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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




When flying on a heading of west, the same things happen.
Inertia from acceleration causes the weight to lag, and the
card rotates toward north. When the aircraft decelerates on
a heading of west, inertia causes the weight to move ahead
and the card rotates toward south.

A mnemonic, or memory jogger, for the effect of acceleration
error is the word "ANDS" (acceleration—north, deceleration—
south). Acceleration causes an indication toward north;
deceleration causes an indication toward south.

Oscillation Error
Oscillation is a combination of all of the other errors, and it
results in the compass card swinging back and forth around
the heading being flown. When setting the gyroscopic
heading indicator to agree with the magnetic compass, use
the average indication between the swings.

The Vertical Card Magnetic Compass
The floating magnet type of compass not only has all the errors
just described, but also lends itself to confused reading. It is
easy to begin a turn in the wrong direction because its card
appears backward. East is on what the pilot would expect to be
the west side. The vertical card magnetic compass eliminates
some of the errors and confusion. The dial of this compass
is graduated with letters representing the cardinal directions,
numbers every 30°, and tick marks every 5°. The dial is rotated
by a set of gears from the shaft-mounted magnet, and the nose
of the symbolic aircraft on the instrument glass represents the
lubber line for reading the heading of the aircraft from the dial.
Eddy currents induced into an aluminum-damping cup damp,
or decrease, oscillation of the magnet. [Figure 7-37]

Lags or Leads
When starting a turn from a northerly heading, the compass
lags behind the turn. When starting a turn from a southerly
heading, the compass leads the turn.

Eddy Current Damping
In the case of a vertical card magnetic
compass, flux from the oscillating permanent magnet
produces eddy currents in a damping disk or cup. The
magnetic flux produced by the eddy currents opposes the flux
from the permanent magnet and decreases the oscillations.

Vertical card compass.
Figure 7-37. Vertical card compass.

Outside Air Temperature (OAT) Gauge
The outside air temperature (OAT) gauge is a simple and
effective device mounted so that the sensing element is
exposed to the outside air. The sensing element consists
of a bimetallic-type thermometer in which two dissimilar
materials are welded together in a single strip and twisted
into a helix. One end is anchored into protective tube and the
other end is affixed to the pointer, which reads against the
calibration on a circular face. OAT gauges are calibrated in
degrees °C, °F, or both. An accurate air temperature provides
the pilot with useful information about temperature lapse rate
with altitude change. [Figure 7-38]

Outside air temperature (OAT) gauge.
Figure 7-38. Outside air temperature (OAT) gauge.

Chapter Summary
Flight instruments enable an aircraft to be operated with
maximum performance and enhanced safety, especially when
flying long distances. Manufacturers provide the necessary
flight instruments, but to use them effectively, pilots need
to understand how they operate. As a pilot, it is important to
become very familiar with the operational aspects of the pitot static
system and associated instruments, the vacuum system
and associated instruments, the gyroscopic instruments, and
the magnetic compass.