuum source is attached to the engine side of the aircraft relief valve.”  I’m wondering what kind of portable vacuum pump I can attach to the system.  Thanks Again. Robert

[carusoam]

For maintenance!!!!   
 

The extra detail helps a lot.  Thanks for clearing that up...

There was a discussion in another thread regarding various tools to measure the vac... (automotive gauge)

The amount of vacuum you are asking for is probably available using ordinary vacuum cleaners...

The challenge is to control the amount of vacuum generated... see what is in the closet at home... many have a method of adjusting the vac available...  slide gates to allow air entry in downstream...

Use extra caution to guard against pulling excess vac through your instruments that are attached to the same system...

Use the gauge first before sucking the guts out of a plane...

PP thoughts only... definitely not a mechanic...

Best regards,

-a-

 

 

[HRM]

1 minute ago, Robert67 said:

Thanks.  For troubleshooting the PC system, thePC service manual states “If desired, an external vacuum pump may be utilized if the vacuum source is attached to the engine side of the aircraft relief valve.”  I’m wondering what kind of portable vacuum pump I can attach to the system.  Thanks Again. Robert

A small vacuum cleaner should work fine. Before energizing, have some sort of relief, could be just a large hole in the hose with a sleeve, that you slowly close while watching your vacuum gauge.

HF carries vacuum pumps for air conditioning work, but those are probably overkill.

[59Moonster]

Question for those who are knowledgeable on this.

I have a Brittain wing leveler(M20A) and it seems almost impossible to get it to track a heading. It always seems to be too much or too little one way or another. Even with super tiny adjustments to the knob. Any tips, or am I expecting too much out of it? 

[Bob R]

10 hours ago, 59Moonster said:

Question for those who are knowledgeable on this.

I have a Brittain wing leveler(M20A) and it seems almost impossible to get it to track a heading. It always seems to be too much or too little one way or another. Even with super tiny adjustments to the knob. Any tips, or am I expecting too much out of it?

 

See this to diagnose.

https://www.knr-inc.com/shoptalk-articles/25-shoptalk/91-201806-pc-systems

I found that in the left wing someone left out a grommet where the tubing exits the last spar before connecting to the servo. It finally cut the tubing I  half.

I reached out to Britain and got a reply. The specs on the tubing are as follows:

(for all servo plumbing in the M20E regardless of color) calls for a Parker P/N E64 “Poly-Flex” tubing or equivalent. That is a 3/8” OD poly tubing available in an array of colors.Bob

 

 

[carusoam]

We have Celia our Brittain person around here...

Brittain is still alive...

Brittain APs have worked incredibly well for decades...

So if yours isn’t working.... don’t give up on it...  dig deeper.

Best regards,

-a-

[sdflysurf]

Hey All, I noticed on one of the diagrams that there is an electric solenoid instead of a vacuum pilot button in some models - I would like to remove the manual pilot valve button on the yoke, and place an electric switch in the panel connecting to an electric solenoid instead, thereby freeing up my yoke for the PTT button.  It is really easy to flick a switch on the panel for comfortable wing-leveling cruise and flick it off when in the pattern.

Any suggestions on parts?  I have searched high and low and at this point may need to get field approval rather than pick up anything used.

[carusoam]

See if @Alan Fox is familiar with the solenoid/vac system switch for the Brittain system...

Best regards,

-a-

[hoot777]

Getting ready for my annual on my G.  I have some new boots to put on.  Anybody know where other parts can be purchased from like o rings for the yoke button.

 

Thanks

[carusoam]

See if you can get an actual catalog from aircraft spruce….

It makes great reading for all the aviation bits and pieces they have in stock, or can order for you…

Not sure if the actual catalog is still available…

PP thoughts only, not a mechanic…

Best regards,

-a-

[N201MKTurbo]

37 minutes ago, carusoam said:

See if you can get an actual catalog from aircraft spruce….

It makes great reading for all the aviation bits and pieces they have in stock, or can order for you…

Not sure if the actual catalog is still available…

PP thoughts only, not a mechanic…

Best regards,

-a-

I’m currently using the Aircraft Spruce catalog as a drilling pad. Much better than a block of wood. It is flatter and doesn’t rock. Most of my drill bits won’t even make it to the middle of the catalog. 
 

And even with the holes, you can still look stuff up if your phone battery dies.