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Why does a barometric pressure above 31.00 InHg trigger a TFR?

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    $\begingroup$ Because most Kollsman Altimeters cannot be adjusted that high, creating a serious risk to aircrews in less than VFR conditions. It's also quite an aberration as the usual air pressure range is between 29.5 and 30.50, making erroneous altimeter reading between 500-1000 ft MSL if aircrews forget to adjust the unit. $\endgroup$ Aug 10 '17 at 1:22
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    $\begingroup$ How often does this happen? $\endgroup$ Aug 10 '17 at 4:14
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That is because most of the legacy altimeter setting barometers cannot handle such high pressures. Most of them, like the one pictured below has a range from 28 to 31 inches of mercury.

Altitude barometer

Kollsman Altimeter Setting Indicator, image from analogweather.com

From Aeronautical Information Manual:

7-2-4. High Barometric Pressure

a. Cold, dry air masses may produce barometric pressures in excess of 31.00 inches of Mercury, and many altimeters do not have an accurate means of being adjusted for settings of these levels.

As a result, the TFR is triggered by 14 CFR 91.144

(a) Special flight restrictions. When ... barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight ...

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    $\begingroup$ Is this for all flights, all IFR flights or just in VMC? Seems like it wouldn't be that much of a problem in VMC. $\endgroup$
    – TomMcW
    Aug 10 '17 at 15:35
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    $\begingroup$ @TomMcW VMC flights use the hemispheric rule to reduce the chances of hitting another airplane. If you can’s set your altimeter, you can’t fly at the appropriate altitude. $\endgroup$
    – JScarry
    Aug 10 '17 at 15:38
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    $\begingroup$ An altimeter is also part of the required VFR equipment under 91.205 if it could not operate in the current atmospheric conditions it may be considered in-op and the plane not airworthy for VFR flight. $\endgroup$
    – Dave
    Aug 10 '17 at 15:57
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    $\begingroup$ @JScarry With a little more digging it appears that the TFR just imposes special rules as per NOTAM. AIM 7-2-2 describes what those procedures are. It appears that VFR flights are not restricted but advised to set 31.00 on the altimeter and use extra caution. $\endgroup$
    – TomMcW
    Aug 10 '17 at 17:52

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