AIM

4/3/14

7−2−3

Altimeter Setting Procedures

7

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2

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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).

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.

The possible result of the above example should

be obvious, particularly if operating at the minimum

altitude or when conducting an instrument approach.

When operating in extreme cold temperatures, pilots

may wish to compensate for the reduction in terrain

clearance by adding a cold temperature correction.