Aircraft altimeters are subject to the following errors and weather factors:
- Instrument error.
- Position error from aircraft static pressure systems.
- Nonstandard atmospheric pressure.
- Nonstandard temperatures.
- The standard altimeter 29.92 inches Mercury (“Hg.) setting at the higher altitudes eliminates station barometer errors, some altimeter instrument errors, and errors caused by altimeter settings derived from different geographical sources.
- Aircraft altimeters are subject to the following errors and weather factors:
Barometric Pressure Altimeter Errors
- High Barometric Pressure: Cold, dry air masses may produce barometric pressures in excess of 31.00 “Hg. Many aircraft altimeters cannot be adjusted above 31.00 “Hg. When an aircraft's altimeter cannot be set to a pressure setting above 31.00 “Hg, the aircraft's true altitude will be higher than the indicated altitude on the barometric altimeter.
Low Barometric Pressure: An abnormal low-pressure condition exists when the barometric pressure is less than 28.00 “Hg. Flight operations are not recommended when an aircraft's altimeter is unable to be set below 28.00 “Hg. In this situation, the aircraft's true altitude is lower than the indicated altitude. This situation may be exacerbated when operating in extremely cold temperatures, which may result in the aircraft's true altitude being significantly lower than the indicated altitude.
EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN IN LOW PRESSURES AND/OR LOW TEMPERATURES.
Altimeter Setting Procedures
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 the nearest available station, or;
Air traffic controllers will furnish this information at least once when en route or on an instrument flight plan within their controlled airspace:
- 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 will be published to define the affected geographic area. The 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.
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.
- 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.
Destination airport altimeter is 31.21 “Hg. The planned approach is an instrument landing system () 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 1/4.
31.00 “Hg. will remain set during the complete instrument approach. The aircraft has arrived at the DA or minimum descent altitude () when the published DA or is displayed on the barometric altimeter.
The aircraft will be approximately 300 feet higher than the indicated barometric altitude using this method.
- These restrictions do not apply to authorized Category II/III ILS operations and certificate holders using approved atmospheric pressure at aerodrome elevation () altimetry systems.
- 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.
- The FAA Flight Procedures & Airspace Group, Flight Technologies and Procedures Division may authorize temporary waivers to permit emergency resupply or emergency medical service operation.
- Barometric pressure is 31.00 “Hg. or less.
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 ENR 1.7-1. Air Traffic Control (ATC) will assign this flight level.
Lowest Usable Flight
29.92 or higher
29.91 to 28.92
28.91 to 27.92
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 ENR 1.7-2. ATC will accomplish this calculation.
29.92 or higher
29.91 to 29.42
29.41 to 28.92
28.91 to 28.42
28.41 to 27.92
27.91 to 27.42
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 ENR 1.7-1) plus 500 feet).
Aircraft operating in an offshore CONTROL AREA should use altimeter-setting procedures as described above, unless directed otherwise by ATC.
Aircraft exiting the oceanic / destined for the U.S. or transitioning through U.S. offshore control areas should use the current reported altimeter of a station nearest to the route being flown. When entering an oceanic / from U.S. offshore control areas, pilots should change to the standard altimeter setting 29.92 “Hg.
- Below 18,000 feet mean sea level (MSL).
- 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: