background image

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.