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AIM

10/12/17

7

−2−3

Altimeter Setting Procedures

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.