T-6A INSTRUMENT NAVIGATION
The standby compass is mounted so that when the aircraft is in straight-and-level unaccelerated
flight, the vertical component of the Earth's magnetic field has no effect on the compass heading
indication. However, when the aircraft is banked on or near a heading of north or south, or when
it is accelerated or decelerated on or near east or west headings, the compass indications are
erroneous. Because of this dip error, precision flight without the use of a gyro-stabilized heading
indicator is difficult, especially in rough air. Another disadvantage is the fluid (acid free white
kerosene), in which the standby compass is immersed to dampen oscillation is subject to swirl,
which may create noticeable error. Additionally, the comparatively small size of the compass
bowl restricts the use of efficient dampening vanes. In extreme latitudes (near the North or
South Poles), the standby magnetic compass is useless because of its proximity to the magnetic
poles. This may cause the compass to spin erratically or display other incorrect indications.
An airspeed indicator is a presentation of the aircraft's forward velocity in KIAS through the
surrounding air mass. Components within the instrument case react to the difference between
ram and static pressure inputs causing a mechanically linked or electronically displayed pointer
to indicate the airspeed on a graduated scale. As the ram/static pressure differential changes, the
pointer indicates a change in airspeed. Depending upon the type instrument, the airspeed
depicted may be either in terms of indicated, true, or displayed as a mach number.
The Vertical Speed Indicator (VSI) measures change of aircraft altitude in Feet Per Minute
(FPM). It indicates the rate of climb or descent by measuring the rate of change in atmospheric
pressure. This information is valuable in maintaining specific rates of descent during instrument
approaches or for maintaining and correcting to a desired altitude.
Cockpit instrumentation includes both a rate of turn, a rate of turn indicator, and a slip indicator.
Though these are usually integrated in one instrument, they will be discussed separately.
The turn indicator is a vertical needle pointer or, in some aircraft, a miniaturized horizontal
sliding bar on the attitude indicator. A gyroscope is used in its operation. The primary function
of the turn indicator is to measure the rate at which the aircraft is turning. A secondary function
is to provide an indication of bank as a backup for the attitude indicator. The needle on the turn
indicator is designed to deflect, in the direction the aircraft is turning. One needle-width
indicates a turn at the rate of 360° every two or four minutes, depending upon the instrument
type. A single needle-width deflection on a two-minute turn needle indicates the aircraft is
turning 3º per second. A single needle-width deflection on a four-minute turn needle indicates
the aircraft is turning 1 1/2° per second.
The slip indicator, called the ball, is a simple inclinometer. It consists of a marble in a slightly
curved clear tube containing a liquid. The ball indicates the relationship between the AOB and
the rate of turn. The forces acting on the ball are gravity and centrifugal force. During a
coordinated turn, these forces are in balance and the ball will remain centered. When the forces
acting on the ball become unbalanced, the ball moves away from center, indicating
uncoordinated flight - a skid or slip. In a skid, the rate of turn is too large for the AOB, and the
excessive centrifugal force causes the ball to move to the outside of the turn. Correcting to
coordinated flight requires either increasing the AOB or decreasing the rate of turn by using less
rudder or a combination of both. In a slip, the rate of turn is too low for the AOB, and the lack of
2-2 INTRODUCTION TO AIRCRAFT FLIGHT INSTRUMENT