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The local air pressure (QFE, and therefore QNH as well) may change throughout the day. ATC will advise pilots of the new altimeter setting on a regular basis. How often do pilots truly update this altimeter setting?

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  • $\begingroup$ what have you searched? why are you not satisfied with what you found? $\endgroup$
    – Federico
    Commented Jun 17, 2022 at 7:23
  • $\begingroup$ Related, but IMO not exact duplicates: aviation.stackexchange.com/q/91330/54557, aviation.stackexchange.com/a/84232/54557. Note for any possible close-voters: I have personally observed a flight crew read back a new altimeter setting and fail to adjust the setting on their instruments. I feel like this is a question about technique rather than policy and deserves answers of its own. $\endgroup$
    – randomhead
    Commented Jun 17, 2022 at 11:12

3 Answers 3

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QNH (or QFE) rarely changes fast enough that a pilot will need to adjust their altimeter during the departure and arrival phases unless they happen to be in the area when a new observation is announced (typically once an hour).

En route is different; assuming you’re below the Transition Altitude, you’ll typically get a new QNH from every ATC sector you pass through. If you’re not talking to ATC, you are still required to set your altimeter to the QNH of a station within 100 miles, so you’ll be changing at least once an hour and likely more often unless your aircraft is exceptionally slow.

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A pilot typically does not make many changes to the pressure setting in flight. The pilot will set the altimeter to the local pressure setting before departure, and set it to the local pressure of the destination airfield before arrival. En route if the pilot is below transition altitude they will use the pressure setting as advised by ATC or using another source of information such as ATIS systems close to the route.

As a private pilot flying relatively short routes, say maximum of 3½ hours, in light aircraft the most I've ever had to change my altimeter setting in a flight is 5 times.

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A pilot will need to set the most accurate pressure setting for the task at hand such as landing or departing at a field or despatching troops or stores. Once above a set altitude know as Transition Altitude a standard pressure setting is used for deconfliction purposes in the cruise however QNH can be used below while being sure it’s updated. The crew will typically change the QNH / QFE when notified by ATC or the ATIS when they are parked or taxiing, or when in a stable flight stage during the initial approach or during the climb. However they are unlikely to mess with the altimeter during the final approach or take off roll.

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