Performance Instruments

CHAPTER TWO

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 3 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