AIM ¶ 7-2-3 — Altimeter Errors and Settings

Manufacturing and installation specifications, along with 14 CFR part 43, Appendix E requirement for periodic tests and inspections, helps reduce mechanical, elastic, temperature, and installation errors. (See Instrument Flying Handbook.) Scale error may be observed while performing a ground altimeter check using the following procedure: Set the current reported airfield altimeter setting on the altimeter setting scale. 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. 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. It is important to set the current altimeter settings for the area of operation when flying at an enroute altitude that does not require a standard altimeter setting of 29.92 “Hg. If the altimeter is not set to the current altimeter setting when flying from an area of high pressure into an area of low pressure, the aircraft will be closer to the surface than the altimeter indicates. An inch Hg. error in the altimeter setting equals 1,000 feet of altitude. For example, setting 29.90 “Hg instead of 30.90 “Hg. To quote an old saying: “GOING FROM A HIGH TO A LOW, LOOK OUT BELOW.” The aircraft cruising altitude or flight level is maintained by referencing the barometric altimeter. Procedures for setting altimeters during high and low barometric pressure events must be set using the following procedures: Below 18,000 feet mean sea level (MSL). Barometric pressure is 31.00 “Hg or less. Set the altimeter to a current reported altimeter setting from a station along the route and within 100 NM of the aircraft, or; If there is no station within this area, use the current reported altimeter setting of an appropriate available station, or; 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. 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: 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. 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. No additional restrictions apply for aircraft operating into an airport that are able to set and measure altimeter settings above 31.00 “Hg. 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.”. VFR aircraft. VFR operating aircraft have no additional restrictions. Pilots must use caution when flight planning and operating in these conditions. IFR aircraft: IFR aircraft unable to set an altimeter setting above 31.00 “Hg. should apply the following: 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. 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. 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. Air Traffic Organization (ATO) Service Center Directors, in their area of jurisdiction, may authorize temporary waivers to high barometric pressure flying NOTAM requirements to permit emergency supply, transport, or medical services, per 14 CFR § 91.144 (b). ATO Service Center contact information can be found at: https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/mission_support/sc . 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 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 Previous | Top | Next AIM | ATPUBS | FAA | Send your comments regarding this website.