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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 by 4
ft to get the amount to be subtracted
from 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
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
instrument is connected to both the
static and dynamic pressure systems.
It's similar to the altimeter inside,
except that the capsule is fed directly
with dynamic pressure, and its size
varies in direct proportion to it. The
needle, connected to the capsule, will
read airspeed directly. Some aircraft,
such as the Bell 407 helicopter, have
a dampened needle, which will
indicate the speed you have been, and
not the speed you are at.
The instrument may be calibrated in
knots or mph, that is, a rate of change
of distance per unit of time. There
are several variations, however:
·  Indicated airspeed (IAS) is read
directly, without corrections.
·  Calibrated airspeed (CAS) is the
IAS corrected for instrument and
position errors, which are highest
at low speeds (IAS and CAS will
be about the same at speeds
above cruise). It's known by
older pilots as the Rectified
Airspeed (RAS).
·  True Air Speed (TAS) is the CAS
corrected for altitude and
temperature (remember its
70 JAR Private Pilot Studies
original calibration is based on
the standard atmosphere). The
slide rule part of the flight
computer is used to calculate
these, discussed below. On
average, the TAS increases by
2% over the IAS for every 1,000
feet. Refer to the Performance
chapter for a discussion on the
effects of air density on TAS.
Various markings are quite useful if
you don't have the flight manual to
hand. The green arc covers the range
of speeds for normal operations.
The yellow arc is the caution range (that
is, not to be used for long periods of
time), and the red line is the speed not
to be exceeded, Vne. A white arc, on
an aeroplane, is for flap operation.
If the pitot tube becomes blocked,
　
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