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·  Angle of Attack Indicator. This is
usually of a vane or a probe
fitted flush with the side of the
aircraft to detect relative air
flow, to give an indication of
the angle of attack. It provides a
basis for the stall warning
system and helps to verify the
aircraft attitude and speed.
Pitot-Static System
This consists of a series of pipes
through which air flows to feed
three common instruments on your
panel, the altimeter, airspeed indicator
and vertical speed indicator.
The system itself starts with a pitot
tube (pronounced pee-toe) connected
to the airspeed indicator (see below),
to measure dynamic pressure, and a
static line connected to all three (or
four), to measure the static pressure, so
called because it remains relatively
static (it's actually the normal
barometric pressure that decreases
with height). The static lines are
connected to static ports or static vents
on the side of the machine, at right
angles to the relative airflow. They
may or may not be heated.
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 indicator 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
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
68 JAR Private Pilot Studies
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
　
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