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6/17/21 

AIM 

Section 2.  Barometric Altimeter Errors and Setting 

Procedures 

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1.  General 

a. 

Aircraft altimeters are subject to the following 

errors and weather factors: 

1. 

Instrument error. 

2. 

Position error from aircraft static pressure 

systems. 

3. 

Nonstandard atmospheric pressure. 

4. 

Nonstandard temperatures. 

b. 

The standard altimeter 29.92 inches Mercury 

(“Hg.) setting at the higher altitudes eliminates 
station barometer errors, some altimeter instrument 
errors, and errors caused by altimeter settings derived 
from different geographical sources. 

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2.  Barometric Pressure Altimeter 

Errors 

a. 

High Barometric Pressure: Cold, dry air masses 

may produce barometric pressures in excess of 31.00 
“Hg. Many aircraft altimeters cannot be adjusted 
above 31.00 “Hg. When an aircraft’s altimeter cannot 
be set to pressure settings above 31.00 “Hg, the 
aircraft’s true altitude will be higher than the 
indicated altitude on the barometric altimeter. 

b. 

Low Barometric Pressure: An abnormal 

low

pressure  condition exists when the barometric 

pressure is less than 28.00 “Hg. Flight operations are 
not recommended when an aircraft’s altimeter is 
unable to be set below 28.00 “Hg. In this situation, the 
aircraft’s true altitude is lower than the indicated 
altitude. This situation may be exacerbated when 
operating in extremely cold temperatures, which may 
result in the aircraft’s true altitude being significantly 
lower than the indicated altitude. 

NOTE

 

EXTREME CAUTION SHOULD BE EXERCISED WHEN 
FLYING IN PROXIMITY TO OBSTRUCTIONS OR 
TERRAIN IN LOW PRESSURES AND/OR LOW TEM-
PERATURES. 

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3.  Altimeter Errors 

a. 

Manufacturing and installation specifications, 

along with 14 CFR Part 43, Appendix E requirement 

for periodic tests and inspections, helps reduce 
mechanical, elastic, temperature, and installation 
errors. (See Instrument Flying Handbook.) Scale 
error may be observed while performing a ground 
altimeter check using the following procedure: 

1. 

Set the current reported airfield altimeter 

setting on the altimeter setting scale. 

2. 

Read the altitude on the altimeter. The 

altitude should read the known field elevation if you 
are located on the same reference level used to 
establish the altimeter setting. 

3. 

If the difference from the known field 

elevation and the altitude read from the altimeter is 
plus or minus 75 feet or greater, 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. 

It is important to set the current altimeter 

settings for the area of operation when flying at an 
enroute altitude that does not require a standard 
altimeter setting of 29.92 “Hg. If the altimeter is not 
set to the current altimeter setting when flying from 
an area of high pressure into an area of low pressure, 
the aircraft will be closer to the surface than the 
altimeter indicates. An inch Hg. error in the altimeter 
setting equals 1,000 feet of altitude. For example, 
setting 29.90 “Hg instead of 30.90 “Hg. To quote an 
old saying: “GOING FROM A HIGH TO A LOW, 
LOOK OUT BELOW.” 

c. 

The aircraft cruising altitude or flight level is 

maintained by referencing the barometric altimeter. 
Procedures for setting altimeters during high and low 
barometric pressure events must be set using the 
following procedures: 

1. 

Below 18,000 feet mean sea level (MSL). 

(a) 

Barometric pressure is 31.00 “Hg or less. 

(1) 

Set the altimeter to a current reported 

altimeter setting from a station along the route and 
within 100 NM of the aircraft, or; 

(2) 

If there is no station within this area, use 

the current reported altimeter setting of an 
appropriate available station, or; 

NOTE

 

Air traffic controllers will furnish this information at least 

Barometric Altimeter Errors and Setting Procedures 

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