Atmospheric Pressure and Aircraft Performance

Atmospheric Pressure

The atmospheric pressure is a variable factor that pilots must know very well since it is not only used in the essential instruments of the flight deck but is one factor that affects aircraft performance. Also, aircraft engineers need to understand all elements in the environment that affects performance, including pressure.

Sadraey defines pressure (2017) “as a normal force exerted by a fluid (gas or liquid) per unit area on which the force acts.” (p. 12). Considering pressure changes with altitude, temperature, and location, the International Civil Aviation Organization (ICAO) established a worldwide standard referred to as the International Standard Atmosphere (ISA). At sea level, the surface pressure is 29.92 inches of mercury or 1013.2 millibars. (FAA, 2016).

Standard sea level pressure.

Moreover, atmospheric pressure decreases as altitude increases. To illustrate, we must understand that air is compressible. Near the surface, the air is compressed by the air above it, but at higher altitudes, the air becomes less dense and pressure becomes less. Consequently, the rate at which pressure decreases with altitude is not linear (Sadraey, 2017). However, for simplicity, under the ISA, the rate at which pressure decreases with altitude is 1 “Hg per 1,000 feet of altitude gain to 10,000 feet.

The rate at which pressure decreases. 

Properties of a standard atmosphere.

As noted above, atmospheric pressure is one factor that affects performance, with it and the standard pressure we calculate pressure altitude. On the performance charts, we use pressure altitude to calculate different numbers, such as takeoff roll and landing roll. However, the performance calculated is valid only in standard atmospheric conditions, which is very unlikely to exist. For that reason, density altitude is calculated. 

References:

Pilot’s handbook of aeronautical knowledge : 2016  (First Skyhorse Publishing edition.). (2016). New York, NY: Skyhorse Publishing.

Sadraey, M. H. (2017). Aircraft performance: An engineering approach. Boca Raton, FL: CRC Press, Taylor & Francis Group. doi:10.1201/9781315366913