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