Density Altitude

 

Density Altitude

Density Altitude (DA) is defined by the Federal Aviation Administration as "pressure altitude corrected for nonstandard temperatures" (FAA, 2008). This is better described by the Aircraft Owners and Pilots Association as the altitude at which the aircraft feels like it is flying (AOPA, 2022). This is important because depending on the environment, the limitations of the aircraft performance chart data may not support the aircraft flight plan. AOPA describes these effects as the "Triple H effect" (AOPA, 2022), as High Altitude, High Temperature, and High Humidity. The FAA simplifies this further by describing it as "High, Hot, and Humid" (FAA, 2008). At high altitudes, the air is thinner and less dense, which causes lift issues. The same environment will provide less oxygen to the engine for the proper mixture, ultimately requiring the engine to work harder. The same theory applies to hot and humid environments in relation to heat. The hotter or more humid the environment is the less dense the air becomes. 

The formula for DA is = PA +120(OAT - ISA)

OAT is Outside Air Temperature, and ISA is Standard Temperature based lines at 35 degrees Celsius and changes 1 degree for every 1,000ft. The difference in the reading needs to be multiplied by 120 and added to the Pressure Altitude (PA). 

One example I have for this topic is from December of 2003. I was in mountain flying school in Nevada, flying to a landing zone at 8,500 ft. MSL. As we were making our way up the side of the mountain, I heard the pilot call for more power, but the Ng level was already reading at 85%. I had seen the pilot moving the collective and remembered thinking to myself, that the aircraft doesn't have enough juice. It was obvious that in those conditions of cooler air and higher temperatures, the aircraft was working harder to maintain flight. The engines didn't have the proper air mixture and the air density was affecting the blade performance. 

References

Aircraft Owners and Pilots Association (2022) Density Altitude. Retrieved from: Density Altitude - AOPA

Aircraft Owners and Pilots Association (2022) Density Altitude - Beware of thin air. Retrieved from: Density Altitude - AOPA

Department of Transportation (2016) Pilot Handbook of Aeronautical Knowledge. FAA-H-8083-25B. Ch 11.  Retrieved from: Pilot's Handbook of Aeronautical Knowledge (25B) (faa.gov)

Federal Aviation Administration (2008) Density Altitude. Retrieved from: P-8740-2.indd (faasafety.gov)