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Hot to cold, look out below!
The altimeter installed in the aircraft you fly is
designed
to sense the atmospheric pressure outside of the aircraft.
As a barometer it
is very accurate. As a gauge to assess our altitude, it has
some
serious shortcomings. Fortunately, all aircraft use the same
basic altimeter and therefore all have the same errors.
However, these errors don't typically cause a problem until
the temperature is very cold.
One of the more common altimeter errors is due to
non-standard
pressure. When the weight of atmosphere above our position
changes, the atmospheric pressure also changes. Less weight
means lower pressure; more weight means higher presssure.
Such a change in pressure is not a
problem since we can adjust our altimeter for non-standard pressure.
This is referred to as the altimeter setting and is adjusted
in
the Kollsman window (which could be an analog or digital gauge
depending on the installed equipment).
We update the altimeter setting periodically as
required by
regulations.
The density of the air below our flight
level also plays an important role. The density of air is a
function of both
temperature and moisture. An increase
in moisture and/or temperature decreases
the air density. Conversely, a decrease
in moisture and/or temperature increases
the air density. Since moisture has a much less significant
role,
we will focus on the role of non-standard temperature in this
discussion. Adjustments to account for non-standard pressure,
however, do not compensate for non-standard temperature.
Air is
a mixture of several gasses. When the temperature of air is
increased, for
example, the molecules in this mixture will achieve a higher kinetic
energy and space themselves farther apart resulting in a decrease in
air density.
As air temperature is decreased, the molecules in the mixture
will achieve
a lower kinetic energy and the molecules in the mixture are spaced
closer together resulting in an increased air density.
Let's assume that you are
flying at a constant pressure level of 850 mb resulting in an indicated
altitude of 5,000 feet. Now, imagine that the temperature in
the
air below
you suddenly increases. How will this change your indicated altitude?
Since the atmosphere is not a closed
container, increasing
the air temperature will cause it to expand in all
directions. To keep the explanation  simple,
let's limit the
discussion to expansion or contraction of air in the vertical.
The only
variable that has changed is the temperature. There has been
no
change in the mass (weight) of the air. In other words, we
have not
added or removed any molecules of air so the pressure at the surface
remains the same. Instead, the
same molecules in this air have simply moved farther apart, and
therefore, decreased
the air density.
In response to the sudden decrease in
density, all of the the pressure surfaces below you have expanded upward.
The 850 mb pressure surface you were flying is now above your current
altitude and now your altimeter is sensing a higher pressure.
This is because the pressure surfaces below you expanded
upward. In order to get back to the 850 mb pressure surface
you will need to climb since it is above you. Consequently,
the sudden decrease in air density due to the increase in temperature
caused your altimeter to now read a lower
altitude. Essentially, you have to increase your true
altitude to stay at a constant indicated
altitude of 5,000 feet. Assuming the earth is completely
flat, this places you farther from the surface of the earth after the
climb.
The opposite is true in response to a sudden
decrease in temperature (increase in density). In this case,
all of the pressure surfaces contract and move downward toward the
earth. This places the 850 mb level below you requiring that
you descend or lose true altitude to stay at a constant indicated
altitude of 5,000 feet. This places you closer to the surface
of the earth after the descent. As you might imagine when the
temperatures are much colder
than standard, this error can reduce your separation with obstacles
when flying close to the earth. This is summarized in the
graphic above. Click here to view a larger image.
This error increases in magnitude the
higher you are above the airport's elevation and only depends on the
temperature of the air below
you. If you are on the surface, there’s no air below you, so
there’s no error. As you increase your altitude above the airport, you
may begin to experience the effect of non-standard temperature. This
error is not exactly linear, but fairly close to being linear at the
altitudes general aviation pilots normally fly. The error is
about 4-percent for every 10°C departure from standard.
For every 10°C above (or below)
standard temperature there's approximately a 4-percent error in your indicated
altitude. Four percent of 3,000 feet is equal to 120 feet of
error in the indicated altitude. Using the table above, interpolate
between 170 feet and 60 feet (numbers from the first two rows in the
table under the 3,000 feet column circled in red), the error depicted
by this table is 115 feet at 5°C (or 10°C below standard assuming a
standard lapse rate up to 3,000 feet).
The magnitude in error increases with
increasing true altitude, but isn't quite enough to cause an issue
while en route. Where it can become an problem is on an
instrument approach to an airport. In very cold temperatures
you can be 50 to 100 feet below your decision height which is charted
as a true altitude based on standard temperature conditions.
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