Aeronautical Information Manual (AIM), page 515

Index   514 -- Page 515 -- 516

Section 2. Altimeter Setting Procedures

7-2-1. General

a. The accuracy of aircraft altimeters is subject to

the following factors:

1. Nonstandard temperatures of the atmosphere.
2. Nonstandard atmospheric pressure.

3. Aircraft static pressure systems (position
error); and

4. Instrument error.
true in extremely cold temperatures that cause a large
differential between the Standard Day temperature
and actual temperature. This circumstance can cause
serious errors that result in the aircraft being
significantly lower than the indicated altitude.
Standard temperature at sea level is 15 degrees Celsius
(59 degrees Fahrenheit). The temperature gradient from
sea level is minus 2 degrees Celsius (3.6 degrees
Fahrenheit) per 1,000 feet. Pilots should apply corrections

for static pressure systems and/or instruments, if
appreciable errors exist.
c. The adoption of a standard altimeter setting at
the higher altitudes eliminates station barometer
errors, some altimeter instrument errors, and errors
caused by altimeter settings derived from different

geographical sources.

7-2-2. Procedures
The cruising altitude or flight level of aircraft must be
maintained by reference to an altimeter which must
be set, when operating:
a. Below 18,000 feet MSL.
1. When the barometric pressure is
31.00 inches Hg. or less. To the current reported
altimeter setting of a station along the route and
within 100 NM of the aircraft, or if there is no station
within this area, the current reported altimeter setting
of an appropriate available station. When an aircraft

is en route on an instrument flight plan, air traffic
controllers will furnish this information to the pilot at

least once while the aircraft is in the controllers area

of jurisdiction. In the case of an aircraft not equipped
with a radio, set to the elevation of the departure
airport or use an appropriate altimeter setting
available prior to departure.

2. When the barometric pressure exceeds
31.00 inches Hg. The following procedures will be
placed in effect by NOTAM defining the geographic
area affected:
(a) For all aircraft. Set 31.00 inches for en
route operations below 18,000 feet MSL. Maintain
this setting until beyond the affected area or until
reaching final approach segment. At the beginning of
the final approach segment, the current altimeter
setting will be set, if possible. If not possible,
31.00 inches will remain set throughout the ap-
proach. Aircraft on departure or missed approach will
set 31.00 inches prior to reaching any mandatory/
crossing altitude or 1,500 feet AGL, whichever is
lower. (Air traffic control will issue actual altimeter
settings and advise pilots to set 31.00 inches in their

altimeters for en route operations below 18,000 feet

MSL in affected areas.)
(b) During preflight, barometric altimeters
must be checked for normal operation to the extent

(c) For aircraft with the capability of setting

the current altimeter setting and operating into
airports with the capability of measuring the current
altimeter setting, no additional restrictions apply.
(d) For aircraft operating VFR, there are no
additional restrictions, however, extra diligence in
flight planning and in operating in these conditions is
(e) Airports unable to accurately measure
barometric pressures above 31.00 inches of Hg. will
report the barometric pressure as "missing" or "in
excess of 31.00 inches of Hg." Flight operations to
and from those airports are restricted to VFR weather

Altimeter Setting Procedures 7-2-1

Page 515 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   514 -- Page 515 -- 516