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Alternate Static Source

Asked by: 31743 views Aircraft Systems, Flight Instructor, General Aviation, Instrument Rating, Weather

How does the Alternate Static Source work? Why do we close all vents and windows and turn on the heater and defoster?

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1 Answers

  1. Best Answer


    John D Collins on Jul 05, 2016

    From the FAA Handbook of Aeronautical Knowledge:

    The static chamber is vented through small holes to the free undisturbed air on the side(s) of the aircraft. As the atmospheric pressure changes, the pressure is able to move freely in and out of the instruments through the small lines which connect the instruments into the static system. An alternate static source is provided in some aircraft to provide static pressure should the primary static source become blocked. The alternate static source is normally found inside of the flight deck. Due to the venturi effect of the air flowing around the fuselage, the air pressure inside the flight deck is lower than the exterior pressure.
    When the alternate static source pressure is used, the following instrument indications are observed:
    1. The altimeter indicates a slightly higher altitude than actual.
    2. The ASI indicates an airspeed greater than the actual airspeed.
    3. The VSI shows a momentary climb and then stabilizes if the altitude is held constant.
    Each pilot is responsible for consulting the Aircraft Flight Manual (AFM) or the Pilot’s Operating Handbook (POH) to determine the amount of error that is introduced into the system when utilizing the alternate static source.

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