Density altitude: what it is and what it does

What density altitude is

Density altitude is pressure altitude corrected for non-standard temperature. It is the altitude in the standard atmosphere at which the air has exactly the density you find right now. Your aeroplane performs as though it were at that altitude, no matter how high it actually is. Only in standard conditions are pressure altitude and density altitude the same. A higher density altitude lowers aircraft performance, a lower one raises it.

It is one of five altitudes you must keep apart:

• Indicated altitude: the altitude shown on the altimeter.
• True altitude: height above mean sea level (MSL).
• Absolute altitude: height above ground level (AGL).
• Pressure altitude: the indicated altitude with the standard pressure of 1013 hPa (29.92 inHg) set.
• Density altitude: pressure altitude corrected for non-standard temperature.

EASA’s PPL(A) syllabus names density altitude under the altimeter and under flight performance (“Effect of density altitude and aeroplane mass”). One example from the FAA pamphlet: if an airfield at 500 ft has a reported density altitude of 5,000 ft, aircraft operating there perform as if the airfield were at 5,000 ft.

What raises it: high, hot, humid

Three things raise density altitude:

1. Altitude: the higher the airfield, the thinner the air.
2. Temperature: the warmer the air, the lower its density. If the temperature is above the standard temperature for the place, density altitude rises.
3. Humidity: moist air is lighter than dry air, so it is thinner. The FAA still does not rate humidity as a major factor, because it affects engine power more than aerodynamics. In very humid air the pamphlet advises adding 10% to the computed take-off distance and expecting a reduced climb rate.

High density altitude and high humidity do not always go together.

What it does to your aeroplane

When the air gets thinner, the FAA handbook PHAK says it reduces:

• power, because the engine takes in less air,
• thrust, because a propeller is less efficient in thin air,
• lift, because the thin air exerts less force on the wings.

For you as the pilot, a higher density altitude means, according to the FAA pamphlet:

• a longer take-off distance,
• a lower rate of climb,
• a higher true airspeed (TAS) at the same indicated airspeed (IAS) on approach and landing,
• a longer landing roll.

On take-off the aeroplane lifts off at the same indicated airspeed as at sea level, but the true airspeed is higher and the thrust is lower. On landing the deceleration stays the same, and the longer distance comes only from the higher true airspeed.

Finding density altitude

1. Set the altimeter to 1013 hPa (29.92 inHg). It then shows pressure altitude.
2. Read the outside air temperature (OAT).
3. Find the intersection of pressure altitude and temperature on a chart, or use a flight computer.

The FAA pamphlet contains a rule-of-thumb table for this. Here is an extract, with temperatures converted to °C and rounded. The cells show density altitude:

Elevation	Standard temperature	27 °C (80 °F)	32 °C (90 °F)	38 °C (100 °F)	43 °C (110 °F)
Sea level	15 °C	1,200 ft	1,900 ft	2,500 ft	3,200 ft
2,000 ft	11 °C	3,800 ft	4,400 ft	5,000 ft	5,600 ft
4,000 ft	7 °C	6,300 ft	6,900 ft	7,500 ft	8,100 ft
6,000 ft	3 °C	8,600 ft	9,200 ft	9,800 ft	10,400 ft