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the Instruments chapter). It measures
the pressure at a given altitude,
which is subtracted from the
pressure at sea level. The difference
is converted to get a height readout.
You would be very lucky to hit the
standard atmosphere more than, say,
25% of the time, so you need a
means of adjusting any instruments
based on it to cope with the
differences. An altimeter has a setting
window in which you can adjust the
figures for the correct pressure on
the ground by turning a knob on the
front (this is actually part of a very
important preflight check, where you
make sure that if you turn the knob
to the right, the height readings
increase, and vice versa. You also need
to check that the reading given
coincides with the airfield elevation,
±50 feet, and that, if you’ve got two
altimeters, they are within ± 50 feet
of each other (in other words, they
can misread by nearly 100 feet and
still be useable).
If you didn’t adjust your
instruments, and were flying
between different areas of air
pressure, you would not be at the
height you thought you were, which
is not that much of a problem if
everybody else uses the same setting,
as your relative height to other
aircraft would be maintained, but it
wouldn’t with respect to hard
objects, that is, obstacles, such as
mountains, television masts, etc.
As an example, if you were flying
from high to low pressure, your
altimeter would be overreading
(from HIGH to LOW, your
instrument is HIGH), so you would
be lower than planned and liable for
a nasty surprise. It’s therefore much
safer to be going the other way (that
is, from LOW to HIGH, where your
instrument is LOW).
124 Canadian Professional Pilot Studies
You can check what the difference is
with simple maths, using the figures
given above of 1" being equal to
1,000 feet. Remember that an
increase in pressure equals a decrease
in altitude, so if you start with 29.92,
then go to where it’s 30.92, the
altimeter reading would be 1,000 feet
less, even though the figures
themselves increase.
To convert from inches to millibars,
in case you have an old altimeter,
start at 29.92 and find the difference
between it and the current pressure.
Multiply that by 3.4 and apply it to
1013.2. For example, if the current
pressure is 30.02, that is, 1 above
29.92", add 3.4 mb and set 1016.6.
The standard atmosphere has a
temperature element that also affects
the altimeter. Remembering that, as
we said above, air density decreases
as it gets warmer, a point in your
imaginary column of air above a
station would be higher on a warm
day than otherwise (see above).
If, therefore, as is typical near the
Rockies in Winter, the air is very
much colder than standard, you will
be lower than you should be
(actually, the phrase above is still
valid, in that going from HIGH
temperature to LOW, your
instruments will be HIGH).
This is serious because, in low
temperatures, combined with other
effects caused by movement of wind
over ridges, you could be as much as
3000 feet below your projected
altitude, which could really spoil
your day. Another factor that arises
from the above diagram is the
creation of a wind purely from the
temperature difference. The cooler
column has a lower pressure at
altitude, and the warmer one has a
higher pressure, causing air to move,
from left to right, in this case.
Applying Buys Ballot's law, low
temperature is on the left in the
Northern Hemisphere if you stand
with your back to the wind.
When the surface temperature is well
below ISA, correct your altitudes by:
Surface Temp (ISA) Correction
–16°C to –30°C + 10%
–31°C to –50°C + 20%
–51°C or below + 25%
See also the Instruments chapter.
The altimeter setting is the station
pressure reduced to mean sea level
under ISA temperatures and
standard lapse rates, that is, taking
the elevation of the aerodrome into
account, expressed in inches of
mercury (Hg) in North America. It is
what your altimeter must be set to
when flying near aerodromes or
other places that may issue it,
because otherwise it will only tell you
your height above the point you
started from.
Its other importance is its use on
weather maps to create isobars. To
Weather 125
adjust station pressure for sea level,
　
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