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checkered, or striped. The maximum airspeed pointer is
actuated by an aneroid, or altimeter mechanism, that moves
it to a lower value as air density decreases. By keeping the
airspeed pointer at a lower value than the maximum pointer,
the pilot avoids the onset of transonic shock waves.
Airspeed Color Codes
The dial of an ASI is color coded to alert the pilot, at a
glance, of the significance of the speed at which the aircraft
is flying. These colors and their associated airspeeds are
shown in Figure 3-15.
Magnetism
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
Mach Number
As an aircraft approaches the speed of sound, the air flowing
over certain areas of its surface speeds up until it reaches the
speed of sound, and shock waves form. The IAS at which
these conditions occur changes with temperature. Therefore,
in this case, airspeed is not entirely adequate to warn the
pilot of the impending problems. Mach number is more
useful. Mach number is the ratio of the TAS of the aircraft
to the speed of sound in the same atmospheric conditions.
An aircraft flying at the speed of sound is flying at Mach
1.0. Some older mechanical Machmeters not driven from
an air data computer use an altitude aneroid inside the
instrument that converts pitot-static pressure into Mach
number. These systems assume that the temperature at any
altitude is standard; therefore, the indicated Mach number is
inaccurate whenever the temperature deviates from standard.
These systems are called indicated Machmeters. Modern
electronic Machmeters use information from an air data
computer system to correct for temperature errors. These
systems display true Mach number.
3-11
Figure 3-15. Color Codes for an Airspeed Indicator.
Figure 3-16. A Magnetic Compass. The vertical line is called the
lubber line.
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.
The Basic Aviation 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 14 CFR part 91 for both VFR and
IFR flight.
Magnetic Compass Overview
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: a north
pole and a south pole. When one magnet is placed in the
field of another, the unlike poles attract each other and like
poles repel.
An aircraft magnetic compass, such as the one in Figure 3-16,
has two small magnets attached to a metal float sealed inside a
bowl of clear compass fluid similar to kerosene. A graduated
scale, called a card, is wrapped around the float and viewed
through a glass window with a lubber line across it. The card
is marked with letters representing the cardinal directions,
north, east, south, and west, and a number for each 30°
between these letters. The final “0” is omitted from these
directions; for example, 3 = 30°, 6 = 60°, and 33 = 330°.
There are long and short graduation marks between the letters
and numbers, with each long mark representing 10° and each
short mark representing 5°.
Magnetic Compass Construction
The float and card assembly has a hardened steel pivot in its
center that rides inside a special, spring-loaded, hard-glass
jewel cup. The buoyancy of the float takes most of the weight
off the pivot, and the fluid damps the oscillation of the float
and card. This jewel-and-pivot type mounting allows the float
freedom to rotate and tilt up to approximately 18° angle of
bank. At steeper bank angles, the compass indications are
erratic and unpredictable.
The compass housing is entirely full of compass fluid. To
prevent damage or leakage when the fluid expands and
contracts with temperature changes, the rear of the compass
case is sealed with a flexible diaphragm, or with a metal
bellows in some compasses.
Magnetic Compass Theory of Operations
The magnets align with the Earth’s magnetic field and the
pilot reads the direction on the scale opposite the lubber line.
Note that in Figure 3-16, the pilot sees the compass card from
its backside. When the pilot is flying north as the compass
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