Density altitude is a measure of atmosphere density int terms of an altitude under standard conditions.
Density altitude is a measure of density and the point of reference is standard air pressure and temperature
At sea level standard temperature is 59F (15C) and standard pressure is 29.92Hg (inches of mercury)
Altitudes:
- True altitude is the altitude above sea level
- Absolute altitude is the height above the ground
Altitudes in terms of air density:
- Indicated altitude is the altimeter set to local barometric pressure
- Pressure altitude is the altimeter set to 29.92
- Density altitude is pressure altitude corrected corrected for nonstandard temperature
On a standard day indicated altitude, pressure altitude, and density altitude are all the same.
Barometric pressure decreases by 1″ Hg per 1000 feet.
At higher altitude the air is thinner than it was at sea level, the airplane will perform poorly.
Higher True Airspeed needed to achieve the same Indicated Airspeed.
High density altitude describes low density air
- Warm air is less dense than cooler air
- Humid air is less dense than drier air
- Low pressure air is less dense than higher pressure air
- Air pressure decreases with altitude
Airplanes perform better in denser air
High density altitude reduces an airplane’s total performance. Propeller efficiency is reduced because it exerts less force at high density altitudes.
Hot, humid air and lower barometric pressure yield high density altitude.
When high density altitude exists (beware of short runway):
- Expect more sluggish takeoffs and climbs
- Expect longer ground rolls and higher ground speed during landings