DENSITY ALTITUDE

Sufficient lift must act upon the wing's airfoil for an aircraft to fly. As the air passes over and under the airfoil, a pressure differential occurs where pressure along the wing's lower surface is greater than that of the upper surface. The amount of lift achieved as the airfoil moves through the atmosphere is dependent on several factors, one being the density of the air itself. There are, of course, many other factors, but this blog focuses primarily on air density as an environmental factor affecting an aircraft's performance. To get a sense of how density altitude can affect aircraft performance, I've included a great video that I found.

What exactly is air density?

First, we must understand that air is a gas and, like any other matter (solids, liquids, plasma), has mass. All matter has density. Density is defined as "the mass of an object divided by its volume" (NASA, n.d.). Therefore, we can derive that air density is how much mass a given volume of air has. The more dense the atmospheric air is, the more molecules it contains.

This is important because the more mass the air has (more dense) the more force it can have upon another object. Simply put, the denser the air surrounding a wing as it flies, the more lift that can be achieved. Conversely, the less dense the air is the less lift that may be achieved. This is, of course, not taking into account any other factors such as speed or the shape of the airfoil.

Air Density vs. Altitude

Air density varies with altitude at a constant rate, at Standard Atmospheric Temperature. But first, we must understand how the atmosphere's temperature and pressure change about a rise in elevation. Standard temperature decreases at a rate of approximately 3.5 degrees Fahrenheit per thousand feet, up to 36,000 feet (Federal Aviation Administration, 2016). Standard pressure changes at a rate of 1 Hg per one thousand feet, up to 10,000 of elevation (Federal Aviation Administration, 2016). These standard lapse rates have been established by the International Civil Aviation Organization (ICAO), and are considered standard temperature and pressure, and any change from this standard is referred to as non-standard temperature and pressure (Federal Aviation Administration, 2016).

At standard temperature, the pressure altitude remains standard. When, at a given altitude, the temperature is no longer standard, air density changes as well. We call this change in air density outside of standard temperatures the Density Altitude. As previously discussed, as air density increases, aircraft performance will also increase. When air density is lower, aircraft performance will also thus be lower (Federal Aviation Administration, 2016). 

Aviators must know the density altitude to determine how the aircraft will perform. An aircraft may be at an elevation of 6,000 feet above sea level, however, due to temperature changes outside of the standard temperature, the air may be as though the aircraft were at 9,000 feet of elevation on a standard day. This can drastically change the performance of the airfoil. If improperly determined, the aircraft may not have enough thrust to sustain flight or it may not have enough runway to get to sufficient speed for rollout.

References

A.V. [AVWeb]. (2012, August 9). Airplane Crash In-Cockpit Footage: Stinson 108–3 [Video]. YouTube. https://www.youtube.com/watch?v=OVM3RRd1vf0&feature=youtu.be

Benson, T. (n.d.). Air Density. NASA. Retrieved July 15, 2022, from https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/fluden.html

Federal Aviation Administration, F. A. (2016). Pilot’s Handbook of Aeronautical Knowledge: FAA-H-8083-25B (Black & White). Independently published. https://www.faa.gov/sites/faa.gov/files/2022-03/pilot_handbook.pdf

Martin, S. (2017, March 21). How Density Altitude Caused A Plane Crash Shortly After Takeoff. Boldmethod Flight Training. Retrieved July 15, 2022, from https://www.boldmethod.com/learn-to-fly/performance/prevent-a-density-altitude-crash-on-takeoff/