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 he airport elevation. It is important
to understand that corrections are based upon the temperature
at the reporting station, not the temperature observed at the
aircraft's current altitude and height above the reporting
station and not the charted IFR altitude.
To see how corrections are applied, note the following
example:
Airport Elevation 496 feet
Airport Temperature 50° C
The Minimum Procedure Turn Altitude of 1,800 feet will
be used as an example to demonstrate determination of
the appropriate temperature correction. Typically, altitude
values are rounded up to the nearest 100foot level. The
charted procedure turn altitude of 1,800 feet minus the airport
elevation of 500 feet equals 1,300 feet. The altitude difference
of 1,300 feet falls between the correction chart elevations of
1,000 feet and 1,500 feet. At the station temperature of 50°C,
the correction falls between 300 feet and 450 feet. Dividing
the difference in compensation values by the difference in
altitude above the airport gives the error value per foot. 
In this case, ISO feet divided by 500 feet = 0.33 feet for each
additional foot of altitude above 1,000 feet. This provides a
correction of 300 feet for the first 1,000 feet and an additional
value of 0.33 times 300 feet, or 99 feet, which is rounded to
100 feet. 300 feet + 100 feet = total temperature correction
of 400 feet. For the given conditions, correcting the charted
value of 1,800 feet above MSL (equal to a height above the
reporting station of 1,300 feet) requires the addition of 400
feet, Thus, when flying at an indicated altitude of 2,200 feet,
the aircraft is actually flying a true altitude of 1,800 feet.
Minimum Procedure Turn Altitude
1,800 feet charted =
Minimum FAF Crossing Altitude
1,200 feel: charted =
Straightin MDA
800 feet charted
Circling MIJA
1,000 feet charted =
Nonstandard Pressure on an Altimeter
Maintaining a current altimeter setting is critical because the
atmosphere pressure is not constant. That is, in one location
the pressure might be higher than the pressure just a short
distance away. Take an aircraft whose altimeter setting is set
to 29.92" of local pressure. As the aircraft moves to an area
of lower pressure (Point A to B in Figure 38) and the pilot
fails to readjust the altimeter setting (essentially calibrating
it to local pressure), then as the pressure decreases, the
true altitude will be lower. Adjusting the altimeter settings
compensates for this. When the altimeter shows an indicated
altitude of 5,000 feet, the true altitude at Point A (the height
above mean sea level) is only 3,500 feet at Point B. The fact
that the altitude indication is not always true lends itself to
the memory aid, "When flying from hot to cold or from a
high to a low, look out below." [Figure 38]
