BASIC INSTRUMENT PROCEDURES
than 9 minutes. Consequently, during a total electrical failure every effort should be made to
expedite recovery to visual meteorological conditions.
Performance Instruments. The performance instruments indicate how the aircraft is
performing as a result of attitude changes.
Standby Magnetic Compass. The standby magnetic compass is simple in construction.
Magnetized needles react to the earth's magnetic field and cause the compass card to indicate
magnetic heading relative to magnetic north. The compass card has letters for cardinal headings
and numbers every 30º in between. The last zero of the degree indication is omitted. Between
these numbers, the card is graduated each 5º. Mounted behind the compass glass face is a lubber
or reference line by which compass indications are read. If the face is broken, the fluid is lost
and the compass becomes inoperative.
The standby compass is used for crosscheck purposes, or when any kind of failure
renders the gyro-stabilized compass unreliable. Variable electrical loads may create
deviation errors for which compass correction cards cannot provide sufficient
The standby compass is so mounted 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 indication. However, when the aircraft is banked, on or near a
heading of north or south, or is accelerated or decelerated on or near east or west
headings, the compass indications are erroneous.
When a checklist or a dynamic phase of flight calls for the "alignment and slaving" of
the BDHI, the standby magnetic compass is the instrument utilized. Again, it is
imperative to realize that straight and level flight must be obtained to ensure an
accurate magnetic heading reading from the "wet compass". In general, the forward
standby magnetic compasses are calibrated more accurately than the aft cockpit
standby magnetic compass. If a discrepancy exists between the forward and aft
cockpit standby magnetic headings, use the forward reading.
Airspeed Indicator. An airspeed indicator is a presentation of an aircraft's forward velocity
in knots through the surrounding air mass. Components within the instrument case react to the
difference between the ram and static pressure inputs causing a mechanically linked indicated
airspeed (solid) pointer to indicate the airspeed on a graduated scale. In addition, the airspeed
indicator in the T-2C incorporates a maximum allowable airspeed (striped) pointer, which in
effect, provides airspeed "red line". During ascent or descent, an altitude sensitive bellows
within the indicator causes the maximum allowable airspeed pointer to move on the calibrated
dial. If airspeed is increased until the pointers meet, the aircraft is then flying at the limit Mach
Vertical Speed Indicator. The vertical speed indicator measures the rate of 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
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