Aviation Glossary - Density Altitude

Density Altitude

This altitude is pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the airplane’s performance.

source: FAA Airplane Flying Handbook (FAA-H-8083-3A)

The altitude in standard air at which the density is the same as that of the existing air.

source: FAA Aviation Maintenance Technician Airframe Handbook (FAA-H-8083-31)

The altitude in standard air at which the density is the same as that of the existing air.

source: FAA Aviation Maintenance Technician Powerplant Handbook (FAA-H-8083-32)

The altitude in the standard atmosphere at which the air has the same density as the air at the point in question. An aircraft will have the same performance characteristics as it would have in a standard atmosphere at this altitude.

source: FAA Aviation Weather for Pilots (AC 00-6A)

As defined in the Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25, “pressure altitude corrected for nonstandard temperature.” However, a more pertinent definition is that in the Airman’s Information Manual, which explains density altitude as being nothing more than a way to comparatively measure aircraft performance. See paragraph 7-5-6 of the AIM for a complete discussion.

source: FAA Balloon Flying Handbook (FAA-H-8083-11)

Pressure altitude corrected for temperature. Pressure and density altitudes are the same when conditions are standard. As the temperature rises above standard, the density of the air decreases, hence an increase in density altitude.

source: FAA Flight Navigator's Handbook (FAA-H-8083-18)

Pressure altitude corrected for nonstandard temperature variations. Performance charts for many older gliders are based on this value.

source: FAA Glider Flying Handbook (FAA-H-8083-13A)

Pressure altitude corrected for nonstandard temperature variations.

source: FAA Helicopter Flying Handbook (FAA-H-8083-21A)

Pressure altitude corrected for nonstandard temperature. Density altitude is used in computing the performance of an aircraft and its engines.

source: FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25A)

Pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the PPC's performance.

source: FAA Weight Shift Control Handbook (FAA-H-8083-5)

Density altitude is the altitude relative to the standard atmosphere conditions (ISA) at which the air density would be equal to the indicated air density at the place of observation. In other words, density altitude is air density given as a height above mean sea level. "Density altitude" can also be considered to be the pressure altitude adjusted for non-standard temperature.

Both an increase in temperature, decrease in atmospheric pressure, and, to a much lesser degree, increase in humidity will cause an increase in density altitude. In hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.

In aviation the density altitude is used to assess the aircraft's aerodynamic performance under certain weather conditions. The lift generated by the aircraft's airfoils and the relation between indicated and true airspeed are also subject to air density changes. Furthermore, the power delivered by the aircraft's engine is affected by the air density and air composition.

Air density is perhaps the single most important factor affecting aircraft performance. It has a direct bearing on:

The lift generated by the wings — reduction in air density reduces the wing's lift.
The efficiency of the propeller or rotor — which for a propeller (effectively an airfoil) behaves similarly to lift on wings.
The power output of the engine — power output depends on oxygen intake, so the engine output is reduced as the equivalent "dry air" density decreases and produces even less power as moisture displaces oxygen in more humid conditions.

Aircraft taking off from a "hot and high" airport such as the Quito Airport or Mexico City are at a significant aerodynamic disadvantage. The following effects result from a density altitude which is higher than the actual physical altitude:

The aircraft will accelerate slower on takeoff as a result of reduced power production.
The aircraft will need to achieve a higher true airspeed to attain the same lift - this implies both a longer takeoff roll and a higher true airspeed which must be maintained when airborne to avoid stalling.
The aircraft will climb slower as the result of reduced power production and lift.

Due to these performance issues, a plane's takeoff weight may need to be lowered or takeoffs may need to be scheduled for cooler times of the day. Wind direction and runway slope may need to be taken into account.

source: Wikitionary / Wikipedia and Related Sources (Edited)