Density altitude and its effect on takeoff performance

Density altitude is the altitude the aircraft believes it is at. Every performance chart is ultimately keyed to air density, and density altitude is how a non-standard day is expressed in a number the charts can use.

The two corrections

Getting to density altitude takes two steps, applied in order. First correct field elevation for non-standard pressure to obtain pressure altitude. Then correct pressure altitude for non-standard temperature to obtain density altitude.

1. Pressure altitude = field elevation + (29.92 minus the altimeter setting) x 1,000.
2. ISA temperature at that pressure altitude = 15 minus 2 degrees Celsius per 1,000 ft.
3. Density altitude = pressure altitude + 120 x (actual OAT minus ISA temperature).

What high density altitude does

• The engine ingests less mass of air, so thrust or power falls.
• The propeller or fan does less work in thinner air.
• The wing produces the same lift only at a higher true airspeed, so the aircraft must accelerate to a higher true airspeed before it will fly.

The three effects compound. Takeoff distance increases, acceleration is slower, climb gradient and rate of climb both fall, and the limiting takeoff weight for a given runway drops. Indicated airspeeds for rotation and climb are largely unchanged, but the true airspeed and the ground roll needed to reach them are not.

Humidity

Water vapour is less dense than dry air, so high humidity reduces air density further. Most standard density altitude computations do not account for it, which means the true performance on a hot and humid day is slightly worse than the computed density altitude suggests. Treat the computed figure as the optimistic case.