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3 Answers

Altimeter Setting

Asked by: 6776 views
Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Instrument Rating, Private Pilot, Student Pilot

altimeter setting must be adjusted as the flight progresses from one station to the next. If ATC service is not available along the route of the flight to update the Kollsman window,  what would be  the maximum distance to trust the previous setting? I also want to know if there is any time validity in case I don't have any means of updating the previous setting. 

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3 Answers



  1. John D Collins on May 26, 2014

    Follow the guidance in 91.121, quoted below for your convenience.

    Sec. 91.121 Altimeter settings.

    (a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating–
    (1) Below 18,000 feet MSL, to–
    (i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
    (ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or
    (iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure;

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  2. Mike Urban on May 26, 2014

    Thank you John for your answer.

    I understand (a) (1) (i) but in (a) (1) (ii), what would be the FAA definition for “an appropriate station”?

    if my destination is 150 nm from the departure airport and there is no source of alt setting available along the route of the trip, is it legal to continue the flight beyond 100 nm from the departure airport?

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  3. Kris Kortokrax on May 26, 2014

    Where in the lower 48 of this country would you find yourself in a position to not able to receive an altimeter setting within 100 NM?

    Given the large number of airports that have an AWOS/ASOS or ATIS coupled with communication capabilities with FSS through VORs and Remote Communication Outlets, it would be next to impossible to find yourself in a position where you could not obtain an altimeter setting. I’m sure someone with time on his hands might find a location more than 100 NM from any airport or VOR, but it would be a rare situation.

    In the course of your pre-flight planning, you should have the altimeter setting for your destination report. Barring any severe weather, the setting should not vary significantly during your trip.

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