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Altimeter Useage During IFR Flight

Asked by: 1775 views General Aviation, Instrument Rating

During IFR flight below Classs A, when ATC assigns an altitude, are they assuming that I will maintain that altitude based on the last altimeter setting they gave me (whether at the departure airport or in flight)?

As an example let's say the altimeter at my departure airport is 29.97, and my clearance is to 9,000. I set 29.97 before takeoff and level off at 9,000. Let's suppose after 15 minutes of cruise flight, I check the AWOS/ASOS/ATIS at an airport I'm passing by and the altimeter setting is 29.87. Center has not given me this new altimeter, I picked it up myself. Obviously if I set my altimeter to 29.87, my indicated altitude would drop to 8900.  Would I stay where I am or climb 100 ft back to my assigned altitude? ATC doesn't know I picked up this "rogue" altimeter (for lack of a better word). What if ATC had reported that new setting to me? Would they expect me to maintain 9,000 using 29.87?

In my mind it would only make sense to fly an IFR assigned altitude based on an altimeter setting only if the setting is provided by ATC, so they can ensure vertical separation of IFR traffic. I'm just wondering if using an altimeter setting that ATC has not provided would be a bad idea because of the loss of vertical separation?

I tried looking in the AIM but couldn't find any guidelines, so any reference to published materials would be appreciated. I understand there is no regulatory requirement for setting your altimeter, but I feel there must be something more to it when operating IFR. 




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1 Answers

  1. John D Collins on Dec 03, 2014

    Normally, the controller provides you with a nearby altimeter setting at each check in and if there is a major change reported along your route. 91.121 specifies how you make the altimeter setting and it is what you are required to comply with by regulation. If you note a significant difference between a nearby altimeter setting and the one provided by ATC, I would discuss it with them so you are both on the same page. When descending for an approach, I will often set my altimeter to the value in the ATIS or ASOS/AWOS when I obtain the airport information. Usually there isn’t enough of a difference between the airport altimeter setting and the one the controller provided, but I have to set the altimeter to the airport setting prior to commencing the approach anyway, so as I said, I normally switch it to that setting so I don’t forget to do it later.

    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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