Direct-Reading Magnetic Compasses
system to the main spring and main shaft. From
here, it goes to the plus and minus assemblies. The
hand assemblies mount on the plus and minus
During the early days of aviation, direction
assemblies. Changes in vertical acceleration cause
of flight was determined chiefly by direct-reading
movement of the mass on the shafts, which
magnetic compasses. Today, the direct-reading
translates into a turning motion of the main shaft.
magnetic compass (fig. 6-41) is used as a standby
The turning motion pivots the indicating hands
compass. Direct-reading magnetic compasses used
in Navy aircraft mount on the instrument panel
around the dial. The hand travels a distance
for use by the pilot. It is read like the dial of a
equivalent to the value, in g units, of the upward
or downward acceleration of the aircraft.
gauge.
The accelerometer operates on the principle
A nonmagnetic metal bowl, filled with liquid,
of Newton's Law of Motion. During level flight,
contains the compass indicating card. The
no forces act to displace the mass from a position
card provides the means of reading compass
midway from the top and bottom of the shafts.
indications. The card mounts on a float assembly
Therefore, the accelerometer pulley system
and is actually a disk with numbers painted on
performs no work, and the indicating hands
its edge. A set of small magnetized bars or needles
remain stationary at +1 g. When the aircraft
fasten to this card. The card-magnet assembly sits
changes from level flight, forces act on the mass.
on a jeweled pivot, which lets the magnets align
This action causes the mass to move either above
themselves freely with the north-south component
or below its midway position. These movements
of the earth's magnetic field. The compass card
cause the accelerometer indicating hands to
and a fixed-position reference marker (lubber's
line) are visible through a glass window on the
change position. When the aircraft goes nose
down, the hands move to the minus section of the
side of the bowl.
dial. When the nose goes up, they move to the
plus section.
An expansion chamber in the compass pro-
vides for expansion and contraction of the liquid
The main hand continuously shows changes
caused by altitude and temperature changes. The
in loading. T h e two other hands on the
liquid dampens, or slows down, the oscillation of
accelerometer show the highest plus acceleration
the card. Aircraft vibration and changes in
heading cause oscillation. If suspended in air, the
and highest minus acceleration of the aircraft
during any maneuver. The indicator uses a ratchet
card would keep swinging back and forth and be
mechanism to maintain these readings. A knob
difficult to read. The liquid also buoys up the float
in the lower left of the instrument face is used to
assembly, reducing the weight and friction on the
reset the maximum- and minimum-reading hands
pivot bearing.
to normal. Thus, the accelerometer keeps an
indication of the highest accelerations during a
Instrument-panel compasses for naval aircraft
particular flight phase or during a series of flights.
are available with cards marked in steps of either
2 degrees or 5 degrees. Such a compass indicates
continuously without electrical or information
Clocks
inputs. You can read the aircraft heading by
looking at the card in reference to the lubber line
The standard Navy clock is a 12-hour, elapsed-
through the bowl window.
time, stem-wound clock with an 8-day movement.
This type of clock is in the cockpit for use by the
pilot or copilot. Clocks maybe located elsewhere
Standby Attitude Indicator
for use by other crew members as well. The pull-
to-set winding stem is at the lower left of the dial.
The standby attitude indicator (fig. 6-41 on
The dial has 60 divisions, which you read as
the pilot instrument panel) consists of a miniature
minutes or seconds, as appropriate. The face has
aircraft symbol, a bank angle dial, and a bank
standard minute and hour hands, a sweep-second
index. It also includes a two-colored drum
hand, and an elapsed-time minute hand. You may
background with a horizon line dividing the two.
start, stop, or reset the elapsed-time minute hand
by pressing a single button at the upper right of
The indicator roll index is graduated in
the dial.
10-degree increments to 30 degrees, with
6-37
