Catastrophic Decompression; Small Place Solo

Mike Rapoport wrote:
"John Gilmer" wrote in message
...

"Mike Rapoport" wrote in message
rthlink.net...

"Ralph Nesbitt" wrote in message
odigy.com...


At altitude A/C cabins are like a pressure vessel. A/C cabins are
pressurized to maintain ~ 12 PSI Gauge, ~ the same as normal

atmospheric

pressure @ 11,000' ASL, in the cabin irrespective of altitude above

11,000'

ASL.
Ralph Nesbitt
Professional FD/CFR/ARFF Type

No, the cabin is not at a constant preasure above 11,000' (and the

number

that you are looking for is 8,000') unless the rate controller is set to

a

rate greater than the climb rate of the aircraft (which is never done).

If

what you say were true then the cabin altitude would not climb with the
airplane above 11,000' (8,000') which it clearly does. If you doubt

this,

go buy an altimeter watch and see for yourself. Or you could buy a
pressurized airplane.

OK, I'll bite.

Just how does the psia in the cabin track the psia "ambient."


What are you asking?

Mike

The comment was made that the pressure inside the cabin is not
necessarily constant above a certain altitude, unless certain
cabin rate settings were made, possibly leading to confusion
over cabin altitude vs. actual aircraft altitude.

It is a simplification to say that cabin altitude
is constant above a given aircraft altitude. In fact, the
cabin altitude will move toward a certain altitude (8,000'
in most cases, it seems) as set by the cabin pressurization
controller (or any of its several other names), IF the
outside pressure altitude is higher than the cabin altitude.
That is to say that if you take off from sea level and climb
to 10,000', the cabin altitude will steadily climb, at a rate
set by the cabin pressure controller, toward 8,000'. If you
were to level off at 5,000' and stay there long enough for
the cabin pressure to catch up, it will maintain 5,000'
until the aircraft begins to climb again, at which it will
again climb at the set rate until it reaches either the set
cabin altitude or the actual ambient pressure altitude. So
the cabin pressure can be below 8,000' (or whatever the
set altitude is) at actual altitudes above the set altitude,
but will be moving toward the set altitude in that case.

Note that cabin altitude cannot be maintained ABOVE the
actual pressure altitude, as air is continually pumped into
the cabin by the bleed air system, and is let out (or leaks
out) to maintain the set altitude or rate of change. If
you descend quickly enough, you may temporarily get a
cabin altitude above ambient, but air will not exit the
aircraft (outside pressure would prevent any air from
leaving through the outflow valve, and in some cases a
safety valve may automatically open to allow air to flow
INTO the aircraft, relieving the negative pressure
differential) and air flowing into the cabin will tend
to increase pressure rapidly until equilibrium is reached.

Having attained the pressure set by the controller,
the system will typically attempt to maintain that
pressure altitude until it is reset to another
altitude, although it may not always be able to.
Descents with low engine power may result in not
airflow into the cabin to match the outflow, due
either to the controller not closing the outflow
valve fast or far enough, or due to leaks in the
airframe letting more air out than the system
can pump in. Also, the cabin controller will
typically not allow a pressure differential above
a certain limit- too much pressure differential
stresses the fuselage. The controller will open
the outflow valve to maintain that maximum
differential, so at that point, if the aircraft
climbs then the cabin altitude will increase as
well.

Prior to or during the descent, the cabin
pressure controller would normally be set to
the field pressure altitude (which is normally
below the cabin altitude during cruise, but
this may not always be the case). The
controller will then begin to decrease the
cabin altitude, again at the set rate. This
prevents rapid pressure changes as the
aircraft descends through the cruise cabin
altitude and is more easier on the passengers
and crew.

Long and convoluted enough for you?

Mike
EC-130H Compass Call
"In Jam, no one can hear you scream"