Density Altitude Versus Pressure Altitude and Its Importance to Aviators (Credit: Federal Aviation Administration, & Meteorologist Jessica Olsen)

DISCUSSION: According to Aircraft Owners and Pilots Associations (AOPA), density altitude is pressure altitude corrected for nonstandard temperature. It has been referred to as the actual altitude at which the plane “feels” it is flying. Whereas pressure altitude is the indicated altitude when an altimeter is set to 29.92 Hg. This density altitude has a direct correlation to aircraft performance. Typical performance is based on 15°C and 29.92 Hg, often found in the Pilots Operating Handbook (POH). However often when flying, conditions are rarely as the published conditions are a brief reference for performance.
 
The Federal Aviation Administration has put out a publication on Density Altitude, in an effort to increase pilot awareness of the subject when flying in other than standard conditions. High density altitude corresponds to reduces air density and thus a reduced aircraft performance. Keeping density altitude in mind, there are 3 factors that contribute to increased density altitude:

• Altitude: Generally, the higher the altitude the less dense the air is, a concern for airports at higher elevations. This coupled with increased temperatures could pose difficulties for operations in airports experiencing extreme temperatures and increased elevation
• Temperature: Typically, the warmer the air, the less dense it is. To see this, when temperature increases above standard (15°C) the density for that location is decreased. A remedy to this would be to push departures and arrivals at the effected airport to times of day when increased temperatures are not prevalent.
• Humidity: Often related to engine power rather than aerodynamic efficiency, humidity is not generally considered a major factor in density altitude computations, but ordinarily high-density altitude and high humidity do not always correlate, however is high humidity does exist, it is common to add 10% to your computed takeoff distance and anticipate a reduces climb rate.

 
Effects on the aircraft are decreased aerodynamic performances, and diminished engine horsepower output. This in turn will influence takeoff distance, power available, climb rate and landing distance. Density altitude is said to increase takeoff distance, decrease rate of climb, increase true airspeed on approach and landing and increased landing roll, all critical elements in a pilot’s operation of the aircraft. The FAA recommend that if the Airplane Flight Manual (AFM)/Pilots Operating Handbook (POH) are unavailable use the Koch Chart to calculate adjustments. In addition to this, this does not account for leaning the mixture may be necessary for maximum power on takeoff, otherwise adding to the decreased performance if mixture is not leaned.
 
© 2019 Meteorologist Jessica Olsen
 
 
 
 
Referenced:
Federal Aviation Administration. (2008). Density Altitude. Retrieved April 15, 2019, from https://www.faasafety.gov/files/gslac/library/documents/2011/Aug/56396/FAA P-8740-02 DensityAltitude[hi-res] branded.pdf