Runway 06R

The calculation of density height requires a few bits of information:

Elevation, QNH and Temperature.

Example

Elevation: 8000 Feet
QNH: 1032 hPa
Temperature: +21 degrees

Use the QNH to work out the pressure height

1013 – 1032 = -19
-19 x 30 = -570
-570 + 8000 = 7430

So our pressure height is 7430 feet, we then use that number to work out the ISA temperature for the given height.

ISA temperature at ground is +15 degrees, and for every 1000 feet of height we loose 2 degrees so we round off to the nearest 500 feet so for our next calculation 7430 becomes 7500. So we then do the following +15 – (2 x 7.5 = 15) = 0 degrees

So our ISA temperature for our height is 0 degrees, however our actual temperature is +21 degrees so we work out the difference which in this case is 0, because our actual temperature is higher then the ISA temperature we refer to the difference with a + sign.

So our temperature is now +21 degrees.

We then apply an ISA deviation so +21 degrees X 120 = 2520 feet.

We then add that to our pressure height 2520 + 7430 = 9950 feet

So 9950 feet is our density height.

There are a number of ways to work out the pressure height of a field or landing strip, with any text book they visualize things I guess so you have some idea of why or what you hope to accomplish. Essentially the pressure height of a field is the starting point for a number of calculations relating to climbing, landing and take off, that is why it is given special consideration.

QNH stands for the atmospheric pressure at mean sea level and changes over time. The QNH is relayed to pilots via the ATIS or 120.9 VHF.

Then you have the ISA to worry about (International Standard Atmosphere), this is a set of standards that have been adopted world wide as an average set of conditions for the whole earth. The sea level pressure is set at 1013 hectopascals (hPa) and sea level temperature is 15 degrees centigrade.

Pressure drops at a rate of 1 hPa for every 30 feet of height gained, likewise temperature drops at a rate of 2 degrees centigrade for every 1000 feet gained in height. So according to ISA if you were flying 1500 feet above sea level you would have an air pressure reading of 963 hPa and a temperature reading of 12.5 degrees centigrade.

That being said the ISA lists averages which are never typically representative of actual conditions so a pilot must convert the actual conditions to its equivalent in ISA in order to run these against aircraft performance charts.

So if a field is 2100 feet above sea level and the QNH is 1002 then the pressure height of the field is 2430 feet. And we work that out by:

ISA sea level reading 1013 (hPa) – the QNH (1002) which equals 11, you then times that number by thirty feet (11 x 30) to get 330 feet. Add that to the field height and you get 2430 feet.

That figure can then be used to consult aircraft performance charts.

Air density, temperature and humidity levels all affect the performance of an aircraft and these must be taken account of when working out your landing and take off. When you are on the ground acquiring this information is pretty simple, switch the radio over to ATIS frequency and wait till they read out the QNH. Simply select the appropriate QNH on the altimeter and it automatically tells you the field pressure height.

However, this doesn’t work for when you are up in the sky and this information has to be calculated manually specifically when landing in adverse weather or at an unfamiliar airport.

Each particle of air contributes to the production of lift and drag, combustion of fuel to produce power and the generation of thrust. Air density suddenly becomes very important and is set by air pressure and temperature.

One way I look at it is that water is dense and as such makes it easy to swim in, it supports our weight. We can’t “swim” in air as it is much less dense then water. It might not be the correct way of explaining it but to me it does make a little more sense.

So above all air density effects the distance required for take off and landing as well as the climb factor. the higher the altitude the less dense the air is, likewise the hotter the day is the less dense the air is. So given that, one could rightfully assume that if you were taking off from a high altitude airstrip on a blazing hot day you would need to allow for greater take off distance.

I’ll jot down the methods of acquiring the correct pressure hight of a given field or landing strip in a future post.