Density height

A more appropriate term relating to aerodynamic performance under non-standard atmospheric conditions is the density height - the height under standard atmospheric conditions corresponding to a particular air density.

The density height is the pressure height corrected by a non-standard temperature. As the density of air increases (lower density height), aircraft performance increases, and conversely, as air density decreases (higher density height), air performance decreases. A decrease in air density means a high density height; an increase in air density means a lower density height. Density height is used to calculate performance. Under standard atmospheric conditions, the air at each level of the atmosphere has a specific density, and under standard conditions, the height of the pressure and the height of the density are the same. Thus, the density height is the vertical distance of a given density location at sea level under standard atmospheric conditions.

The calculation of the density height must take into account the pressure (pressure height) and temperature. Since aircraft performance at any altitude is based on air density under standard daytime conditions, this performance data applied to the air density altitude may be inconsistent with the altimeter indication. These heights cannot be calculated directly from the altimeter above or below the standard.

The density height is first calculated from the first measured pressure height and then corrected for non-standard temperature changes. Since the density varies directly with pressure and varies inversely with temperature, a given pressure height may be present over a wide range of temperatures when density is allowed to vary. However, a known density can occur at any temperature and pressure level. Of course, the density of the air has a significant impact on aircraft and engine performance. Regardless of the actual altitude at which the aircraft is operating, it will appear as if it were running at a height equal to the current density height.

For example, when set to 29.92, the altimeter may indicate a pressure height of 5000 feet. According to the Aircraft Flight Manual/Pilot Operating Manual, a ground run at take-off at standard temperature conditions may require a distance of 790 feet. However, if the temperature is 20 degrees Celsius above the standard, the expansion of the air increases the density height. Using the temperature correction data in a table or chart or using a computer to derive the density height, you may find that the density height is 7,000 feet and the required ground run distance may be close to 1000 feet.

Air density is affected by changes in height, temperature and humidity. High density height refers to lean air and low density height refers to dense air. Conditions that result in high density heights are high altitude, low atmospheric pressure, high temperature, high humidity, or some combination of these factors. Low altitude, high atmospheric pressure, low temperature and low humidity are more obvious signs of low density.

Using the flight calculator, the density height can be calculated by inputting the pressure height and the outside air temperature at the flight altitude. The density height can also be calculated by referring to the tables and charts in Figures 9-3 and 9-4.