What is pitch on?

[Kerrville]

I am looking at buying a Mooney. Flew it the other day. Couple questions since it was the first time I had even been in a Mooney. 
 

1)there was a push pull knob in the center of the panel labeled “pitch on.”  What is that?

 

2)There is another switch “pc on.”  I love read what Mooney positive control is, and assume that is what it was. When I switched it on the plane wanted to roll right pretty hard. Tired me out flying it with it on. There is also a Brittan b5 autopilot. Is pc separate from the autopilot?  Pc seemed to roll the plane even with ap off. Ap on seemed to do nothing. Are these old systems usually worthless, and if not can they be repaired somewhere, or is it’s destiny the garbage can?

Thanks for the info.

—prospective Mooney owner

[Fly Boomer]

6 minutes ago, Kerrville said:

I am looking at buying a Mooney. Flew it the other day. Couple questions since it was the first time I had even been in a Mooney. 
 

1)there was a push pull knob in the center of the panel labeled “pitch on.”  What is that?

 

2)There is another switch “pc on.”  I love read what Mooney positive control is, and assume that is what it was. When I switched it on the plane wanted to roll right pretty hard. Tired me out flying it with it on. There is also a Brittan b5 autopilot. Is pc separate from the autopilot?  Pc seemed to roll the plane even with ap off. Ap on seemed to do nothing. Are these old systems usually worthless, and if not can they be repaired somewhere, or is it’s destiny the garbage can?

Thanks for the info.

—prospective Mooney owner

Many fly with (and seem to love) both PC and the Brittain autopilot.  Parts are scarce, but may be available to a scrounger.

[Utah20Gflyer]

The PC system pulling to one side is a classic indication there is a leak in one of the vacuum lines.    

[Hank]

16 minutes ago, Utah20Gflyer said:

The PC system pulling to one side is a classic indication there is a leak in one of the vacuum lines.    

The PC system will fly in the direction of the heading bug on the DG, or in the direction indicated on the dial of the PC box, depending on which version you have.

So if the pointer is set on 270, and you're flying 180 and just turn it on, it will want to turn right pretty  hard.

Or it could be a leak in the system.

[Kerrville]

Is the PC a wholly separate system from the Britain?  There is a switch for pc on and the Britain box has an on off dial as well. 
 

and as far as the heading bug goes, there is not one. However, like you said the Britain box has a dial with 360 degrees on it.  How does it “know” the heading?   Does it have some kind of internal compass?

[Vance Harral]

The "Pitch On" knob enables the pitch damping system in a Brittain autopilot.  It is a poor man's pitch hold device.  It does not know the actual pitch attitude of the airplane because it is not connected to any kind of gyro.  It is instead connected to a box which contains both a "G sensing device" (a weighted mass in the middle of a rubber sheet), as well as a calibrated leak (i.e. a VSI).  The control system attempts to keep the airplane at 1G and constant vertical speed when enabled.  The idea is that you manually set up the airplane for a certain power/pitch/performance state, which it would naturally hold in smooth air.  In turbulent air, the airplane will oscillate around that state.  The pitch hold system dampens those oscillations.

In our airplane, the pitch hold system is part of the altitude hold system.  In addition to the box with the G sensing device/calibrated leak, there is also a pressure chamber which is sealed when you enable altitude hold.  That chamber provides a reference which the altitude hold system attempts to match against static air pressure.  But the response time of that system would be too slow by itself to achieve precise altitude control.  By working in concert with the pitch damping system, altitude hold is more precise, because the system immediately corrects for pitch deviations before the static pressure system indicates much change.

It is a very clever system that works nicely when everything is exactly tuned, and all the static and vacuum system is very well sealed.  Unfortunately, it is rather difficult to keep the system well-tuned and well-sealed.  We've been fiddling off and on with it for 20 years.  Ours "mostly" works.  We generally know when/why it's out of tune, and what could be done to improve it.  But it's a constant battle of chasing down small leaks in the system, cleaning dirty potentiometers, etc.

Regarding your other questions, the Brittain B5 has its own heading knob.  It relies on a remote compass in the tailcone to know the heading of the airplane.  There is a procedure for calibrating the system, provided of course that the remote compass in the tailcone is actually working.  In addition to heading mode, the B-5 can integrate with a nav head to provide course tracking for enroute and approach.  This requires the device to be electrically connected to a CDI.  As for how the B5 integrates with the PC system, they come together in the turn coordinator, which is the critical component of the PC system.  A Brittain TC is connected to the vacuum system, and has a shuttle valve that routes vacuum to "left" and "right" lines when the TC indicates a turn to the right/left, respectively.  Any time the TC senses a turn, the PC system turns the airplane back against the turn.  In airplanes with a B5 or other Brittain autopilots, the lateral outputs of the autopilot electrically connect to the shuttle valve of the PC, such that it can move the valve to command a turn.

[Kerrville]

If I buy the plane I will have a bunch of fiddling around to do. More excuses to fly and hang out at the airport!

