Altimeter Setting During Instrument Approaches

4 Answers

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   John D. Collins on Jul 29, 2011

   I update the altimeter setting to the airport when I listen to the ATIS, ASOS, or AWOS.  It is usually within a few hundredths of the last enroute setting.  Any altimeter setting has more uncertainty the further one is away from the source of the reading and the higher one is above the source location. Waiting until cleared for the approach or the FAF inbound is too late in my opinion. I will normally get the local weather when in the terminal area in between when I switch to the final approach controller.

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   Andy Neumann on Jul 30, 2011

   Great question!  Just today I had a situation in which my descent altimeter setting (given to me by ATC) was about 0.2 inHg higher than the arrival airport altimeter setting.  I had my arrival altimeter setting because I listened to the ATIS about 30 minutes before landing.  I switched altimeter settings to local when I was about 15 minutes out from landing.  I think the most important thing here would be to set it before reaching the IAF (the FAF is too late).  If you are flying the minimum altitudes on the approach chart, you want the local altimeter setting. 
   I think this is a matter of personal preference, but I would consider incorporating an “instrument flow check” into your approach routine.  This comes AFTER you brief the approach, but before you actually begin the approach.  In addition to setting your altimeter for the approach, you would verify all radios are tuned and ID’d, all flight instruments adjusted properly (directional gyro, attitude indicator bars, etc.) and no flags showing, DME’s set up properly, course selector set, and so forth.  You could do this about 2 to 3 minutes before the IAF–say maybe 5 to 10 miles out?  Then you will have a reminder to double check your altimeter and have it set to the local setting. 

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   Matthew Waugh on Aug 04, 2011

   Check out 91.121 – once you’re within 100 miles of your destination set the destination altimeter setting and you’ll be good to go. I wouldn’t recommend fumbling around setting altimeters while you’re actually flying the approach.

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   Andy Neumann on Aug 04, 2011

   I don’t fully agree with Matthew on this one for two reasons.  Although that is indeed what the regulations specify, I take it as a maximum, but not necessarily the best procedure to follow.  Here are my reasons: the altimeter setting can change in the time it takes to fly 100 NM, especially if you fly a slow airplane.  I know you’re thinking, geez it wouldn’t change that much, but as an instrument pilot I want to have the most current and most accurate altimeter setting I can get.  That means I will listen to the weather just before I begin the approach and set my altimeter just before I begin the approach.  The second reason is that the geography and weather patterns can set up situations where the local altimeter varies fairly widely within that 100 NM radius.  For instance, this morning, Palm Springs was reporting 29.72 and Riverside (just 38 NM away) was reporting 29.89.  That’s approximately 170′–if you were to set the Riverside altimeter while over Palm Springs, you would be about 170′ low (all other things equal).  So, you have to use judgement and use the most appropriate altimeter setting for your location. 

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