Pressure Calculation

When making calculations in altimetry, you can assume that 1 hPa corresponds to 27 ft for the JAR exams, even though in the real atmosphere the pressure lapse rate decreases as altitude increases, as discussed in the previous chapter.

Converting Between Height and Altitude

As discussed above, the vertical distance above aerodrome level is known as height. The vertical distance above mean sea level is altitude.
QFE is the pressure at aerodrome level. QNH is QFE reduced to sea level using ISA conditions. Therefore, if the airfield is above sea level, the QFE is of a lower pressure than the QNH, and the height is lower than the altitude.
If the airfield is below sea level (a rare occurrence, but not impossible) the QFE is higher than the QNH and the altitude is lower than the height.

Example 1:

An aircraft is flying at an altitude of 3500 ft. The QNH is 1010 hPa. The QFE is 988 hPa. What is the aircraft’s height?
1010 – 988 = 22 hPa
Using 27 ft per hPa, the elevation of the airfield must be 22 × 27 = 594 ft.
Hence the aircraft height must be 3500 – 594 = 2906 ft.

This situation is depicted graphically below:

Example 2:

An aerodrome has an elevation of 1500 ft. The QFE is 965 hPa. Calculate an approximate QNH.
1500 / 27 = 56 hPa
The airfield is above sea level so the QNH will be higher, hence:
QNH = 965 + 56 = 1021 hPa.

This situation is depicted graphically below:


Flight level and pressure altitude is based on a pressure setting of 1013 hPa. For example, if you have 1013 set on your altimeter and your reading is 35 000 ft, you are at a pressure altitude of 35 000 ft. Flight level is simply pressure altitude divided by 100. In this example, you are at Flight Level 350.

Altitude is based on the QNH at any particular time. This varies from place to place and with time. Look at the following example. As you can see, if QNH is lower than 1013 hPa, the altitude is lower than the pressure altitude. If QNH is higher than 1013 hPa, the altitude is higher than the pressure altitude.

As before, use 27 ft per hPa.

The diagram below shows the corresponding altitudes for FL 350 with a low QNH and with a high QNH.


Behnam Bidar

Commercial Pilot Of I.R.I And The Civil Aviation Training Group Supervisor

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Pressure Calculation

by Behnam Bidar time to read: 1 min