[ckb]

I am a new owner of a 1966 M20E.  In 2004, the previous owner had the Brittain pitch control system installed.  I am interested in adjusting it so that in level flight when the pitch valve is pulled the pitch indicator lines remain aligned thus requiring minimal trim adjustment to maintain level flight.  At the moment, when I pull the pitch value the left line drops below and the plane descends aggressively.  I can trim the plane for level flight.   With this background I have three questions.

1.       Does the centering adjustment at the top of the pitch/altitude sensor assembly enable one to get the pitch trim indicator lines to align in level flight?

2.       If so, what is the easiest way to adjust?  According to the manual you connect a manometer to the pitch/altitude sensor and adjust so there is zero differential between the blue and yellow output ports.  Silly question, but the plane must be running right?

3.       Is there a way to adjust it without a manometer?  I read on Mooneyspace you can sort of wing it and a clockwise turn causes an upward pitch movement and a counterclockwise will cause a downward pitch movement.  Is this correct?  if so, anyone know the effect of one turn?

Any insight is super helpful!

Thanks

[Echo]

Earlier Mooneys had a pc disconnect (wing leveler) disconnect in the pilot yoke. It simply dumped vacuum and shut off the pc wing leveler. It is always on and can be defeated with yoke movement, but makes control inputs stiffer. Brittain has an electric solonoid dump with switch and also a manual pull button dump. If you have the sight window for pitch they are rare and have value. I had same issue with horizontal lines and need to trim for level flight. My altitude hold never worked well so I just trimmed for level flight. Early Mooneys had pc system in tail. Later used the turn and bank with system. There should be a manual trim wheel ( mine is on yoke to achieve wings level. If it won’t level there is a leak or leaks in lines or rubber aileron or elevator boots. 

[Echo]

8 hours ago, ckb said:

I am a new owner of a 1966 M20E.  In 2004, the previous owner had the Brittain pitch control system installed.  I am interested in adjusting it so that in level flight when the pitch valve is pulled the pitch indicator lines remain aligned thus requiring minimal trim adjustment to maintain level flight.  At the moment, when I pull the pitch value the left line drops below and the plane descends aggressively.  I can trim the plane for level flight.   With this background I have three questions.

1.       Does the centering adjustment at the top of the pitch/altitude sensor assembly enable one to get the pitch trim indicator lines to align in level flight?

2.       If so, what is the easiest way to adjust?  According to the manual you connect a manometer to the pitch/altitude sensor and adjust so there is zero differential between the blue and yellow output ports.  Silly question, but the plane must be running right?

3.       Is there a way to adjust it without a manometer?  I read on Mooneyspace you can sort of wing it and a clockwise turn causes an upward pitch movement and a counterclockwise will cause a downward pitch movement.  Is this correct?  if so, anyone know the effect of one turn?

Any insight is super helpful!

Thanks

Get the manual from Brittain. They will email it to you if you call. Thete is a very small rubber gasket on box for altitude hold box that must be turned (depending on pitch up or down) and then fly and readjust to achieve neutral pitch and proper altitude hold. The accutrak and accuflite are nav ad ons. Accuflite uses a direction heading bug and accutrak uses left and right gps input 

[Echo]

On 11/13/2023 at 8:19 AM, Kerrville said:

I am looking at buying a Mooney. Flew it the other day. Couple questions since it was the first time I had even been in a Mooney. 
 

1)there was a push pull knob in the center of the panel labeled “pitch on.”  What is that?

 

2)There is another switch “pc on.”  I love read what Mooney positive control is, and assume that is what it was. When I switched it on the plane wanted to roll right pretty hard. Tired me out flying it with it on. There is also a Brittan b5 autopilot. Is pc separate from the autopilot?  Pc seemed to roll the plane even with ap off. Ap on seemed to do nothing. Are these old systems usually worthless, and if not can they be repaired somewhere, or is it’s destiny the garbage can?

Thanks for the info.

—prospective Mooney owner

Pitch on engages pitch control. Trim for level flight. Altitude hold on then holds the altitude you set with pitch. Ideally the eyeball lines should be even

[Kerrville]

The people at Brittain were super nice and gave me all the manuals.  I still ended up pulling it out and getting rid of the whole thing though...  I wish I had known that the one with the "eye" had value though...

[takair]

Unfortunately, the diaphragms in the dynertial and alt hold age, stick, and leak.  To test the system, trim the aircraft in level flight.  Turn the dynertial (pitch) on and retrim for level flight.  The indicator may or may not be level.  It should not take a lot of trim.  Assuming you can trim it, turn on altitude hold.  It should stay fairly level.  If not, retrim.  It should again take little to get it level.  If you send me a PM with your email, I can send you a detailed test, based on my interpretation of the manual.  You can attempt adjusting the centering on the dynertial, but more than a turn can damage the unit.

[ckb]

Thanks for everyone's input.  As a follow up, I ended up buying a manometer and followed the adjustment instructions in the manual. This corrected the pitch trim indicator (the lines now line up in level flight) and minimized the need to retrim when the pitch valve is engaged.  You could probably use trial and error, turn the centering adjustment screw and  see what happens... But following the Brittain instructions seems like the way to go... shocking. 

I now need to figure out why the altitude hold doesnt work.  Ive tested for leaks to the alt hold chamber and there were none.  So Im thinking the issue might be in the altitude hold sensor.