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Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Gyroscopic Flight Instruments

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Heading Indicator
The heading indicator is fundamentally a mechanical
instrument designed to facilitate the use of the magnetic
compass. Errors in the magnetic compass are numerous,
making straight flight and precision turns to headings difficult
to accomplish, particularly in turbulent air. A heading
indicator, however, is not affected by the forces that make
the magnetic compass difficult to interpret. [Figure 7-25]

A heading indicator displays headings based on a 360°
Figure 7-25. A heading indicator displays headings based on a 360°
azimuth, with the final zero omitted. For example, "6" represents
060°, while "21" indicates 210°. The adjustment knob is used to
align the heading indicator with the magnetic compass.

The operation of the heading indicator depends upon the
principle of rigidity in space. The rotor turns in a vertical
plane and fixed to the rotor is a compass card. Since the rotor
remains rigid in space, the points on the card hold the same
position in space relative to the vertical plane of the gyro. The
aircraft actually rotates around the rotating gyro, not the other
way around. As the instrument case and the aircraft revolve
around the vertical axis of the gyro, the card provides clear
and accurate heading information.

Because of precession caused by friction, the heading
indicator creeps or drifts from a heading to which it is set.
Among other factors, the amount of drift depends largely
upon the condition of the instrument. If the bearings are worn,
dirty, or improperly lubricated, the drift may be excessive.
Another error in the heading indicator is caused by the fact
that the gyro is oriented in space, and the Earth rotates in
space at a rate of 15° in 1 hour. Thus, discounting precession
caused by friction, the heading indicator may indicate as
much as 15° error per every hour of operation.

Some heading indicators referred to as horizontal situation
indicators (HSI) receive a magnetic north reference from
a magnetic slaving transmitter, and generally need no
adjustment. The magnetic slaving transmitter is called a

Attitude and Heading Reference System (AHRS)
Electronic flight displays have replaced free-spinning gyros
with solid-state laser systems that are capable of flight at
any attitude without tumbling. This capability is the result
of the development of the Attitude and Heading Reference
System (AHRS).

The AHRS sends attitude information to the PFD in order
to generate the pitch and bank information of the attitude
indicator. The heading information is derived from a
magnetometer which senses the earth's lines of magnetic
flux This information is then processed and sent out to the
PFD to generate the heading display. [Figure 7-26]

Attitude and heading reference system (AHRS).
Figure 7-26. Attitude and heading reference system (AHRS).

The Flux Gate Compass System
As mentioned earlier, the lines of flux in the Earth's magnetic
field have two basic characteristics: a magnet aligns with
them, and an electrical current is induced, or generated, in
any wire crossed by them.

The flux gate compass that drives slaved gyros uses the
characteristic of current induction. The flux valve is a small,
segmented ring, like the one in Figure 7-27, made of soft iron
that readily accepts lines of magnetic flux An electrical coil
is wound around each of the three legs to accept the current
induced in this ring by the Earth's magnetic field. A coil
wound around the iron spacer in the center of the frame has
400 Hz alternating current (AC) .owing through it. During
the times when this current reaches its peak, twice during
each cycle, there is so much magnetism produced by this
coil that the frame cannot accept the lines of flux from the
Earth's field.