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only pressure that should be there is
that from the pitot-static system.
Inside the instrument are two aneroid
capsules (vacuums), which are
corrugated for strength and kept
open with a large spring. They are
very sensitive, and their movements
as you go up and down are
magnified by a linkage that connects
them directly to the pointer. If they
expand, as they would when you go
up, the pointer increases the reading.
Outside, there is a small knob, which
is linked to a subscale, visible
through a small window. Rotating
the knob causes the subscale to
move and adjust the instrument to
whatever altimeter setting you are
flying through (see Weather).
Only in standard conditions will the
true altitude be indicated. For
example, when it is extremely cold, it
will be a lot lower than indicated, so
corrections must be applied
(altitudes given with radar vectors
from ATC are corrected already). If
this is something you need to take
note of, you could perhaps mark the
corrections directly on to the
approach chart, next to the heights
they refer to.
The dials work in a similar way to a
clock, in that the needles represent
different scales:
The long, thin pointer indicates
hundreds of feet and the short, wide
one, thousands. A very thin one,
maybe with an inverted triangle at
the end, like the one above, shows
feet in ten thousands.
Height is the vertical distance from a
particular datum, usually in the case
of aviation from the surface of an
airfield (QFE is used more in
Europe). Altitude is height above sea
level. Elevation is the vertical distance
of a point on the Earth's surface
from mean sea level.
Indicated altitude is what is shown on
the dial at the current altimeter
setting. Calibrated altitude is the
indicated altitude corrected for
instrument and position error (see
below). True Altitude is the actual one
above mean sea level, taking the
above errors into account, plus the
air temperature and density.
You can calculate true altitude with a
formula. First, subtract the ground
elevation from the indicated altitude,
and divide by 1,000 feet to get a
single decimal number. Next,
multiply that figure by the difference
between the ISA temperature and
the indicated one. Multiply that
figure by 4 ft to get the amount to be
subtracted from the indicated altitude.
Thus:
Ind Alt-Elevation x OAT-ISA x 4 ft
1,000
On the flight computer, put the PA
against the OAT in the appropriate
window and read the true altitude on
the outer scale against the indicated
one on the inner scale.
Pressure altitude is the height of a
particular pressure setting,
commonly 29.92 inches of mercury.
Instruments 69
Density altitude is the pressure altitude
corrected for non-standard
temperature.
The altimeter setting (QNH) is the
pressure at a point (or station, to be
technical), corrected for temperature
and reduced to mean sea level under
standard conditions, so if you set it
on your scale, you will see your
altitude (height above mean sea level),
or the airfield elevation when on the
ground. In the latter case, to be
serviceable, the altimeter should read
within ± 50 feet (multiple altimeters
should also be within ± 50 feet of
each other, so they can misread by
nearly 100' and still be OK).
Instrument and position errors will
have been calculated by the
manufacturers of the instrument and
aircraft, respectively, and will be
found in the flight manual. Position
error arises because there is no
perfect place to put the static ports
(or the pitot tube, for that matter,
when it comes to the ASI).
Altimeters also suffer from mechanical
error, due to misalignment in the
linkages and gears, temperature error,
particularly when cold, causing it to
over-read, elastic error (hysteretic), a
lag from stretching in the materials
used in the capsule, found after large
or rapid altitude changes, and reversal
error, a momentary display in the
wrong direction after an abrupt
attitude change.
An encoding altimeter is used with a
transponder in a Mode C system so
that a height readout can be shown
on a radar display.
Airspeed Indicator
To find airspeed, you need to
compare the general pressure outside
(the static pressure) with the dynamic
pressure from the aircraft's
movement through the air, so this
　
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