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Aero Academic
Density altitude is pressure altitude corrected for nonstandard temperature, and it is the altitude your aircraft performs as though it were at. When air is thin, from high elevation, low pressure, high temperature or high humidity, density altitude rises and engine power, propeller thrust and wing lift all fall.
Checked against: FAA-H-8083-25C Pilot's Handbook of Aeronautical Knowledge, Chapter 4 (Principles of Flight: The Atmosphere, Pressure Altitude and Density Altitude) and Chapter 11 (Aircraft Performance: Structure of the Atmosphere).
| Factor | Condition | Effect on density altitude | Effect on performance |
|---|---|---|---|
| Elevation | Higher | Higher | Performance decreases |
| Atmospheric pressure | Lower | Higher | Performance decreases |
| Temperature | Higher | Higher | Performance decreases |
| Humidity | Higher | Higher (a contributing factor) | Performance decreases |
| Elevation | Lower | Lower | Performance increases |
| Atmospheric pressure | Higher | Lower | Performance increases |
| Temperature | Lower | Lower | Performance increases |
| Quantity | Standard value |
|---|---|
| Sea level temperature | 59 degrees F (15 degrees C) |
| Sea level pressure | 29.92 inches of mercury (1,013.2 mb) |
| Temperature lapse rate | About 3.5 degrees F (2 degrees C) per 1,000 ft, up to 36,000 ft |
| Pressure lapse rate | About 1 inch of mercury per 1,000 ft, up to 10,000 ft |
Aircraft instruments are calibrated, and aircraft performance data are written, for a standard atmosphere. At sea level that atmosphere has a temperature of 59 degrees Fahrenheit (15 degrees Celsius) and a pressure of 29.92 inches of mercury, which is 1,013.2 millibars. The International Civil Aviation Organization established this as a worldwide standard, often called the International Standard Atmosphere.
The standard atmosphere also has standard rates of change. Temperature decreases at about 3.5 degrees Fahrenheit, or 2 degrees Celsius, per thousand feet up to 36,000 feet. Pressure decreases at about 1 inch of mercury per thousand feet of altitude gain up to 10,000 feet. Any temperature or pressure that differs from these rates is called nonstandard.
The real atmosphere is almost never standard. That is why a set of related terms exists: pressure altitude, which accounts for nonstandard pressure, and density altitude, which then accounts for nonstandard temperature. Performance charts use them because the engine, the propeller and the wing respond to air density, not to the number on the altimeter.
Pressure altitude is the height above the standard datum plane, a theoretical level where the weight of the atmosphere is 29.92 inches of mercury. An altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If you set the altimeter to 29.92 and read the indicated altitude, you have the pressure altitude.
The handbook gives two methods. The first is setting the barometric scale to 29.92 and reading the indicated altitude. The second is applying a correction factor to the indicated altitude according to the reported altimeter setting. Both give the same quantity, which is the starting point for density altitude and for assigning flight levels at or above 18,000 feet.
On a day when the altimeter setting is below 29.92, the pressure altitude at the airport is higher than the field elevation, because the standard datum plane sits below sea level. That already puts the aircraft in thinner air than the elevation suggests, before temperature is considered.
Density altitude is the vertical distance above sea level in the standard atmosphere at which a given density is found. The handbook says it is determined by first finding pressure altitude and then correcting that altitude for nonstandard temperature variations. Because density varies directly with pressure and inversely with temperature, warm air at a given pressure altitude is less dense than standard air.
The practical meaning is stated plainly in the handbook: regardless of the actual altitude of the aircraft, it performs as though it were operating at an altitude equal to the existing density altitude. A field at 5,000 feet on a hot day can behave like a much higher airport.
To work a number, find the pressure altitude, compare the outside air temperature with the standard temperature for that altitude, and use your flight computer or the performance chart to correct for the difference. A common approximation, which is not a handbook figure, is to add about 120 feet for each degree Celsius above standard. Treat it as a quick check and use the chart or computer for planning.
