FAR/AIM: Section 2. Barometric Altimeter Errors and Setting Procedures

1. Instrument error.
2. Position error from aircraft static pressure systems.
3. Nonstandard atmospheric pressure.
4. Nonstandard temperatures.

NOTE-

EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN IN LOW PRESSURES AND/OR LOW TEMPERATURES.

1. Set the current reported airfield altimeter setting on the altimeter setting scale.
2. Read the altitude on the altimeter. The altitude should read the known field elevation if you are located on the same reference level used to establish the altimeter setting.
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.

1. Below 18,000 feet mean sea level (MSL).
   1. Barometric pressure is 31.00 “Hg or less.
      1. Set the altimeter to a current reported altimeter setting from a station along the route and within 100 NM of the aircraft, or;
      2. If there is no station within this area, use the current reported altimeter setting of an appropriate available station, or;

         NOTE-

         Air traffic controllers will furnish this information at least once when en route or on an instrument flight plan within their controlled airspace:

      3. If the aircraft is not equipped with a radio, set the altimeter to the elevation of the departure airport or use an available appropriate altimeter setting prior to departure.
   2. When the barometric pressure exceeds 31.00 “Hg., a NOTAM will be published to define the affected geographic area. The NOTAM will also institute the following procedures:
      1. All aircraft: All aircraft will set 31.00 “Hg. for en route operations below 18,000 feet MSL. Maintain this setting until out of the affected area or until reaching the beginning of the final approach segment on an instrument approach. Set the current altimeter setting (above 31.00 “Hg.) approaching the final segment, if possible. If no current altimeter setting is available, or if a setting above 31.00 “Hg. cannot be made on the aircraft's altimeter, leave 31.00 “Hg. set in the altimeter and continue the approach.
      2. Set 31.00 “Hg. in the altimeter prior to reaching the lowest of any mandatory/crossing altitudes or 1,500 feet above ground level (AGL) when on a departure or missed approach.

         NOTE-

         Air traffic control will issue actual altimeter settings and advise pilots to set 31.00 “Hg. in their altimeters for en route operations below 18,000 feet MSL in affected areas.

      3. No additional restrictions apply for aircraft operating into an airport that are able to set and measure altimeter settings above 31.00 “Hg.
      4. Flight operations are restricted to VFR weather conditions to and from an airport that is unable to accurately measure barometric pressures above 31.00 “Hg. These airports will report the barometric pressure as “missing” or “in excess of 31.00 “Hg.”.
      5. VFR aircraft. VFR operating aircraft have no additional restrictions. Pilots must use caution when flight planning and operating in these conditions.
      6. IFR aircraft: IFR aircraft unable to set an altimeter setting above 31.00 “Hg. should apply the following:
         1. The suitability of departure alternate airports, destination airports, and destination alternate airports will be determined by increasing the published ceiling and visibility requirements when unable to set the aircraft altimeter above 31.00 “Hg. Any reported or forecast altimeter setting over 31.00 “Hg. will be rounded up to the next tenth to calculate the required increases. The ceiling will be increased by 100 feet and the visibility by 1/4 statute mile for each 1/10 “Hg. over 31.00 “Hg. Use these adjusted values in accordance with operating regulations and operations specifications.

            EXAMPLE-

            Destination airport altimeter is 31.21 “Hg. The planned approach is an instrument landing system (ILS) with a decision altitude (DA) 200 feet and visibility 1/2 mile (200-1/2). Subtract 31.00 “Hg. from 31.21 “Hg. to get .21 “Hg. .21 “Hg rounds up to .30 “Hg. Calculate the increased requirement: 100 feet per 1/10 equates to a 300 feet increase for .30 “Hg. 1/4 statute mile per 1/10 equates to a 3/4 statute mile increase for .30 “Hg. The destination weather requirement is determined by adding the 300-3/4 increase to 200-1/2. The destination weather requirement is now 500-1 ¼..

         2. 31.00 “Hg. will remain set during the complete instrument approach. The aircraft has arrived at the DA or minimum descent altitude (MDA) when the published DA or MDA is displayed on the barometric altimeter.

            NOTE-

            The aircraft will be approximately 300 feet higher than the indicated barometric altitude using this method.

         3. These restrictions do not apply to authorized Category II/III ILS operations and certificate holders using approved atmospheric pressure at aerodrome elevation (QFE) altimetry systems.
      7. The FAA Flight Procedures & Airspace Group, Flight Technologies and Procedures Division may authorize temporary waivers to permit emergency resupply or emergency medical service operation.
2. At or above 18,000 feet MSL. All operators will set 29.92 “Hg. (standard setting) in the barometric altimeter. The lowest usable flight level is determined by the atmospheric pressure in the area of operation as shown in TBL 7-2-1. Air Traffic Control (ATC) will assign this flight level.

   TBL 7-2-1
   Lowest Usable Flight Level

   Altimeter Setting (Current Reported)	Lowest Usable Flight Level
   29.92 or higher	180
   29.91 to 28.92	190
   28.91 to 27.92	200

3. When the minimum altitude per 14 CFR Section 91.159 and 14 CFR Section 91.177 is above 18,000 feet MSL, the lowest usable flight level must be the flight level equivalent of the minimum altitude plus the number of feet specified in TBL 7-2-2. ATC will accomplish this calculation.

   TBL 7-2-2
   Lowest Flight Level Correction Factor

   Altimeter Factor	Correction Factor
   29.92 or higher	None
   29.91 to 29.42	500 feet
   29.41 to 28.92	1000 feet
   28.91 to 28.42	1500 feet
   28.41 to 27.92	2000 feet
   27.91 to 27.42	2500 feet

   EXAMPLE-

   The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and 29.43 “Hg, the lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude (TBL 7-2-1) plus 500 feet).