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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.
·  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 Canadian Private Pilot Studies
　
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