79 cent Brittain P.C. Tune Up

[takair]

I’ve been a big fan of of the PC with Accu-Trak and AccuFlite, but lately have been tempted by one of the new autopilots coming to market.  Both, the Accu-Trak and Accu-Flite just seemed a little sloppy in holding headings and track.  I convinced myself that this is the way it always was.  After annual, where I did the yoke shaft AD, it seemed a little worse.  This past week, the system seemed exceptionally lazy  for much of the flight, at times not even leveling the wings.

This weekend I removed and cleaned the vacuum pilot valve....no real findings there.  I started tracing things back, applying light vacuum to look for leaks and found the yoke disengage line was leaky.  Pulling the yoke cover off, first thing I noticed was I had put the line on the wrong tube, it belongs on the innermost tube, however, I still had a leak.  Focusing on the button, a visual inspection at the o rings revealed no obvious issues, however putting the button into the yoke revealed that the large (1/4”I’d x 1/16”) o ring was loose in the hole.  Replaced it with a new one and it sealed up nicely.

Test flight today in bumpy conditions and what a surprise.  The system worked fantastic, has not performed this well in years...if ever.  Instant response in wing level mode and held heading and track perfectly.  It seems that a partial leak in the yoke button will allow the shuttle in the pilot valve to move part way....allowing some semblance of functionality, but not optimal functionality.

In summary, sloppy wing leveler, check the o rings on the button.  As a side note, in another thread I suggested removing the button if you want to disable the wing leveler for training or other reasons.  Turns out, this likely wears the o ring in question, causing this issue.  Otherwise this o ring remains static.  Hope this saves someone a few $$.

[0TreeLemur]

Good job!  The previous owner of my M20C went through the same process as you with the same result.   The PC system w/ Accutrak II is amazing.