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

"Ralph Nesbitt" wrote in message
m...
Hopefully all realize when referring to "Air Pressure" at altitude this
is
an "absolute" pressure value inside the fuselage irrespective of
ambient.
When referring to air pressures at ground level the pressure reading is
above unadjusted ambient barometric pressure.

This is incorrect, pressurization is the differential between ambient and
cabin preasure.

Mike
MU-2

Please reconsider your statement above as it applies to pressurization of
A/C cabins at altitude.

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

A/C pressure systems are set to operate from departure point. If set
manually from info provided by ATC this is "Unadjusted Barometric
Presser"
read from an instrument at the base of the Tower, a specified height
above
the ramp. Most modern commercial A/C are equipped with automatic systems
that capture relevant data upon command/that is reset for each flight.
Ralph Nesbitt
Professional FD/CFR/ARFF Type