Piper Navajo PA-31 Service Manual page 638

The attitude gyro is essentially an air driven gyroscope rotating in a horizontal plane and
is operated by the same principle as the directional gyro. Because of the gyroscopic inertia,
the spin axis continues to point in the vertical direction, providing a constant visual
reference to the attitude of the airplane relative to pitch and roll axis. A bar across the face
of the indicator represents the horizon. A miniature adjustable airplane is mounted to the
case, and aligning the miniature airplane to the horizon bar simulates the alignment of the
airplane to the actual horizon. Any deviation simulates the deviation of the airplane from
the true horizon. The attitude gyro is marked for different degrees of bank.
The gyro pressure gauge indicates the pressure drop across the gyros or the differential
pressure between the gyro inlet and the gyro outlet (vent). Should one of the pressure
pumps fail, the gauge will also indicate this and which one. On airplanes with serial numbers
31-7612019 and up, pneumatic source malfunction is indicated not at the gauge but rather
at the pneumatic source malfunction lights on the left instrument panel.
10-5. PITOT-STATIC INSTRUMENTS. The airspeed indicator provides a means of
indicating the speed of the airplane passing through the air. The airspeed indication is the
differential
pressure reading between pitot air pressure and static air pressure. This
instrument has the diaphragm vented to the pitot air source and the case is vented to the
static air system. As the airplane increases speed, the pitot air pressure increases, causing the
diaphragm to expand. A mechanical linkage picks up this motion and moves the instrument
pointer to the indicator speed. The instrument dial is calibrated in knots and miles per hour,
and also has the necessary operating range markings for safe operation of the airplane.
The altimeter indicates pressure altitude in feet above sea level. The indicator has three
pointers and dial scale. The long pointer is read in hundreds of feet. The middle pointer is
read in thousands of feet and the short pointer in ten thousand feet. A field pressure
window is located on the right side of the indicator dial and is set by the knob located on
the lower left corner of the instrument. The altimeter consists of a sealed diaphragm that is
connected to the pointers through a mechanical linkage. The instrument case is vented to
the static air system, and as static air pressure decreases the diaphragm expands, causing the
pointers to move through the mechanical linkage.
The rate of climb indicator measures the rate of change in static pressure when the
airplane is climbing or descending. By means of a pointer and dial, this instrument will
indicate the rate of ascent or descent of the airplane in feet per minute.
10-6. ELECTRICAL INSTRUMENT. The turn and bank indicator
is an electrical
instrument used for making correctly controlled turns. The turn portion of the indicator is
an electrically driven gyroscope, while the bank portion is a ball sealed in a curved glass tube
filled with damping fluid. The indicator is connected directly to the main distribution bus
through its own circuit breaker.
The ammeter will indicate the electrical output-input of the battery. The ammeter is
also equipped with two selector switches which enable an independent output check of each
alternator.
Reissued: 10/12/79
INSTRUMENTS
PIPER NAVAJO SERVICE MANUAL
3C24