Density Altitude Calculation

DEFINITIONS:

DENSITY ALTITUDE (DA) - the height your aircraft 'thinks' it is at. Atmospheric conditions determine this. Your engine will behave accordingly.

ALTITUDE with QNH set - your actual height above sea level.

HEIGHT QFE - your height above ground level. This is specific to the actual spot where you set the QFE.


WHY CALCULATE DENSITY ALTITUDE?


Because there are days that, although you and your aircraft are at 5500ft AMLS, your aircraft will perform as though it is at 8500ft, or higher, on take-off.

Because there are days that, at Sea Level, your aircraft will behave as though it is already at 3000ft on take-off.

It's not the aircraft's fault. It's D-e-n-s-i-t-y A-l-t-i-t-u-d-e.

I am going to keep this DA calculation very practical and user friendly. Of course there are different methods to do the same thing.


HOW TO CALCULATE DENSITY ALTITUDE:

Step 1.

Determine the Pressure Altitude:

Look in the aircraft, and set the Altimeter to the Standard setting, (QNE), 1013 hPa or 29.92inches of Mercury . Read off the corresponding altitude on your Altimeter. This is the Pressure Altitude where you are, for today.

If there is a high pressure system around, your Altimeter will read a lower altitude than your actual height AMSL, if there is a low pressure system around, your altimeter will read as though you are higher than your actual height AMSL. It is possible for the PA to be the same as Standard, in which case your altimeter will read correctly. (1 hPa = 30ft)

Step 2.

Work out the effect of Temperature:

Look at the Outside Air Temperature (OAT) There should be one somewhere in the cockpit. Calculate what ISA would deem as your actual ISA temperature at your current altitude AMSL. eg. at Sea Level in Richards Bay, it should be 15° C. Truth is, it usually isn't. Lets assume it is mid summer, and is 35° outside, excluding the effect of the 80% humidity. Yes, it feels like a warming drawer, and every sane person is hiding inside a building with the air-con on full blast. So it should be 15° C but it is 35° C; a good 20° C hotter than ISA.

The calculation to work out your DA is take the difference between the ISA temperature and the actual temperature, (in this case 20° C) and multiply that number by a constant 120 ft (we round up from 118.8 ft per ° C to 120 ft for easier calculations... it's close enough and errs on the side of safety).

20 x 120 = 2400'

Altitude Temperature according to ISA
8000 ft -1°C
7000 ft 1°C
6000 ft 3°C
5000 ft 5°C
4000 ft 7°C
3000 ft 9°C
2000 ft 11°C
1000 ft 13°C
0 ft 15°C

Step 3.

Put it all together:

Now add 2400' to your Pressure Altitude you got in step 1 above, and VOILA, you have the Density Altitude. Roughly.

Please note, this calculation EXCLUDES the effect of humidity.

I haven't been able to find a formula for humidity, but would recommend adding 20% to your take off distance if your humidity is 80%.

Remember, when it is humid, water particles are taking up the space that would otherwise have been air particles, in effect making the air less dense, and giving your propeller less air to "bite" into, thereby reducing the performance, (both engine and aerodynamic), of your aircraft.

It is always a good idea to make a quick density altitude calculation during your preflight.

From Density Altitude back to Plane Talk

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