We make a few assumptions before we begin
this test. They are, the Turbos are spinning at the proper
speed and providing a full 31.5 in. at 2400 rpm up to 24,000
feet. This will insure that there is sufficient volume
available to pressurize the cabin. Check all hoses from the
intercoolers to the mix valve for rub holes, good clamps,
etc. etc. Use Skeet (2 ply) vs. Scat hose.
Given the fact that we have sufficient
volume and air pressure available at the mix valve, we can
check the downstream side from the mix valve.
The way to approach the subject is to
eliminate the elements that could be leaking. I'll give you
a list that if you check off each one, by the time you get
to the end you should have it fixed.
If your A&P has a large capacity air
compressor that will be a great help. An alternate
pressurization device could be a large capacity shop vacuum
connected to the blow side. I use a 6hp Sears shopvac.
First---Check the outflow valve.
Electrically and mechanically.
Remove the outflow valve and check the
electrical canon plug against the wiring diagram. Especially
check the dump switch and the gear handle microswitch.
Turning off the door seal will depressurize faster than any
Connect the electrical canon plug.
With power on, operate the pressurization
controller to both upscale and down scale positions.
The outflow should open and close and fit
snugly against the seat. The bladder will not be extended
but you should be able to determine if the center collar is
that the end of the shaft.
Reinstall the valve and make the absolutely
certain that the nose cone of the pressurization valve is
free and clear of all lines fittings etc. etc. under the
bucket. I had this problem and it drove me crazy. Use
silicone around the bottom of the valve itself to seal into
Check the filter on the outflow valve for
I also would remove the vacuum source line
and cap it. I do not believe it does any good and will be a
problem source down the road. I put my controller to the
highest range as soon as I descend below 5000 feet. This
puts the outflow valve open and stops the rattle of my NACA
scoop air inlet valve on the pressure bulkhead. And allows
full airflow in from the NACA scoop for cooling. I then
close off the engine air inlet (which puts all engine bleed
air overboard). All this is assuming no Air Conditioning.
For takeoff I have maximum ventilation until
I climbed airplane to 8000 feet (approximately) I roll the
altitude selector down to 8000 feet and open the engine air
valve slowly. This keeps a cabin cool, (as possible) and I
have less ear problems.
Remove the static source aluminum line and
cap the portion that goes overboard. This allows the cabin
pressure to force the outflow valve closed. If there's any
doubt that the outflow valve is not operating properly put a
plate over the bucket and seal that.
Remove the control wire from the master
Check for leaks
Check the mix valve to have adequate tension
on the temperature disk that rotates (controls hot vs. cold
source air) (pre intercooler vs. post air cooler air). Boy
if that isn't a joke! Both are hot. Disconnect both inlets.
Put your finger in the holes and make sure the disk is in
the proper position related to the cable handle connection
(and labeled properly). Make sure or shaft has a spring and
it is tightened down so that coils will barely allow a
business card between them.
Prepare the interior
Instant glue a nylon thread to the bat
(handle) of the door seal switch. Run the thread out the
door and tape it into position so that pulling on the thread
can deflate (turn off) the door seal.
Turn on door seal switch.
Check the outflow valve static port
(aluminum line) is opened to atmosphere--and the outside
port is capped.
Also remove and cap the pressure port to the
elevator compensator. (You may want to connect this to an
Check the over pressure valve is tightened
and installed in the proper direction. If in doubt-- put a
piece of tape over it.
Turn on the master switch.
Prepare the exterior
Close and latch door handles.
Connect master switch control wire to relay.
Door seal should pump up. Connect vacuum cleaner or air
compressor and start pressurization. A mechanic's
stethoscope will help. A microphone on a wand with that a
headset to listen is also very useful.
Vacuum cleaner should give over 2 psid. Air
compressor ovthaner 4 psid. Look in the window at the
differential pressure gauge. Or---connect an extension hose
to the elevator compensator line and connect your own
pressure gauge. The 0-10 psi range would be correct.
To check for air leaks:
Put your hand around the door. Check the
door seal for leaks. And check around the door handles. Lock
(not much you can do about this one).
Check the firewall.
Especially check the overboard dump valve
(adjustment) (1 in. line) coming from the left side of the
Minor leaks from the throttle- prop- mixture
cables are okay.
Leaks around wiring hydraulic fittings fuel
fittings etc. should be sealed.
Remove the lower fuel line access plate at
the wing root. Check for leaks of around fittings (pitot,
nav-antenna etc.) and wiring pass-throughs. Check around the
Open the gear doors and check side skirts
and seat bottom. Check for outflow valve leaks and
Check elevator boots.
Check around safety belt pass through
cutouts. This is a common leak source.
Disconnect and check for air passing into
the fresh air inlet door in the upper center of the pressure
You can probably think of a few other places
When finished--- disconnect the master
switch relay control wire.
Pull thread that is connected to the door
seal switch. The seal should deflate. If it pulls off or
becomes disconnected for some reason, take the rest of the
day off and come back tomorrow. It will take that long for
the seal to deflate on its own. Don't try to force door open
with seal inflated.
Once you have exhausted your ability to find
leaks from the outside, have someone sit inside the cabin
with the seats, and sidewalls removed. Pressurize -- and try
a smoke source around the potential leak areas.
To seal difficult areas, I coated fiberglass
cloth material on both sides with silicone (wet) and used it
to seal around areas such as safety belt and hardware etc.
For all other areas, I used the straight GE
clear silicone from Wal-Mart.