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although this will tend to come from
an auxiliary system.
The altimeter therefore has two
connections, the difference between
them providing the basis of airspeed,
assuming they are not blocked.
If the pitot tube and its drain get
blocked, the airspeed and Mach
indicators will read high in the climb,
low in the descent and not change at
all when airspeed varies. This is
because only the static pressure is
changing, so they are behaving like
altimeters (a typical icing situation).
If the drain hole remains open,
however, the IAS will read zero,
because there is no differential
between static and dynamic
pressures, due to the drain hole
allowing the pressure in the lines to
drop to atmospheric.
The pitot tube should be parallel to
the relative airflow for best effect. It
will be hot on most aircraft, as it
needs to be protected against icing
up, and a heating element will be
switched on at all times, so be
careful on your preflight and warn
your passengers not to touch it.
Aircraft intended for IFR work will
have an alternate static source, which
takes its feed from inside the aircraft
in case the main one gets blocked,
either through ice, a bird strike, or
whatever. If that is the case, the
238 Canadian Professional Pilot Studies
pressure read will be slightly lower,
and will cause the airspeed and
altimeter to read high.
The Altimeter
This is actually a barometer with the
scale marked in feet rather than
millibars. As you go up, the pressure
will be less, which is the same effect
as the pressure reducing at sea level.
The altimeter, however, will be
better sealed, so that air pressure in
the cockpit doesn't affect it. The
only pressure that should be there is
that from the pitot-static system.
Inside a sensitive altimeter are two
aneroid capsules (vacuums), which are
corrugated for strength and kept
open with a large spring. Their
movements as you go up and down
are magnified by a linkage that
connects them directly to the
pointer, using jewelled bearings. If
they expand, as they would when
you go up, the pointer increases the
reading. There is also a temperature
compensation system to correct
spring and linkage tensions.
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 ISA conditions will
true altitude be indicated. For
example, in extreme cold, it will be a
lot lower than indicated, so
corrections must be applied
(altitudes given with radar vectors
from ATC are corrected already).
The dials work like a clock:
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, as above, shows feet in ten
thousands. Caution: The three-needle
display can be easily misread.
The servo-assisted altimeter typically
uses a digital readout to minimise
errors. In this instrument, the
aneroid capsules are connected to
one end of a pivoting magnet (an Ibar)
which influences an E-bar that
has windings on each of its arms:
An AC current is fed to the primary
winding on the centre arm, and as
long as the gaps between the E and
the I bar are equidistant, equal AC
voltages will be induced in the other
arms. Once the capsules move,
however, the gaps vary in size and
the difference is calculated to give
the readout.
Height is the vertical distance from a
particular datum, usually in the case
of aviation from the surface of an
Instruments 239
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 air
temperature and density (absolute
altitude is the actual height above the
surface, and is produced by a radio
altimeter, below).
You can calculate true altitude with a
formula. First, subtract the ground
elevation from the indicated altitude,
　
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