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Re: [LML] Re: Airspeed sensing switch: Pressurization
Of course I wasn't suggesting manually blocking the
outflow valve. I'm saying you should consider all failure modes in the
analysis.
Based on the evidence presented so far, I think I'm
convinced that you can't get the plane stopped and have pressure left in the
cabin, even if the outflow valve were blocked.
Based on the evidence presented so far, I'm not
convinced it's impossible to land and have a little differential, given all
failure modes.
anyway, thanks for the info. that's
interesting about the mvp-50 pressure sensor.
----- Original Message -----
Sent: Monday, February 01, 2010 7:18
PM
Subject: [LML] Re: Airspeed sensing
switch: Pressurization (IV-P)
Colyn,
Yes,
to pressurize the cockpit, upper deck pressure must exceed ambient. I THINK
(but have not verified) that the pressurization controller will close
completely to achieve the commanded cabin altitude. If anyone knows about
this, please advise. I know there are some small leaks in my cabin that
provide some air exchange, but they are not very large. I say this because I
had a piece of masking tape over a ¼” shoulder harness attachment hole that
dumped into the baggage compartment (don’t ask!). The tape blew through the
hole one day, and in addition to the whistle (Judy said SHRIEK), the cabin
differential pressure dropped to less than 1 PSI…because of the leak. Power
was at low cruise (60% or so) so I might have done better with more power, but
I was surprised that a ¼” hole effectively kept the cabin from
pressurizing…
I
don’t know about the results of plugging the outflow valve, but it may be
academic? At least theoretically, a pressure differential can be generated
from any ambient (outside) pressure, but not sure how, or why one would try
plugging the Dukes controler outlet to do this? I actually covered and
sealed the whole pressure “bucket” in the rear seat when I was looking for
leaks, but my input was from the vacuum cleaner. As this effectively blocks
the emergency dump valve as well, I’d not consider doing this with another
pressure source, and particularly not with the engine, nor
in-flight…
On
the “failure condition” you ask about: I have a cabin pressure sensor hooked
to my MVP-50 engine monitoring system. It shows cabin altitude in feet, and is
alarmed to annunciate a cabin altitude below -500’ (overpressure on the
ground—but only at sea level) and a cabin altitude above 10,000’. I’ve only
seen it alarm during intentional tests, but any discrepancy between the cabin
altitude displayed on the MVP-50 and what’s set on the Dukes controller would
indicate a failure/problem, noting that this will occur when the differential
gets to 5.5 and the aircraft continues to climb. BTW, I don’t have a separate
cabin altitude gauge…the MVP works well for this, AND can be alarmed.
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