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MINIMUM FLIGHT LEVEL ADJUSTMENT

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FAA Regulations, Instrument Rating

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The following is stated in the air traffic control procedures; what does it mean to the pilots?

4−5−5. ADJUSTED MINIMUM FLIGHT LEVEL

When the prescribed minimum altitude for IFR operations is at or above 18,000 feet MSL and the atmospheric pressure is less than 29.92”, add the appropriate adjustment factor from table 4−5−3 to the flight level equivalent of the minimum altitude in feet to determine the adjusted minimum flight level.

Altimeter Setting        Adjustment Factor

29.92” or higher          0000 feet

29.91” to 29.42”         0500 feet

29.41” to 28.92”         1,000 feet

28.91” to 28.42”         1,500 feet

28.41” to 27.92”         2,000 feet

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1 Answers



  1. Wes Beard on Jun 30, 2013

    The altimeter setting you place in the Kollsman window is based on the sea level pressure for the area you are flying in. Hence, if you were to be at sea level the altimeter will read zero.

    The sensitive altimeter is calibrated to sense decreasing pressure and through fears and linkages show a different altitude than zero. For every 1″mb drop in pressure, the altimeter will read one thousand feet higher.

    At 18,000 ft MSL the outside pressure will be around 12″mb. Since the altimeter is set to read sea level pressure we set inside the Kollsman window the sea level pressure and the alyimeter will read 18,000 feet.

    If the sea level pressure is lower than 29.92, the flight levels (based on 29.92) will start higher. See 91.121 showing the minimum flight level based on sea level pressure.

    Lastly, aircraft flying in the flight levels aren’t worried about hitting terrain but other aircraft. To solve that problem, all aircraft set the same altimeter setting so they can compare their altitude with other airplanes.

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