(f) For aircraft operating IFR and unable to set
the current altimeter setting, the following restrictions apply:
(1) To determine the suitability of departure alternate airports, destination airports, and
destination alternate airports, increase ceiling
requirements by 100 feet and visibility requirements
by 1/4 statute mile for each 1/10 of an inch of Hg., or
any portion thereof, over 31.00 inches. These
adjusted values are then applied in accordance with
the requirements of the applicable operating
regulations and operations specifications.
EXAMPLE-
Destination altimeter is 31.28 inches, ILS DH 250 feet
(200-1/2 ). When flight planning, add 300-3/4 to the
weather requirements which would become 500-11/4 .

(2) On approach, 31.00 inches will remain
set. Decision height (DH) or minimum descent
altitude must be deemed to have been reached when
the published altitude is displayed on the altimeter.
NOTE-
Although visibility is normally the limiting factor on an
approach, pilots should be aware that when reaching DH
the aircraft will be higher than indicated. Using the
example above the aircraft would be approximately
300 feet higher.

(3) These restrictions do not apply to
authorized Category II and III ILS operations nor do
they apply to certificate holders using approved QFE
altimetry systems.
(g) The FAA Regional Flight Standards
Division Manager of the affected area is authorized to
approve temporary waivers to permit emergency
resupply or emergency medical service operation.

7-2-2

b. At or above 18,000 feet MSL. To 29.92 inches of mercury (standard setting). The lowest usable
flight level is determined by the atmospheric pressure
in the area of operation as shown in TBL 7-2-1.
TBL 7-2-1

Lowest Usable Flight Level
Altimeter Setting
(Current Reported)

Lowest Usable
Flight Level

29.92 or higher
29.91 to 29.42
29.41 to 28.92
28.91 to 28.42
28.41 to 27.92

c. Where the minimum altitude, as prescribed in
14 CFR Section 91.159 and 14 CFR Section 91.177,
is above 18,000 feet MSL, the lowest usable flight
level must be the flight level equivalent of the
minimum altitude plus the number of feet specified in
TBL 7-2-2.
TBL 7-2-2

Lowest Flight Level Correction Factor
Altimeter Setting

Correction Factor

29.92 or higher

none

29.91 to 29.42

500 feet

29.41 to 28.92

1000 feet

28.91 to 28.42

1500 feet

28.41 to 27.92

2000 feet

27.91 to 27.42

2500 feet

EXAMPLE-
The minimum safe altitude of a route is 19,000 feet MSL
and the altimeter setting is reported between 29.92 and
29.42 inches of mercury, the lowest usable flight level will
be 195, which is the flight level equivalent of 19,500 feet
MSL (minimum altitude plus 500 feet).

Altimeter Setting Procedures