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:

7-2-2

[a] The suitability of departure alternate
airports, destination airports, and destination alter
nate 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 ..

[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) The FAA Flight Procedures & Airspace
Group, Flight Technologies and Procedures Division
may authorize temporary waivers to permit emergen
cy 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.

Barometric Altimeter Errors and Setting Procedures