Altimeter Useage During IFR Flight
Asked by: Matthew Drooyan 3565 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.
Thanks,
Matt
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