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Why FAR 91.144? Barometric pressure in excess of 31.00 or below 28.00Hg

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General Aviation

It is my understanding that FAR 91.44 restricts flights when barometric pressure exceeds 31.00Hg.  Does anyone know the basis for this regulation?  High pressure is generally associated with good weather.  Therefore, I would think this regulation exists due to a limitation of the altimeter...though I'm not sure.

I also think flights are restricted when the barometric pressure falls below 28.00.  Again, limitation of altimeter?

I wouldn't think the pressure would drastically fall below 28.00 or above 31.00 while in flight.  But should this occur, what would a pilot do?

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3 Answers



  1. John D. Collins on Feb 11, 2011

    The standard Baro altimeter is adjustable thru the barometric pressure range of 28.1 to 31.0 inches, so you just run out of adjustment capability on the altimeter.  Last week there was a large section of the country that had barometric pressure above 31.0 inches and recently in Alaska there was a commercial flight delayed for the same reason, it was sever blue VFR in both cases. Read section 7-2-2 in the AIM for a more complete what to do when it occurs.  I have included an excerpt for your convienence.
     
     
     
     
     
     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     


    2. When the barometric pressure exceeds 31.00 inches Hg.

     

     

     
    The following procedures will be placed in effect by NOTAM defining the geographic area affected:


    (a) For all aircraft.

     

     

     

    (b)

     

     
     
    (c) For aircraft with the capability of setting the current altimeter setting and operating into airports with the capability of measuring the current altimeter setting, no additional restrictions apply.


    (d)

     

     

     

    (e)

     

     

    Airports unable to accurately measure barometric pressures above 31.00 inches of Hg. will report the barometric pressure as “missing” or “in excess of 31.00 inches of Hg.” Flight operations to and from those airports are restricted to VFR weather conditions.For aircraft operating VFR, there are no additional restrictions, however, extra diligence in flight planning and in operating in these conditions is essential.During preflight, barometric altimeters shall be checked for normal operation to the extent possible. Set 31.00 inches for en route operations below 18,000 feet MSL. Maintain this setting until beyond the affected area or until reaching final approach segment. At the beginning of the final approach segment, the current altimeter setting will be set, if possible. If not possible, 31.00 inches will remain set throughout the approach.
     
    Aircraft on departure or missed approach will set 31.00 inches prior to reaching any mandatory/crossing altitude or 1,500 feet AGL, whichever is lower. (Air traffic control will issue actual altimeter settings and advise pilots to set 31.00 inches in their altimeters for en route operations below 18,000 feet MSL in affected areas.)

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  2. John D. Collins on Feb 11, 2011

    Sorry about the formatting of the previous post.
     
    The standard Baro altimeter is adjustable thru the barometric pressure range of 28.1 to 31.0 inches, so you just run out of adjustment capability on the altimeter.  Last week there was a large section of the country that had barometric pressure above 31.0 inches and recently in Alaska there was a commercial flight delayed for the same reason, it was sever blue VFR in both cases. Read section 7-2-2 in the AIM for a more complete what to do when the baro pressure is above 31.0 inches and 7-2-5 for what to do for barometric pressures below 28.0 inches  I have included an excerpt for your convenience.
     2. When the barometric pressure exceeds 31.00 inches Hg. The following procedures will be placed in effect by NOTAM defining the geographic area affected:
    (a) For all aircraft. Set 31.00 inches for enroute operations below 18,000 feet MSL. Maintain this setting until beyond the affected area or until reaching final approach segment. At the beginning of the final approach segment, the current altimeter setting will be set, if possible. If not possible, 31.00 inches will remain set throughout the approach. Aircraft on departure or missed approach will set 31.00 inches prior to reaching any mandatory/ crossing altitude or 1,500 feet AGL, whichever is lower. (Air traffic control will issue actual altimeter settings and advise pilots to set 31.00 inches in their altimeters for en route operations below 18,000 feet MSL in affected areas.)
    (b) During preflight, barometric altimeters shall be checked for normal operation to the extent possible.
    (c) For aircraft with the capability of setting the current altimeter setting and operating into airports with the capability of measuring the current altimeter setting, no additional restrictions apply.
    (d) For aircraft operating VFR, there are no additional restrictions, however, extra diligence in flight planning and in operating in these conditions is essential.
    (e) Airports unable to accurately measure barometric pressures above 31.00 inches of Hg. will report the barometric pressure as “missing” or “in excess of 31.00 inches of Hg.” Flight operations to and from those airports are restricted to VFR weather conditions.
    (f) For aircraft operating IFR and unable to set the current altimeter setting, the following restrictions
    apply:
     
    (1) To determine the suitability of departure alternate airports, destination airports, and destination alternate airports, increase ceiling requirements by 100 feet and visibility requirements by 1/4 statute mile for each 1/10 of an inch of Hg., or any portion thereof, over 31.00 inches. These adjusted values are then applied in accordance with the requirements of the applicable operating regulations and operations specifications.
    EXAMPLE− Destination altimeter is 31.28 inches, ILS DH 250 feet (200−1/2 ). When flight planning, add 300−3/4 to the weather requirements which would become 500−11/4.
    (2) On approach, 31.00 inches will remain set. Decision height (DH) or minimum descent altitude shall be deemed to have been reached when the published altitude is displayed on the altimeter.
    NOTE− Although visibility is normally the limiting factor on an approach, pilots should be aware that when reaching DH the aircraft will be higher than indicated. Using the example above the aircraft would be approximately 300 feet higher.
    7−2−5. Low Barometric Pressure When abnormally low barometric pressure conditions occur (below 28.00), flight operations by aircraft unable to set the actual altimeter setting are not recommended.
     

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  3. Anyonymous on Feb 12, 2011

    Interesting!
    So, if the pressure was too high (31.00Hg) or too low (28.00Hg) for flight, I would assume this information would be distributed in the form of a NOTAM, a weather briefer, or ATC.

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