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Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Compass Systems

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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




A separate unit, the magnetic slaving transmitter is mounted
remotely, usually in a wingtip to eliminate the possibility of
magnetic interference. It contains the flux valve, which is
the direction-sensing device of the system. A concentration
of lines of magnetic force, after being amplified, becomes
a signal relayed to the heading indicator unit, which is also
remotely mounted. This signal operates a torque motor in
the heading indicator unit that processes the gyro unit until
it is aligned with the transmitter signal. The magnetic slaving
transmitter is connected electrically to the HSI.

There are a number of designs of the remote indicating
compass; therefore, only the basic features of the system are
covered here. Instrument pilots must become familiar with
the characteristics of the equipment in their aircraft.
As instrument panels become more crowded and the pilot's
available scan time is reduced by a heavier flight deck
workload, instrument manufacturers have worked toward
combining instruments. One good example of this is the
RMI in Figure 7-30. The compass card is driven by signals
from the flux valve, and the two pointers are driven by an
automatic direction finder (ADF) and a very high frequency
(VHF) omni-directional radio range (VOR).

Driven by signals from a flux valve
Figure 7-30. Driven by signals from a flux valve, the compass card
in this RMI indicates the heading of the aircraft opposite the upper
center index mark. The green pointer is driven by the ADF.

Heading indicators that do not have this automatic
north seeking capability are called "free" gyros, and require
periodic adjustment. It is important to check the indications
frequently (approximately every 15 minutes) and reset the
heading indicator to align it with the magnetic compass
when required. Adjust the heading indicator to the magnetic
compass heading when the aircraft is straight and level at a
constant speed to avoid compass errors.

The bank and pitch limits of the heading indicator vary
with the particular design and make of instrument. On some
heading indicators found in light aircraft, the limits are
approximately 55° of pitch and 55° of bank. When either of
these attitude limits is exceeded, the instrument "tumbles"
or "spills" and no longer gives the correct indication until
reset. After spilling, it may be reset with the caging knob.
Many of the modern instruments used are designed in such
a manner that they do not tumble.

An additional precession error may occur due to a gyro not
spinning fast enough to maintain its alignment. When the
vacuum system stops producing adequate suction to maintain
the gyro speed, the heading indicator and the attitude indicator
gyros begin to slow down. As they slow, they become more
susceptible to deflection from the plane of rotation. Some
aircraft have warning lights to indicate that a low vacuum
situation has occurred. Other aircraft may have only a vacuum
gauge that indicates the suction.

Instrument Check
As the gyro spools up, make sure there are no abnormal
sounds. While taxiing, the instrument should indicate turns in
the correct direction, and precession should not be abnormal.
At idle power settings, the gyroscopic instruments using the
vacuum system might not be up to operating speeds and
precession might occur more rapidly than during flight

Compass Systems

The Earth is a huge magnet, spinning in space, surrounded
by a magnetic field made up of invisible lines of flux These
lines leave the surface at the magnetic north pole and reenter
at the magnetic South Pole.

Lines of magnetic flux have two important characteristics:
any magnet that is free to rotate will align with them, and
an electrical current is induced into any conductor that cuts
across them. Most direction indicators installed in aircraft
make use of one of these two characteristics.

Magnetic Compass
One of the oldest and simplest instruments for indicating
direction is the magnetic compass. It is also one of the basic
instruments required by Title 14 of the Code of Federal
Regulations (14 CFR) part 91 for both VFR and IFR flight
A magnet is a piece of material, usually a metal containing
iron, which attracts and holds lines of magnetic flux.

Regardless of size, every magnet has two poles: north and
south. When one magnet is placed in the field of another, the
unlike poles attract each other, and like poles repel.