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lower marks indicates a bank angle of 15 degrees, which produces a standard rate turn. When the aircraft
turns right, the minature aircraft in the instrument indicated a right bank. When turning left, it indicates a
left bank.
Turn Quality
The inclinometer in both the Turn and Slip Indicator and the Turn Coordinator measures the turn quality.
As mentioned previously, when in a turn, part of the lift of the wing goes into "turning" the aircraft. This
is called the Horizontal Component of Lift (HCL). This HCL is directed toward the center of the turn.
Also, a force directed outward and away from the center of the turn exists. This is called Centrifugal
Force (CF). For the turn to be coordinated these two opposing forces must be equal. When they are
equal, the ball in the inclinometer will remain in the middle.
When too much rudder is applied, a skid results. The Centrifrugal Force is bigger than the Horizontal
Component of Lift (HCL). This makes the ball go toward the outside of the inclinomenter (i.e. the ball in
NOT in the middle).
When insufficient rudder is applied, a slip results. The Horizontal Component of Lift (HCL) is larger
than the Centrifrugal Force (CF). The ball rolls toward the lower side( or inside) on the inclinometer.
The term step on the ball is often used as a memory aid in overcoming a slip or skid. In actuality, more
rudder pressure (or less bank) must be applied in a slip. Less rudder pressure (or more bank) must be
applied in a skid. In both cases correction must be made so that HCL = CF to keep the ball centered.
Back to Home Back to Table of Conents To Magnetic Compass
GYROSCOPIC INSTRUMENTS
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Magnetic Compass
Magnetic Compass
The magnetic compass is the only instrument in the aircraft by which the pilot determines the direction of flight.
Magnets in the compass cause it to align with the Magnetic North Pole.
The compass card has the four cardinal headings shown as N, E, S, and W. Numbers appear every 30 degrees.
Long vertical marks occur in 10 degree increments, with intervening short marks at 5 degree points.
The compass card containing the magnets are mounted on a small pivot point in the center of the card assembly.
This allows the compass card to rotate and float freely. It is somewhat like suspending a paper cup, upside down,
on a pencil point located at the center of the cup bottom. The enclosure is filled with white kerosene to provide a
medium to dampen out some vibration and unwanted oscillations. A "lubber line" in etched on the glass face of
the instrument to enable exact reading of the compass.
Magnets in the compass align themselves along a Magnetic North-South orientation. Whenever the aircraft is
headed toward magnetic North, the compass will indicate N. If the aircraft turns from this direction, the magnets
in the compass still align to this N-S direction. Similar to a gyro, the case of the compass and the lubber line is
fixed to the aircraft. Thus when the airplane turns, the the case turns about the compass card. The lubber line will
then show a reading other than North.
Compass Errors
Magnetic lines of force surround the Earth, flowing from the North to South Magnetic poles. The magnetic field
strength is greatest near the magnetic poles and weakest at the Equator. Several compass errors can occur.
These are:
1. Magnetic Variation
2. Compass Deviation
3. Magnetic dip
4. Compass Card oscillation
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Magnetic Variation
The Magnetic North Pole and the True North Pole are not at the same location on the surface of the earth.
Magnetic Variation at any given location on the earth’s surface is the difference between the Compass North and
True North. The map below shows the magnetic variation at various locations in the US.
The Agonic Line is the line of zero degree variation. It proceeds from upper Michigan through central South
Carolina. Variation values to the East of the agonic line are called Westerly Variation; i.e. the magnetic north
pole is West of True North. Likewise, the variation values west of the agonic line are known as Easterly
Variation; i.e. the Magnetic North Pole is East of True North.
Magnetic North changes in small amounts each year. Aeronautical charts are updated periodically to correct for
this yearly change.
When plotting a course on an aeronautical chart, the degrees of heading are measured against latitude and
longitude lines. This is called a True Course (TC) because it is being measured relative to the True North Pole.
Since the pilot relies on the magnetic compass for direction, the pilot will be steering the aircraft relative to the
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