You may have heard about Density Altitude, or vaguely remember working it out as part of your pilot training or exams, but haven’t heard much about it since.
What is Density Altitude?
Density Altitude is the altitude relative to standard atmospheric conditions. It tells you the air density as a height above sea level, or the pressure altitude for non-standard temperature.
If that sounds confusing, simply put it is the altitude that the aircraft will feel like it is at given the current atmospheric conditions, such as temperature. If the density altitude is 3,500ft, the aircraft will operate as if it is at 3,500ft when you take off, even though your airfield may be at sea level.
Why is Density Altitude Important?
If you fly from a big long runway you probably won’t think much about density altitude. But if you’re operating from short strips or in areas of high terrain, you’re going to want to think about it seriously as understanding the performance of your aircraft is important for operating it safely in these locations.
For example, on a hot and humid day the density altitude may be high, say 5,000ft. Your take-off run is therefore going to be more than double what it would be at sea level. With a particularly short strip and a heavy aircraft, this may mean you can’t get into the air safely in the distance available, or clear obstacles beyond the runway safely.
You need to be thinking about the combination of effects such as high temperature, high humidity, wind conditions, aircraft weight, airfield altitude and runway length available when these factors are likely to combine against you.
How to Calculate Density Altitude
You probably used your E6B computer when training to calculate density altitude. If you still remember this then that is great.
There are also a number of apps available for smartphones and tablets which can instantly give you the calculations needed. Examples include DensityAltitude+ and AutoDens, which can be found by searching your app store of choice.
Alternatively there’s a hand web calculator at this link to use: https://wahiduddin.net/calc/calc_da.htm
Another handy way of working out density altitude if you’re sitting in your aircraft is to turn the altimeter to the standard pressure setting (either 1013hp or 29.92mb) and see what altitude the dial reads at. This is the current density altitude where you are.