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“Standard atmosphere at sea level”

Asked by: 2918 views General Aviation

The CAS is equal to TAS in a standard atmosphere at sea level. Isn't the "at sea level" redundant since standard atmosphere implies a constant temperature and pressure decrease as the altitude changes (eg: -2 deg C and 1''HG)?

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  1. Skyfox on Sep 22, 2016

    Standard atmosphere refers to 15°C (59°F) and 29.92″Hg (1013.25 mb) at sea level. What you\’re referring to is the standard lapse rate of 2°C (3.5°F) per thousand feet. Calibrated airspeed (CAS) is just indicated airspeed (IAS) corrected for installation and instrument errors. Here\’s something pasted from the Pilot’s Handbook of Aeronautical Knowledge:

    “True airspeed (TAS)—CAS corrected for altitude and nonstandard temperature. Because air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, for a given CAS, TAS increases as altitude increases; or for a given TAS, CAS decreases as altitude increases. A pilot can find TAS by two methods. The most accurate method is to use a flight computer. With this method, the CAS is corrected for temperature and pressure variation by using the airspeed correction scale on the computer.”

    So to answer your question, no, the CAS and TAS won’t remain the same as altitude increases above sea level, and the FAR definition of CAS isn’t being redundant by including “at sea level” in the definition.

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