"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

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