What is the altimeter setting procedure when flying at or above 18,000 feet MSL?

7-2-3. Altimeter Errors
a. 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:
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.
b. 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
Barometric Altimeter Errors and Setting Procedures 7-2-1

AIM 2/20/25
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."
c. 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:
1. Below 18,000 feet mean sea level (MSL).
(a) 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.
(b) 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:
[a] 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)
7-2-2 Barometric Altimeter Errors and Setting Procedures

AIM2/20/258/7/25 AIM
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..
[b] 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.
[c] 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) 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.
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 Lowest Usable Flight
(Current Reported) 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).
Barometric Altimeter Errors and Setting Procedures 7-2-3

2/20/25 AIM
Section 3. Cold Temperature Barometric Altimeter
Errors, Setting Procedures and Cold Temperature
Airports (CTA)