Jeff,
At 5.5 PSI differential, I start to lose cabin pressurization
between 27-28” of manifold pressure. Testing done between 16,000 and 17,000’. I
can maintain some pressurization down to about 24” but below that the cabin is
at ambient. I’ve actually pulled the power up high and verified that the cabin
air back-flows through the firewall mounted controller into the intake system.
Bob
PS: I don’t have a cabin dump switch on my instrument panel for
the reasons you lay out below.
Bob Pastusek
Upper deck has to be higher than the indicated MAP, at least
by 10" if we want to siphon off 10" for the 5 psi cabin differential
(assuming a perfectly sealed cabin).
The TBM has a back flow valve that limits the venting of
cabin pressure, something the LIVP does not have. Also, the outside
attendant can not open the cabin no matter what until we drop the door
seal. I can't even throw the door cleats back over center (from inside)
when the door seal is inflated.
The ultimate cabin dump switch is the door seal; it takes
less than a second to normalize cabin-to-ambient pressure. Throw that
anytime and everything vents out the door. If I had smoke in the cabin,
would I throw the Cabin Dump switch (on the panel) and open the Duke's Valve,
inviting all that smoke to travel past me towards the rear sear, or should I
deflate the door seal (switch by my elbow on the door)? In my mind (and
my POH), the door seal does the trick. So why even have a "Cabin
Dump" switch on the panel? I've asked myself this, but still I have
one.
I would love for every LIVP to fly level at 16000', cabin
differential 5psi (fully pressurized), 31" MAP, and start pulling MAP off
and observe when the cabin (and Duke's Valve) begins to lose
pressurization. I think mine is 28" MAP, but I'm working that
currently with the annual. If other's hold cabin 5 psi differential down
to 18", wow...that's a huge difference. You'd have bragging
rights. Post your numbers for the community.