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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
A charted IFR approach to the airport provides the following
data:
Minimum Procedure Turn Altitude 1,800 feet
Minimum FAF Crossing Altitude 1,200 feet
Straight-in Minimum Descent Altitude 800 feet
Circling MDA 1,000 feet
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 100-foot 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, 150 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 = 2,200 feet corrected
Minimum FAF Crossing Altitude
1,200 feet charted = 1,500 feet corrected
Straight-in MDA
800 feet charted = 900 feet corrected
Circling MDA
1,000 feet charted = 1,200 feet corrected
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 3-8) 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
3-7
Figure 3-8. Effects of Nonstandard Pressure on an Altimeter of an
Aircraft Flown into Air of Lower Than Standard Pressure (Air is
Less Dense).
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 3-8]
Altimeter Enhancements (Encoding)
It is not sufficient in the airspace system for only the pilot
to have an indication of the aircraft’s altitude; the air traffic
controller on the ground must also know the altitude of the
aircraft. To provide this information, the aircraft is typically
equipped with an encoding altimeter.
When the ATC transponder is set to Mode C, the encoding
altimeter supplies the transponder with a series of pulses
identifying the flight level (in increments of 100 feet) at
which the aircraft is flying. This series of pulses is transmitted
to the ground radar where they appear on the controller’s
scope as an alphanumeric display around the return for the
aircraft. The transponder allows the ground controller to
identify the aircraft and determine the pressure altitude at
which it is flying.
A computer inside the encoding altimeter measures the
pressure referenced from 29.92" Hg and delivers this data to
the transponder. When the pilot adjusts the barometric scale
to the local altimeter setting, the data sent to the transponder
is not affected. This is to ensure that all Mode C aircraft are
transmitting data referenced to a common pressure level. ATC
equipment adjusts the displayed altitudes to compensate for
local pressure differences allowing display of targets at correct
　
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