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AIM 

6/17/21 

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 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 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) 

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. 

7

2

Barometric Altimeter Errors and Setting Procedures