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Aeronautical Information Manual (AIM), page 517

Index   516 -- Page 517 -- 518



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 and the elevation above the
altitude setting reporting source. It is these
"differences" that cause the error in indicated
altitude. When the column of air is warmer than
standard, you are higher than your altimeter indicates.
Subsequently, when the column of air is colder than
standard, you are lower than indicated. It is the
magnitude of these "differences" that determine 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 terrain,

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.

NOTE-
Non-standard temperatures can result in a change to

effective vertical paths and actual descent rates while
using aircraft Baro-VNAV equipment for vertical guidance
on final approach segments. A higher than standard
temperature will result in a steeper gradient and increased
actual descent rate. Indications of these differences are

often not directly related to vertical speed indications.
Conversely, a lower than standard temperature will result
in a shallower descent gradient and reduced actual descent
rate. Pilots should consider potential consequences of
these effects on approach minimums, power settings, sight
picture, visual cues, etc., especially for high-altitude or

terrain-challenged locations and during low-visibility
conditions.

d. TBL 7-2-3, derived from ICAO formulas,
indicates how much error can exist when operating in
cold temperatures. To use the table, find the reported
temperature in the left column, 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). The intersection of
the column and row is how much lower the aircraft
may actually be as a result of the possible cold
temperature induced error.

e. Pilots are responsible to compensate for cold
temperature altimetry errors when operating into an
airport with any published cold temperature
restriction and a reported airport temperature at or
below the published temperature restriction. Pilots
must ensure compensating aircraft are correcting on
the proper segment or segments of the approach.
Manually correct if compensating aircraft system is
inoperable. Pilots manually correcting, are respons-
ible to calculate and apply a cold temperature altitude
correction derived from TBL 7-2-3 to the affected
approach segment or segments. Pilots must advise the
cold temperature altitude correction to Air Traffic
Control (ATC). Pilots are not required to advise ATC
of a cold temperature altitude correction inside of the
final approach fix.

Altimeter Setting Procedures 7-2-3

Page 517 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   516 -- Page 517 -- 518