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needle registers a change in vertical direction. When the
pressure differential stabilizes at a definite ratio, the needle
registers the rate of altitude change.
12-3
ways. If the ram air inlet is clogged, but the drain hole
remains open, the airspeed indicator registers zero, regardless
of airspeed. If both the ram air inlet and the drain hole
become blocked, pressure in the line is trapped, and the
airspeed indicator reacts like an altimeter, showing an
increase in airspeed with an increase in altitude, and a
decrease in speed as altitude decreases. This occurs as
long as the static port remains unobstructed.
If the static port alone becomes blocked, the airspeed
indicator continues to function, but with incorrect readings.
When you are operating above the altitude where
the static port became clogged, the airspeed indicator
reads lower than it should. Conversely, when operating
below that altitude, the indicator reads higher than the
correct value. The amount of error is proportional to
the distance from the altitude where the static system
became blocked. The greater the difference, the greater
the error. With a blocked static system, the altimeter
freezes at the last altitude and the VSI freezes at zero.
Both instruments are then unusable.
Some helicopters are equipped with an alternate static
source, which may be selected in the event that the main
static system becomes blocked. The alternate source generally
vents into the cabin, where air pressures are slightly
different than outside pressures, so the airspeed and
altimeter usually read higher than normal. Correction
charts may be supplied in the flight manual.
GYROSCOPIC INSTRUMENTS
The three gyroscopic instruments that are required for
instrument flight are the attitude indicator, heading
indicator, and turn indicator. When installed in helicopters,
these instruments are usually electrically powered.
Gyros are affected by two principles—rigidity in space and
precession. Rigidity in space means that once a gyro is
spinning, it tends to remain in a fixed position and resists
external forces applied to it. This principle allows a gyro to
be used to measure changes in attitude or direction.
Precession is the tilting or turning of a gyro in response to
pressure. The reaction to this pressure does not occur at
the point where it was applied; rather, it occurs at a point
that is 90° later in the direction of rotation from where the
pressure was applied. This principle allows the gyro to
determine a rate of turn by sensing the amount of pressure
created by a change in direction. Precession can also
create some minor errors in some instruments.
ATTITUDE INDICATOR
The attitude indicator provides a substitute for the natural
horizon. It is the only instrument that provides an
immediate and direct indication of the helicopter’s
pitch and bank attitude. Since most attitude indicators
installed in helicopters are electrically powered, there
may be a separate power switch, as well as a warning
flag within the instrument, that indicates a loss of
power. A caging or “quick erect” knob may be
included, so you can stabilize the spin axis if the gyro
has tumbled. [Figure 12-5]
HEADING INDICATOR
The heading indicator, which is sometimes referred to
as a directional gyro (DG), senses movement around
the vertical axis and provides a more accurate heading
reference compared to a magnetic compass, which has
a number of turning errors. [Figure 12-6].
Bank Index
Gyro
Gimbal

Rotation
Roll

Gimbal
Pitch

Gimbal
Horizon

Reference

Arm
Figure 12-5. The gyro in the attitude indicator spins in the
horizontal plane. Two mountings, or gimbals, are used so
that both pitch and roll can be sensed simultaneously. Due to
rigidity in space, the gyro remains in a fixed position relative
to the horizon as the case and helicopter rotate around it.
Adjustment Gears
Adjustment

Knob
Gimbal

Rotation
Gimbal Gyro
Main

Drive Gear
Compass

Card Gear
Figure 12-6. A heading indicator displays headings based on
a 360° azimuth, with the final zero omitted. For example, a 6
represents 060°, while a 21 indicates 210°. The adjustment
knob is used to align the heading indicator with the magnetic
compass.
12-4
Due to internal friction within the gyroscope, precession
is common in heading indicators. Precession
causes the selected heading to drift from the set value.
Some heading indicators receive a magnetic north reference
　
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