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consists of setting the barometric scale to the local altimeter
setting. The altimeter should indicate the surveyed elevation
3-5
Figure 3-6. The loss of altitude experienced when flying into an area where the air is colder (more dense) than standard.
of the airport. If the indication is off by more than 75 feet from
the surveyed elevation, the instrument should be referred
to a certificated instrument repair station for recalibration.
Differences between ambient temperature and/or pressure
causes an erroneous indication on the altimeter.
Inherent Altimeter Error
When the aircraft is flying in air that is warmer than standard,
the air is less dense and the pressure levels are farther apart.
When the aircraft is flying at an indicated altitude of 5,000
feet, the pressure level for that altitude is higher than it would
be in air at standard temperature, and the aircraft is higher
than it would be if the air were cooler. If the air is colder
than standard, it is denser and the pressure levels are closer
together. When the aircraft is flying at an indicated altitude
of 5,000 feet, its true altitude is lower than it would be if the
air were warmer. [Figure 3-6]
Cold Weather Altimeter Errors
A correctly calibrated pressure altimeter indicates true
altitude above mean sea level (MSL) when operating within
the International Standard Atmosphere (ISA) parameters of
pressure and temperature. Nonstandard pressure conditions are
corrected by applying the correct local area altimeter setting.
Temperature errors from ISA result in true altitude being
higher than indicated altitude whenever the temperature is
warmer than ISA and true altitude being lower than indicated
altitude whenever the temperature is colder than ISA.
True altitude variance under conditions of colder than ISA
temperatures poses the risk of inadequate obstacle clearance.
Under extremely cold conditions, pilots may need to add an
appropriate temperature correction determined from the chart
in Figure 3-7 to charted IFR altitudes to ensure terrain and
obstacle clearance with the following restrictions:
• Altitudes specifically assigned by Air Traffic Control
(ATC), such as “maintain 5,000 feet” shall not be
corrected. Assigned altitudes may be rejected if the
pilot decides that low temperatures pose a risk of
inadequate terrain or obstacle clearance.
• If temperature corrections are applied to charted
IFR altitudes (such as procedure turn altitudes, final
approach fix crossing altitudes, etc.), the pilot must
advise ATC of the applied correction.
ICAO Cold Temperature Error Table
The cold temperature induced altimeter error may be
significant when considering obstacle clearances when
temperatures are well below standard. Pilots may wish to
increase their minimum terrain clearance altitudes with a
corresponding increase in ceiling from the normal minimum
when flying in extreme cold temperature conditions. Higher
altitudes may need to be selected when flying at low terrain
clearances. Most flight management systems (FMS) with
air data computers implement a capability to compensate
for cold temperature errors. Pilots flying with these systems
should ensure they are aware of the conditions under which
the system will automatically compensate. If compensation
is applied by the FMS or manually, ATC must be informed
that the aircraft is not flying the assigned altitude. Otherwise,
vertical separation from other aircraft may be reduced
creating a potentially hazardous situation. The table in
Figure 3-7, derived from International Civil Aviation
3-6
Figure 3-7. ICAO Cold Temperature Error Table.
Organization (ICAO) standard formulas, shows how much
error can exist when the temperature is extremely cold. To
use the table, find the reported temperature in the left column,
and then read across the top row to the height above the
airport/reporting station. Subtract the airport elevation from
the altitude of the final approach fix (FAF). The intersection
of the column and row is the amount of possible error.
Example: The reported temperature is -10° Celsius and the
FAF is 500 feet above the airport elevation. The reported
current altimeter setting may place the aircraft as much as 50
feet below the altitude indicated by the altimeter.
When using the cold temperature error table, the altitude
error is proportional to both the height above the reporting
station elevation and the temperature at the reporting
station. For IFR approach procedures, the reporting station
elevation is assumed to be airport elevation. It is important
　
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