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Instruments
Before we start:
The instrument experience you get on your
basic licence course does not qualify you for
proper instrument flying! You must learn
to overcome many of your body's
limitation to do it properly.
OK, having said that, flight
instruments are in a common
format, called a T arrangement:
I know it doesn't look like one, but
ignore the bottom right and left for
the moment. The artificial horizon is in
the centre, because it is a primary
instrument (it tells you which way is
up), the heading indicator is below, No
1 altimeter at the top right, with the
vertical speed indicator below, and the
airspeed indicator is at the top left with
the turn coordinator underneath.
A primary instrument is one which
gives instant and constant readouts
(also direct). A secondary instrument is
one that you have to deduce things
from, such as the altimeter
increasing, telling you that the pitch
must have changed (you might also
say that the altimeter gives you an
indirect indication of pitch attitude).
The ASI and VSI also give indirect
indications of pitch, and the HI and
TC indicate bank. Note also that a
primary instrument will tell you at
what rate things are changing, but a
secondary one will only indicate that
change is taking place.
They are further grouped under the
headings of pitch, bank and power.
Pitch
· Artificial Horizon (Attitude
Indicator). The most important
pitch instrument, because it
gives direct, instantaneous
readings.
66 JAR Private Pilot Studies
· Altimeter. Although it indicates
pitch indirectly, it is a primary
pitch instrument.
· Airspeed Indicator. A secondary
pitch instrument.
· Vertical Speed Indicator (VSI). A
secondary pitch instrument.
Bank
· Artificial Horizon (Attitude
Indicator). Also the most
important bank instrument, for
similar reasons to pitch.
· Heading Indicator. An indirect
instrument, because if you
change heading, pitch must be
involved somewhere.
· Turn Coordinator. As it shows a
rate of turn (3° per second for
rate 1), it is an indirect
indication of bank.
Power
Not in the traditional T, but you
have to keep an eye on them anyway.
· Engine RPM. Direct indication
of power. Turbines rotate so
fast that the numbers are too
large to make sense of, so
percentages are used instead
(that is, 100% means full
power). In a helicopter, the
engine and rotor RPM needles
usually sit on top of each other
in the same instrument,
although they can be separate.
In powered flight, the needles
are joined; in autorotation, they
are split. Both conditions have a
range outside which the rotor
needle should not go. A turbine
helicopter will also have a
smaller gauge showing "gas
producer" RPM.
· The Manifold Pressure gauge
shows the pressure inside the
inlet manifold, measured in
inches. The theory is that the
higher the pressure, the more
the amount of fuel/air mixture
that is potentially available. The
equivalent of the manifold
pressure gauge in a turbine
helicopter is the torquemeter,
usually expressed as a
percentage. Although 100% is
the usual maximum, it may
often be increased for a few
seconds (check flight manual).
When the engine is running,
MAP is below atmospheric
because of the pressure drop
across the throttle plate
(butterfly valve). As the throttle
is closed, the pressure drop will
increase, and MAP will fall.
When the engine is stopped, the
MAP will be atmospheric.
You keep MAP constant with
altitude by opening the throttle.
Power will increase because
exhaust back pressure falls,
improving scavenging.
· The Cylinder Head Temperature
(CHT) gauge shows you the
temperature of a selected
cylinder in a piston engine, but
not necessarily the hottest (it's
usually a rear one in a
horizontally opposed engine).
Operating an engine at a higher
than intended temperature will
cause loss of power, excessive
oil consumption and damage to
the cylinders.
Instruments 67
· Knowing the Exhaust Gas
Temperature (EGT) is useful
when leaning mixtures.
· Airspeed Indicator. A secondary
power instrument - it changes
with power application.
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