Pressure Altitude
How to find Pressure Altitude (also known as Pressure Height - PHT) for an aerodrome at an elevation of 1500ft and a sea level pressure of 1003 hPa (QNH).
Simply follow these steps:
- From the QNE minus the QNH. (The answer is a difference of hPa)
- Multiply the difference by 30ft. (There is a 1 hPa change of pressure over 30ft of vertical distance in the atmosphere. This will change the difference of hPa into a difference of feet)
- Add the elevation to the difference of feet.
- The answer is the Pressure Height 1800ft.
Density Altitude
Find Density Altitude (also known as Density Height - DHT) for an aerodrome using the calculated Pressure Height of 1800 ft and an OAT of +20.
Simply follow these steps:
- The OAT is +20° at a PHT of 1800ft. Using the ISA temperature scale we find an ISA temperature of +11° at 1800ft PHT. Therefore the OAT is +9° warmer than the ISA temperature. (The ISA Variation is +9°.)
- OAT +20° - +11 ISA = ISA + 9° Difference
- The answer will be a difference of temperatures to the nearest 500ft. (Using the standard lap’s rate of 2° per 1000ft to find the ISA temperature at the calculated Pressure Height of 1800ft).
- Multiply the difference by 120ft. (There is a 1° degree change in temperature per 120ft of vertical distance through the atmosphere. (This will convert temperature into feet).
- Add the Pressure Height to the difference of feet.
- The answer is the Density Height 2880ft.