Flight Training - October 2014 - 43

When you set your altimeter on the
ground to the correct setting, it should
read the elevation of the ramp on which
you're sitting. If it isn't within 75 feet,
your altimeter might need calibration.
If you're sure your altimeter is correctly
calibrated and a nearby altimeter setting
isn't available, you can set your altimeter
to show the airport elevation when you
are still on the ground.
Think of the altimeter setting as the
atmospheric pressure measured at the
bottom of a hole dug to sea level at the
airport. In pre-computer days, weather
observers calculated the altimeter setting using the weather station's elevation
and the station's actual barometric pressure, called the station pressure. Today's
automated weather observing systems
do these calculations.
SETTING YOUR ALTIMETER IN FLIGHT.

When you fly more than a few miles
from your takeoff point, you need to set
your altimeter to the nearest airport at
which you intend to land. In fact, on
a trip of more than an hour or so it's a
good idea to set it regularly, using settings from airports along your route.
5,00

If you receive VFR
5,000 feet
radar advisories
from air traffic
25.74 inches
4,000 feet
of Mercury
control, you'll be
5,000 feet
indicated al
titude
provided a current
25.74 inches
of Mercury
altimeter setting
every time you're
handed off to
another controller.
Airport A
Airport B
Sea level
30.74 altimeter setting
29.74 altimeter setting
The illustration
at right shows what
happens when you
don't do this when flying into an area
orange line labeled "5,000 feet, indicated
of low atmospheric pressure. To keep
altitude."
it simple, we're assuming both airports
Let's assume you're so busy holding an
are at sea level. The flight path would be indicated altitude of exactly 5,000 feet
the same if the airports were at different and enjoying the view that you neglect to
elevations.
obtain the altimeter setting for Airport
At Airport A, on the left, you set your
B and do not reset your altimeter. As you
altimeter to 30.74 and, since you're at sea arrive over B, your altimeter still reads
level, it reads 0 for the altitude. As you
5,000 feet. But, how high are you actuclimb, the altimeter senses lower and
ally above the sea-level airport? When
lower pressure until the pressure is 25.74 you reset your altimeter to Airport B's
inches of mercury. The altimeter reads
29.74 reading, you see that you are really
5,000, and you level off and head for
4,000 feet above the ground.
Airport B on the right.
The saying "high to low, look out
Since the surface pressure is lower
below" helps you remember what hapat B than at A, the pressure levels aloft
pens when you fly into an area of lower
slope down from A to B following the
atmospheric pressure. Your airplane will

0-fo

ot pr
essu

re le
vel

4,000
-

foot p
ressur
e

level

3,000-f
oot pre

1,000-foot pressure
level

True altitude

level

True altitude

2,000-foo
t pressure

True altitude

ssure le
vel

Sea level

30°C

15°C

0°C

WHEN THE AIR IS COLDER, the column of air shrinks.

OCTOBER 2014 FLIGHT TRAINING

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Flight Training - October 2014

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