BASIC INSTRUMENT PROCEDURES
rates of descent during instrument approaches or for maintaining and correcting to a desired
Vertical speed indicators are subject to two types of error, lag and reversal. After
entering or completing an altitude change, approximately 6 seconds are required for
the pressure differential within the instrument itself to equalize. This time delay is an
inherent error called lag. The vertical speed indicator is also subject to reversal error.
This error is caused by inducing false static pressure into the static system. This
normally occurs during sudden or abrupt pitch changes. The reversal error is not
synonymous with lag error, however, both may occur simultaneously. Making small
and/or smooth pitch changes minimizes the magnitude of this error.
The VSI is invaluable as a trend instrument. During smooth flight conditions, the
VSI will indicate a deviation from altitude prior to the altimeter needle moving.
Turn and Slip Indicator. The turn indicator is a vertical needle pointer utilizing a
gyroscope driven by pressurized air from the engine compressor. The primary function of the
turn indicator is to measure the rate at which the aircraft is turning. The secondary function is to
provide an indication of bank as a backup for the attitude indicator. If the aircraft is in a turn, the
turn needle will be deflected in the direction of the turn. The turn needle is calibrated for a one-
half standard-rate turn at one needle width deflection.
The above description applies to the four-minute turn needle in a
2F101/T-2C. Other aircraft may be equipped with a two-minute
The slip indicator, called the ball, is a simple inclinometer. It consists of a marble ball in a
slightly curved clear tube containing a liquid. The ball indicates the relationship between the
angle of bank 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 the center, indicating
uncoordinated flight: a skid or slip.
Angle Of Attack Indicator. Angle of attack is the angle between the wing's mean
aerodynamic chord of a moving aircraft and the relative wind. The angle of attack instrument is
a visual indication of this relationship. A sensor mounted on the outside of the aircraft measures
angle of attack. This sensor aligns itself with the relative wind and transmits an electric signal to
the cockpit instrument, which is a pointer needle against a fixed dial. Optimum angle of attack
for any phase of flight does not vary with gross weight, bank angle, or density altitude (as does
airspeed). Some angle of attack equivalents for the T-2C in various flight conditions are:
BASIC INSTRUMENT PROCEDURES 1-7