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Instrument Flying Handbook
Flight Instruments
Basic Aviation Magnetic Compass

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Instrument Flying


Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Using an Electronic Flight

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency

Vertical card Magnetic Compass.
Figure 3-22. Vertical card Magnetic Compass.

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 3-23, made of soft
iron that readily accepts lines of magnetic flux. An electrical
coil is wound around cacti 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) flowing 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.

flux valve
Figure 3-23. The soft iron frame of the flux valve accepts the flux
from the Earth's magnetic field each time the current in the center
coil reverses. This flux causes current to flow in the three pickup coils.

three pickup coils
Figure 3-24. The current in each of the three pickup coils changes
with the heading of the aircraft.

But as the current reverses between the peaks, it demagnetizes
the frame so it can accept the flux from the Earth's field. As
this flux cuts across the windings in the three coils, it causes
current to flow in them. These three coils are connected in
such a way that the current flowing in them changes as the
heading of the aircraft changes. [Figure 3-24]

The three coils are connected to three similar but smaller coils
in a synchro inside the instrument case. The synchro rotates
the dial of a radio magnetic indicator (RMI) or a horizontal
situation indicator (HST).

Remote Indicating Compass
Remote indicating compasses were developed to compensate
for the errors and limitations of the older type of heading
indicators. The two panel-mounted components of a typical
system are the pictorial navigation indicator and the slaving
control and compensator unit, [Figure 3-25] The pictorial
navigation indicator is commonly referred to as a HSI.

Pictorial Navigation Indicator
Figure 3-25. Pictorial Navigation Indicator (HIS Top),
Slaving Control and Compensator Unit.