Operating Environment and Aircraft Performance

Atmospheric Pressure

Atmospheric pressure is different than many of the other environmental factors for aviators.  Because it is not easily seen, like precipitation, atmospheric pressure can be (and has been) a contributing factor in aviation mishaps.  

Pressure altitude is the term used when referring to the height above the standard sea level pressure (SDP or Standard Datum Plane).  The standard atmospheric pressure at sea level is 29.92 ”Hg or 1,013.2 mb (pictured below).  In the cockpit of an aircraft, 2992 is dialed in to the barometric altimeter when above a certain altitude.

When high pressure altitude is combined with other factors, such as high temperature and high humidity, the result can be what is known as a high density altitude (Goodman & Small, 2018).  

Chapter 4 of the Pilot’s Handbook of Aeronautical Knowledge states: 

“Density altitude is pressure altitude corrected for nonstandard temperature.”

A simpler way to say this is – It is the atmospheric pressure that is corrected for:

• Field Elevation
• Temperature
• Humidity

This means the runway that is listed at 6,000 feet on the chart supplemental may not feel like it is at 6,000 feet when operating an aircraft because the plane could behave like it is at 10,000 feet once the other factors affect the aircraft.  

Those factors include:

• Decreased lift – thin air means less fluid passing over and around the wing
• Reduced engine performance – the engine is receiving less air to the compressor
• Reduced thrust – the propeller produces less thrust with less fluid to push or pull

A good mitigation strategy in order to prevent aviation mishaps include:

• Proper mission planning (know the conditions of the airfield)
• Plan take off and landings when the Outside Air Temperature (OAT) is at its lowest point
• Plan for longer distances to take off and land (see chart below) to compensate for higher velocity to offset reduced lift (see chart below)

Word Count: 301

References:

Federal Aviation Administration. (2019, February 22). Mountain Flying and High Density Altitude in 57 Seconds [Video]. YouTube. https://www.youtube.com/watch?v=k1qylDKtr6M

Federal Aviation Administration. (2016). Pilot’s Handbook of Aeronautical Knowledge (PHAK). https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/

Goodman, C. J., & Small Griswold, J.,D. (2018). Climate Impacts on Density Altitude and Aviation Operations. Journal of Applied Meteorology and Climatology, 57(3), 517-523. http://dx.doi.org/10.1175/JAMC-D-17-0126.1