Fuel System Description - Piper Navajo Chieftain PA-31-350 Service Manual

NAVAJO CHIEFTAIN SERVICE MANUAL
SECTION IX
FUEL SYSTEM
9-1. INTRODUCTION. The fuel system components covered in this section consist of fuel cells, fuel
selector valves, filters and electric fuel pumps.
This section provides instructions for removal, cleaning, inspection and repair, reassembly, testing and
adjustment of the various fuel system components. A troubleshooting chart to assist in isolating and
correcting troubles which may occur is also included.
9-2. DESCRIPTION. The fuel system is contained in two independent units that allow each engine to have
its own fuel supply. The systems are connected only by a crossfeed that will allow fuel to be drawn from one
set of fuel cells to the engine of the opposite side, in the event of an emergency.
The fuel cells are of the bladder type. The inboard cells and the outboard cells are installed in cavities in
the wings, with each inboard (main) cell holding a capacity of 56 U.S. gallons. The outboard (auxiliary) cells
hold a capacity of 40 U.S. gallons each. There are (optional) nacelle fuel cells; each cell holds 27 U.S. gallons.
This gives a total fuel load of 246 U.S. gallons when (optional) fuel cells are installed.
Fuel is taken from each cell through a screen located in the cell outlet fitting and then onto the shutoff
selector valve. From the selector valve, fuel is drawn in a series configuration through the fuel filter (fuel
boost pump on S/N 31-7405479 and up or aircraft with Kit No. 760 873 installed), electric fuel pump,
emergency shutoff valve and onto the engine-driven pump. These units, except for the engine-driven pump,
are accessible through a panel located between the underside of each wing and the fuselage. The fuel filter and
electric and engine pumps incorporate a bypass that will open in the event of fuel stoppage through their nor-
mal avenue of flow. The fuel in the nacelle fuel cells is transferred to the inboard fuel cells by gravity feed. As
mentioned above on aircraft S/N 31-7405479 and up or aircraft with Kit No. 760 873 installed, two extra fuel
boost pumps have been placed in the fuel system. Each pump is an electric, continuous duty, inline type pump
located between the fuel filter and the electric fuel pump. These pumps are provided to maintain fuel under
pressure to the other fuel pumps, improving the altitude performance of the fuel system. Each pump is con-
trolled by a separate circuit braker located in the circuit braker control panel. These pumps are activated
when the master switch is turned on and continue to operate until the master switch is turned off or the circuit
breakers are pulled (off). Red fuel boost pump warning lights, mounted at the base of the windshield divider
panel, provide a visual indication of an inoperative pump. Each warning light is controlled by a sensor switch
located above the fire wall shutoff valve, forward of the crossfeed fuel line, and is connected to the fuel boost
pump. This pressure switch will activate when the pressure produced by the fuel boost pump falls to a
pressure of 2-4 psi. (Refer to Figure 9-9.) Refer to the electrical schematic for the fuel boost pumps and the
fuel flow warning circuit.
Left and right fuel flow warning lights are mounted at the base of the windshield divider post and will
illuminate to warn the pilot of an impending fuel flow interruption. Each light is controlled by a sensing
probe mounted near each inboard fuel tank outlet. In the event the fuel level near the tank outlet drops to
a point where a fuel flow interruption and power loss could occur, the sensing probe will activate and
energize the gated relay. This relay, located aft of bulkhead station 81.00 and bolted to the windshield
channel bracket, delays the illumination of the warning light for 2 seconds. If after the 2 second delay the
sensing probe is still activated, the relay will then close the circuit to the warning light for a minimum of 10
seconds and will remain closed if the cause is not corrected. The lights are provided with a "press-to-test"
feature. To test, depress the button mounted in the center of the light cluster for 3 seconds; the lights
should illuminate and remain on for 10 seconds. During this test, the fuel boost pump inoperative lights
mounted below the fuel flow warning lights should illuminate when the button is depressed (no hold
required on this circuit) and go out when the button is released.
Revised: 3/11/81
FUEL SYSTEM
2K3