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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", but could be any
other, such as 700 mb, as on high
level weather charts. 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).
Errors
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, or instrument error, due to
misalignment in the linkages and
gears, temperature error, particularly
when cold, causing it to over-read,
elastic error (hysteresis), or a time 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.
Radio Altimeter
This figures out the height from the
time interval between a transmitted
and received pulse from the aircraft,
based on the constant speed of
electromagnetic waves. You might
240 Canadian Professional Pilot Studies
get a logarithmic scale, where
divisions are longer at lower heights,
or a digital readout.
Radio altimeters are used in GPWS,
and as a crosscheck for the normal
altimeter. They can also be used on
approaches for a more accurate
determination of DH or MDA, and
are particularly useful on aerial
survey, or with certain external loads
on a helicopter, such as the EM bird,
which may also use a laser altimeter.
There is an adjustable marker that
can be set to a certain height, and act
as a warning when you reach it.
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
will vary in direct proportion to it.
The otherwise gas-tight instrument
casing (outside the capsule) is fed
with static pressure, so dynamic
pressure is isolated. The needle,
being connected to the capsule, will
therefore read airspeed directly.
Some aircraft, such as the Bell 407,
have a dampened needle, which will
indicate the speed you have been, and
not the speed you are at.
The ASI suffers from position and
attitude errors, plus those arising
from the instrument itself. It may be
calibrated in knots or mph, that is, a
rate of change of distance per unit of
time. There are several variations on
the theme, 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
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