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Why do you have to put the current altimeter setting in the kollsman window? Why can't you just set it to field elevation when you're on the ground? I understand why you would have to do it in the air.

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    $\begingroup$ I have flown into places where there were no altimeter settings or weather for that matter. If there are a lot of traffic in the area, the pilots would agree on a altimeter setting to use to help separate traffic. Pressure constantly changes at an airport. It is conceivable, a plane practicing landings in the pattern may have a different altimeter setting (based on field elevation) than another plane taking off an hour later (based on field elevation). This could cause conflicts in the air if the same altimeter setting is not used. $\endgroup$ – wbeard52 Feb 19 at 13:45
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Good question. The reason is things fail.

If you set the altimeter to show field elevation, but the altimeter is faulty, you could end up flying differently from the rest, leading to loss of separation or worse.

But if you set the QNH, and then the reading is within 75 feet of the field elevation, you're good to go.

3. Altimeter Setting Procedures

3.1.3 If the difference from the known field elevation and the altitude read from the altimeter is plus or minus 75 feet or greater, the accuracy of the altimeter is questionable and the problem should be referred to an appropriately rated repair station for evaluation and possible correction. (USA AIP ENR 1.7)

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  • $\begingroup$ Besides the obvious IFR issues, I think the main reason that's so important is that if you are in controlled airspace flying an assigned alt and your altimeter is off, your xponder's encoder will be reporting your real alt and oops... busted! $\endgroup$ – John K Feb 19 at 2:58
  • $\begingroup$ @JohnK - for clarification, a transponder reports pressure altitude (based on 29.92 and does not depend on local altimeter/barometric setting) so the transponder response will always be correct. The important point about the correct barometric setting is so the pilot's altimeter will be accurate so he or she can fly the assigned altitude. $\endgroup$ – user2070305 Feb 19 at 3:52
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    $\begingroup$ Unless you are at a flight level, if a controller gives you an altitude assignment it will be local baro alt and will expect to see the equivalent pressure alt on his radar. If your altimeter is way off, when you fly a the assigned baro alt the encoder will be broadcasting the correct pressure alt, which will be off. Has to be significant, because the controller only sees nearest 100 ft. $\endgroup$ – John K Feb 19 at 5:17
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    $\begingroup$ @user2070305: Transponder response can be wrong. Some commercial pilot got "terrain terrain pull up", "stall", and "overspeed" over the ocean, called ATC to verify altitude, and they read his transponder back. He was at 50 feet at the time but that's not what the transponder said ... $\endgroup$ – Joshua Feb 19 at 19:43
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Because air pressure is not constant over wide areas. Weather provides us with high and low pressure areas, and you may not know the pressure at your destination when departing. Indeed, it may have changed en route. OTOH, barring a major earthquake, the field elevation is not going to change significantly.

In point of fact, I DO almost always set the altimiter to field elevation when departing, mainly because a lot of my flying is from smaller fields that don't even have Unicom. If I'm at a larger field, I might cross-check with the reported pressure, just to make sure things are working correctly.

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