On-ground Pressurization Test

Stratus337 · #1 12-20-15, 09:26 AM

I've read several it is possible to hook a blower up to pressurize the cabin on the ground to look for leaks. I'm only getting about 1.5-1.7 psi on my 1973 P337.
Can someone tell me how to use the blower to pressurize on the ground so I can locate the leaks? It does not look like there is any ground sensing dump valve (squat switch that dumps pressure on landing), so maybe it is as simple as putting the blower at the right place?

Not expecting a perfect pressurization, but would like to be able to get more benefit from the system.

hharney · #2 12-20-15, 08:59 PM

I would suggest you find a reputable shop that is familiar with pressurized aircraft and save yourself a lot of grief. The pressurization system is pretty much the same on most Cessna's so there should be a shop in the area around you that can take care of your problem. It could be the valves are not closing completely or a door alignment issue. You might pay a little more in the beginning but at least you have a shop behind the work. Get some references and find some knowledgeable folks out there.

Stratus337 · #3 01-13-16, 09:04 AM

I'm trying to trouble shoot a bit more before starting the work to hopefully point the mechanic in the right direction. I flew yesterday on a 200 mile trip each direction (out and back). Because of winds I went down at 11,500 and came back at 6,500.

At 11,500 I was getting a 1.5x dif with a cabin around 8,000.

At 6,500 I was getting a 1.0x dif with a cabin around 4,000.

If I had a true leak I would have expected the dif to get to the same maximum level. With the lower dif at the lower altitude it makes me think that something may not be communicating correctly between the outflow valve and the controller. I don't have the service manual, but are those the only two components that ultimately control the outflow valve and the differential pressure?

To me, it appears the outflow valve is not working correctly to maintain the inputted pressure commands.

Any help would be appreciated if the facts above point to something someone else has dealt with.

JamesC · #4 01-13-16, 09:25 PM

TC output efficiency will increase (to a point) with altitude (colder air) and since the TC supplies the upper deck pressure and cabin pressure, there may be more air left over for at altitude for keeping the cabin pressure up. Otherwise diff should not change with altitude, but you are far enough from max diff that you can see it. That is my guess.
re 'home' pressurization test - do-able if you have the electric door seal option. Otherwise it is tough for a shop vac to blow enough PSI to inflate the door seal. Some have used one or two leaf blowers and a Halloween fog generator running inside the cabin on an inverter pack.
I agree with Herb - take it to a good shop with a huffer cart - they will find and seal the leaks much more quickly.

Stratus337 · #5 01-14-16, 10:49 AM

Thanks James. I have an inverter pack and fog machine (kids' haunted house). What I'm trying to figure out is the best way to use the leaf blower to get air into the cabin to provide some pressure. Can I simply "connect it" to the air inlet near the cabin on the leading edge of either wing?

I assume the master switch will need to be on to allow the outflow valve to close? I'm at 1,000 feet here so I assume I dial it to the lowest position before trying to pressurize.

I should be able to fill the cabin with the fog and shut the door before blowing in air. Making sure the outflow valve closes and actually getting the air into the cabin are really the only two things that I need to figure out.

I'm likely going to end up at a shop, but would like to do a basic test to see if anything obvious jumps out before committing to spend that time and money.

JeffAxel · #6 01-15-16, 01:05 AM

The outflow valve is closed with the power off. When you put the pressurization switch to "on", it takes power off that valve and the valve is closed, allowing pressurization. When you switch to "off", it sends power to the valve opening it and preventing pressurization. The P337 pressurization system is completely pneumatic, doesn't need any electricity to pressurize the cabin.

JamesC · #7 01-15-16, 07:32 PM

System is passive so battery switch can be off unless you have an electric door seal. That is why power failures at altitude in unpressurized mode can cause the cabin to pressurize suddenly, when the dump solenoid loses power.
I haven't done it but if I was doing it I would connect the blower via some kind of adaptor hose to the hose (after the inline venturi) entering the cabin through the firewall. eg. see attached for what the front engine part looks like. Note you would likely have to pull the rear engine dump valve as this also closes the connection from the cabin to the non-running engine.
Good luck - let us know how it goes.