Yellow tube -where does it connect?

[JoeM]

On my 65 M20E, the yellow tube (circled in red) is connected to a small black box (also circled in red in the 3rd pic) behind my copilot panel. Where should it be connected? I have a Brittain with Alt hold and Pitch control. My altitude hold isn't working well but pitch control does. Also, my heading selector is off quite a bit. I have tested and repaired the boots and lines to 5" of vacuum and the PC system works normally. 

[Vance Harral]

It's hard to say without the manuals for your specific model of autopilot, but that box and the tubing attached to it don't look like Brittain autopilot components at all.  The box appears to be plastic, and the tubing appears to be clear poly-vinyl that has oxidized and tinted with age.  Every Brittain component I'm familiar with has a metal case, not plastic; and the pneumatic lines in Brittain systems are opaque with bright colors (green and red for lateral, yellow and blue for vertical, orange for altitude reference), not clear.  I've attached the pneumatic schematic for the Brittain B-5 autopilot in our airplane, but note that you have something different than a B-5.

I don't see a data plate or sticker on this box you're asking about.  If this is a certified airplane part, it should have identifying information somewhere on the case.  If it doesn't, I'd bet you're looking at some sort of black/gray-market, home-made or experimental market thingie.  Hard to tell what it might be intended to do.  One of the lines attached to it is disconnected, as you note.  Maybe that line used to connect to something else which has been removed from the aircraft.  But it also may be deliberately open, intended to measure cabin altitude.  There is a second line connected to the box.  What does that second line connect to?

Independent of that mystery, if you want to look for obvious problems in the altitude hold system, trace the orange tube I can see in your photos.  One end of it should connect to an altitude hold control unit, and the other to an altitude reference chamber.  Somewhere in between there should be a valve and/or solenoid that connects to the ALT HOLD button on your controller head.  When altitude hold is disengaged, the altitude reference chamber is open to ambient pressure.  When engaged, the valve/solenoid seals the altitude reference chamber, providing a reference pressure to the altitude control unit, which climbs or descends as necessary to make the ambient pressure the same as the pressure in the sealed box.  So if your pitch control works - meaning the elevator servos are working - but altitude hold doesn't, one possible explanation is that the altitude reference chamber isn't sealing when altitude hold is engaged.  This can be caused by the orange tubing getting brittle so that it doesn't make airtight connections, and/or by the O-rings in the valve/solenoid getting brittle such that they don't make a good seal when closed.  It's also possible that the altitude control box itself is broken in some way.   If that's the case, you're outta luck, because no qualified technicians are available to repair them.  But O-rings and seals are more pedestrian work, that some A&Ps would be willing to sign off on.

B5vacuum.pdf

[0TreeLemur]

That box is not Brittain.   It looks like it might be some kind of differential pressure transducer to me.   Maybe some aftermarket airspeed based gear alarm system?   A digital airspeed readout?   Something that was once really cool, but now partially removed?  As @Vance Harral suggested, it kind of looks like a project box/homebuilt experiment.

[N201MKTurbo]

That part on the top of it is a Motorola differential pressure transducer. It would be more suited for an airspeed sensor. If it was, one tube would need a static source and the other a pitot source. What is that box connected to?

BTW

Electronics International uses boxes like that. Do you have a UBG-16 or some other EI gauge?

Edited October 1, 2021 by N201MKTurbo

[Vance Harral]

1 hour ago, N201MKTurbo said:

Electronics International uses boxes like that. Do you have a UBG-16 or some other EI gauge?

That was my first thought as well: looks like an EI project box, including the gauge and color of wiring.  Back when the UBG-16 was their top-of-the-line product, they had a bunch of add-on modules for it, that made various sensors "look like" a thermocouple output, so they could be interfaced to display (and record) things like fuel flow and pressure, manifold pressure, etc.

So... I went looking on the product page for the UBG-16, and there's a "Gyro Vacuum Pressure" add-on module for it.  @JoeM, does this airplane have (or did have in the past) a UBG-16 engine monitor?  If so, and if the other tube of that differential pressure gizmo is connected to the gyro vacuum manifold, we may have solved the mystery.

[JoeM]

Thanks all! I do have an UBG-16 and the gyro pressure is inop. I will try connecting this to that unit. It is likely that's where it goes. I appreciate all of your help.

 

[Ragsf15e]

Jeez, you guys were amazing identifying that.

[carusoam]

The power of MS in full force… 

Now, Joe has some homework to complete…

Because that tube being static or pitot related…. Means that one of those systems is now open to the atmosphere…

So… there are no obvious issues related to this loose tube… probably means it is an open static system… like having the alt static source open…

 

See if testing the alt stat, like you usually do, shows the proper response… if there is no difference… this open tube is probably the cause…

PP thoughts only…

Best regards,

-a-

[Yetti]

all that laying on the control shaft is a big no no per my IA.   maybe whoever took it apart should take pictures prior to disassembly

[carusoam]

6 hours ago, Yetti said:

all that laying on the control shaft is a big no no per my IA.   maybe whoever took it apart should take pictures prior to disassembly

That probably explains why the flexible pvc tubing has become detached…

It has a pretty sticky surface when new or warm… that can grab things that move by it…

In bumpy weather… anything leaning on the yoke shaft might get tugged…

PP thoughts only, not a mechanic or CFI…

Best regards,

-a-