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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 180
29.91 to 29.42 185
29.41 to 28.92 190
28.91 to 28.42 195
28.41 to 27.92 200
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 shall 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).
2/19/04 AIM
Altimeter Setting Procedures 7−2−3
7−2−3. Altimeter Errors
a. Most pressure altimeters are subject to
mechanical, elastic, temperature, and installation
errors. (Detailed information regarding the use of
pressure altimeters is found in the Instrument Flying
Handbook, Chapter IV.) Although manufacturing
and installation specifications, as well as the periodic
test and inspections required by regulations (14 CFR
Part 43, Appendix E), act to reduce these errors, any
scale error may be observed in the following manner:
1. Set the current reported altimeter setting on
the altimeter setting scale.
2. Altimeter should now read field elevation if
you are located on the same reference level used to
establish the altimeter setting.
3. Note the variation between the known field
elevation and the altimeter indication. If this variation
is in the order of plus or minus 75 feet, the accuracy
of the altimeter is questionable and the problem
should be referred to an appropriately rated repair
station for evaluation and possible correction.
b. Once in flight, it is very important to obtain
frequently current altimeter settings en route. If you
do not reset your altimeter when flying from an area
of high pressure into an area of low pressure, your
aircraft will be closer to the surface than your
altimeter indicates. An inch error in the altimeter
setting equals 1,000 feet of altitude. To quote an old
saying: “GOING FROM A HIGH TO A LOW,
LOOK OUT BELOW.”
c. Temperature also has an effect on the accuracy
of altimeters and your altitude. The crucial values to
consider are standard temperature versus the ambient
(at altitude) temperature. It is this “difference” that
causes the error in indicated altitude. When the air is
warmer than standard, you are higher than your
altimeter indicates. Subsequently, when the air is
colder than standard you are lower than indicated. It
is the magnitude of this “difference” that determines
the magnitude of the error. When flying into a cooler
air mass while maintaining a constant indicated
altitude, you are losing true altitude. However, flying
into a cooler air mass does not necessarily mean you
will be lower than indicated if the difference is still on
the plus side. For example, while flying at 10,000 feet
(where STANDARD temperature is −5 degrees
Celsius (C)), the outside air temperature cools from
+5 degrees C to 0 degrees C, the temperature error
will nevertheless cause the aircraft to be HIGHER
than indicated. It is the extreme “cold” difference that
normally would be of concern to the pilot. Also, when
flying in cold conditions over mountainous country,
the pilot should exercise caution in flight planning
both in regard to route and altitude to ensure adequate
en route and terminal area terrain clearance.
d. TBL 7−2−3, derived from ICAO formulas,
indicates how much error can exist when the
temperature is extremely cold. To use the table, find
the reported temperature in the left column, then read
across the top row to locate the height above the
airport/reporting station (i.e., subtract the airport/reporting
elevation from the intended flight altitude).
　
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