pressurization system

[angola1]

Any of you guys ever seen a pressuized piston twin breathe when in level flight? Cabin is near max diff. and the cabin vsi starts going up 500-1000', down 500-1000', again and again.

The pressure vessel will hold max diff when pumped up on the ground, with no abnormal leakage.

Variable-rate controller (Cessna) has bench checked normal.

Safety and out-flow valve both have fresh yellow tags.

Sometimes the system functions normally, sometimes it does not.

I can't make the airplane do this when I want it to, so it's hard for a mechanic to diagnose.

What gives?

-pilotwithsinuspain

 

[jergar999]

What is the make and model of aircraft?

Edit: sorry, just noticed the Cessna in parentheses

 

[avbug]

Check the regulator filter. Also check out the static reference line for bugs that may swell in moisture or humidity.

Sounds like a reference line leak.

 

[bart]

I agree with avbug. Check the reference line.

You might also pay close attention to your manifold pressures and get the wastegates looked at. I flew an Aerostar that did some funny stuff in cruise and the problem turned up in the wastegate regulators.

 

[SabreFlyR]

I know you said the outflow valve has been checked, but it might be worth looking at it once more.
I had the same problem on a Sabre 40 and it turned out to be the valve was sticking on the shaft it slideson and would not move freely and was causing the same bumps in the pressure. We cleaned it up good and it's been ok ever since.

 

[340drvr]

Check the o-rings and sealing of the big butterfly ram air valves, we had trouble with those once.

 

[SuperFLUF]

Just ask Fletch.

Its the seventh svetzer valve. All you need to fix it is some gauze pads, anti freeze (preferrably Prestone) and some ball bearings.

Don't you know everything is ball bearings these days?

Wheeech!

 

[angola1]

well,

the o-rings/ valves are good.

the out-flow valve once had a case of the sticky shaft, but that has long since been healed by modern medicine.

the wastegate variable rate controllers are rock solid, MAP steady.

regulator fliter is new.

static reference line okay.

"ball bearings" greased.

any other guesses? this has got me totally stumped, which is not that hard to do. i am going to "borrow" a controller out of another airplane to see if maybe that will help. any other guesses appreciated.

-scratchingmyhead

 

[avbug]

Have you trouble-shot it using the maintenance manual? That's always a good place to start.

You may have a bad controller. You may have an intermittant pressure vessel leak; one that will hold at a certain differential pressure, but then act as a pressure relief valve, causing pressure fluctuations. Often the culprit is a bad door seal.

 

[angola1]

I have had two mechanics, both following the mtc manual, attempt to repair the problem.

The limited trouble shooting I am doing now, is just guess work on my part, before it goes to a different shop for a third attempt.

The controller is my #1 suspect. The pressure vessel holds very well throughout its entire range, and the door seal is in very good condition.

You guys continue with the ideas, if there are any others, and I'll post the solution when I finally get it fixed.

Thanks, and best regards.