Piper PA-24 Comanche, Service Manual

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PIPER COMANCHE SERVICE MANUAL
XI - INSTRUMENTS 04/01/09
3F18
The altimeter indicates pressure altitude in feet above sea level. The indicator has three pointers and a
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.
11-5.
Electrical Instruments.
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 dampening fluid. The electrical switch for the turn and bank indicator is
located on the switch panel under the left instrument panel.
The ammeter is mounted in the instrument cluster located on the instrument panel. This instrument
measures the amount of current received and the amount of current drain on the battery.
Two fuel quantity gauges are used on the Comanche (PA-24-150, Serial Nos. 24-1 and up, and PA-24-
250, Serial Nos. 24-1 to 24-2843 inclusive) and are mounted in the cluster on the instrument panel.
These instruments are calibrated in fractional divisions of one fourth, one half, three fourths and full. A
transmitter unit is installed in each fuel cell. This unit contains a resistance strip and a movable control
arm. The position of this arm is controlled by a float in the fuel cell and this position is transmitted
electrically to the indicator gauge to show the amount of fuel in the cell. If the auxiliary fuel system is
installed, two auxiliary fuel quantity gauges will be installed in the instrument cluster.
In the Comanche, PA-24-250, Serial Nos. 24-2844 and up, PA-24-260, and PA-24-400, only one fuel
quantity gauge is incorporated in the system. This gauge will indicate the amount of fuel in the cell that
is selected. This arrangement is made possible by a selector switch mounted on the fuel selector valve.
The switch is located directly below the fuel selector plate. An override system is incorporated so that it
is possible to check the amount of fuel available in the remaining cells without moving the selector
handle to that cell position. This accomplished by depressing the red button (located on the fuel selector
plate) under the desired fuel cell position. The fuel gauge will indicate the amount of fuel available in
that cell. When the red button is released the indicating system will return to its normal operation of
indicating the amount of fuel in the tank selected.
The oil temperature indicator is mounted in the instrument cluster on the instrument panel. This
instrument will provide a temperature indication of the engine oil in degrees Fahrenheit. The instrument
has a temperature bulb located in the oil screen assembly on the engine accessory section.
11-6.
Miscellaneous Instrument.
The magnetic compass installed in the Comanche is a self-contained instrument and is mounted above
the instrument panel on the windshield center strip. This instrument has an individual light which is
connected to the instrument lighting circuit. The compass correction card is located in the card holder
mounted on the instrument panel.
The clock is a hand wound, 8-day time piece. It incorporates a stem wind with the knob at the lower left-
hand side of the dial. This knob, when pulled and turned, also adjusts the hands. In addition, two dummy
hands are controlled by a center knob and they function as elapsed time hands.