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AIM

10/12/17

7−2−1

Altimeter Setting Procedures

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.

b. EXTREME CAUTION SHOULD BE EXER-

CISED WHEN FLYING IN PROXIMITY TO

OBSTRUCTIONS OR TERRAIN IN LOW TEM-

PERATURES AND PRESSURES. This is especially

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.

NOTE−

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

possible.

(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

essential.

(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

conditions.