Computes pressure altitude from an indicated altitude and QNH altimeter setting.
Pressure altitude is the altitude referenced to the ISA standard datum (29.92126 inHg /
1013.25 hPa) rather than local QNH. It is used for flight level assignment and
performance calculations.
The calculation derives the actual static pressure at the indicated altitude (using the
given QNH as the sea-level reference), then inverts the ISA pressure-altitude formula
to find the equivalent altitude on the standard datum. Uses the troposphere formula
throughout, which is accurate for all practical QNH use. QNH is only set below
FL180 in the US (and equivalent transition altitudes internationally); above that,
standard pressure (29.92 inHg) is used and indicated altitude equals pressure altitude
by definition.
Parameters
indicatedAltitudeFt: number
Indicated altitude in feet (read from altimeter set to QNH).
Computes pressure altitude from an indicated altitude and QNH altimeter setting. Pressure altitude is the altitude referenced to the ISA standard datum (29.92126 inHg / 1013.25 hPa) rather than local QNH. It is used for flight level assignment and performance calculations.
The calculation derives the actual static pressure at the indicated altitude (using the given QNH as the sea-level reference), then inverts the ISA pressure-altitude formula to find the equivalent altitude on the standard datum. Uses the troposphere formula throughout, which is accurate for all practical QNH use. QNH is only set below FL180 in the US (and equivalent transition altitudes internationally); above that, standard pressure (29.92 inHg) is used and indicated altitude equals pressure altitude by definition.