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8.4 Density Altitude


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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:

  1. From the QNE minus the QNH. (The answer is a difference of hPa)
  2. 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)
  3. Add the elevation to the difference of feet.
  4. 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:

  1. 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°.)
  2. OAT +20° - +11 ISA = ISA + 9° Difference
  3. 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).
  4. 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).
  5. Add the Pressure Height to the difference of feet.
  6. The answer is the Density Height 2880ft.