The handbook lists three losses as air becomes less dense. Power falls because the engine takes in less air, so it burns less fuel and produces less power. Thrust falls because a propeller is less efficient in thin air. Lift falls because thin air exerts less force on the airfoils, so the wing needs a higher true airspeed to produce the same lift.
Together these change every phase of flight. Takeoff roll lengthens because acceleration is lower and a higher groundspeed is needed to lift off. Climb rate and climb gradient decrease, which matters when the departure end of the runway has terrain or obstacles. Landing roll lengthens because the touchdown groundspeed is higher for the same indicated airspeed.
The effect is not a small adjustment. On a hot, high airport, an aircraft can have noticeably less performance than the same aircraft at sea level on a standard day. The remedy is to know the number before you taxi.
Pressure, temperature and humidity all change density, and the handbook explains how each acts. At constant temperature, density is directly proportional to pressure: halve the pressure and the density halves. At constant pressure, density varies inversely with temperature, so heating the air makes it less dense.
In the atmosphere both pressure and temperature fall with altitude and they have conflicting effects, but the fairly rapid drop in pressure usually dominates, so density decreases with altitude. That is why a high airport has a higher density altitude than a low one even on a cool day.
Humidity matters less but is a contributing factor. Water vapor is lighter than air, so moist air is less dense than dry air, and as the water content increases, density altitude rises and performance falls. The handbook notes that humidity alone is usually not considered an important factor in calculating density altitude, yet warm and moist air, which is the lightest combination, is where it stacks on top of heat.
Take an airport with a field elevation of 5,000 feet, an altimeter setting of 29.92 and an outside air temperature of 25 degrees Celsius. With the altimeter at 29.92, the pressure altitude equals the field elevation, 5,000 feet.
The standard temperature at 5,000 feet follows from the standard lapse rate of 2 degrees Celsius per thousand feet: 15 degrees at sea level minus 10 degrees is 5 degrees Celsius. The outside air temperature of 25 degrees is therefore 20 degrees Celsius above standard, which is what pushes the density altitude above the pressure altitude.
Using the common approximation of about 120 feet per degree above standard, the correction is about 2,400 feet and the density altitude is about 7,400 feet. The aircraft will perform as though it were at roughly 7,400 feet, even though the altimeter reads 5,000. Your flight computer or the manufacturer chart gives the exact figure; the point of the example is to see the size of the effect and which direction it goes.
Planning starts before you reach the airport. Look at the forecast temperature and altimeter setting for the time you will depart, work out the density altitude, and then use the performance charts in the aircraft flight manual or pilot operating handbook for takeoff and landing distances and climb. Compare those figures with the runway length and with obstacles, with a margin.
Several habits help in hot or high conditions. Depart in the cooler part of the day when you can. Reduce weight, since weight lengthens the takeoff roll and reduces climb. Lean the mixture for best power as the flight manual directs for the altitude. Be ready to abandon the takeoff if acceleration is poor, and plan a climb that keeps you clear of terrain.
On the knowledge test, expect questions that ask which conditions result in the highest density altitude, which way density altitude moves when temperature or humidity changes, and how performance responds. The answers follow from one idea: less dense air is a higher density altitude and means worse performance.
Pressure altitude is the height above the standard datum plane, read by setting the altimeter to 29.92. Density altitude is pressure altitude corrected for nonstandard temperature, and it is the altitude at which the aircraft performs.
High elevation, low atmospheric pressure, high temperature, high humidity, or some combination of these. Lower elevation, high pressure, low temperature and low humidity indicate a low density altitude.
Yes. As air becomes less dense, the engine takes in less air and produces less power, the propeller is less efficient in thin air, and the wing produces less lift.
Yes, as a contributing factor. Water vapor is lighter than air, so moist air is less dense than dry air. Increased water content raises density altitude and decreases performance.
A temperature of 59 degrees Fahrenheit (15 degrees Celsius) and a pressure of 29.92 inches of mercury (1,013.2 millibars).
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