Piper Navajo PA-31 Service Manual page 578

PIPER NAVAJO SERVICE MANUAL
SECTION IX
FUEL SYSTEM
9-1. DESCRIPTION. The fuel system is contained in two independent systems that allow each engine to
have its own fuel supply. The systems are connected by a cross feed valve that allows fuel to be drawn from
one set of fuel cells to the engine on the opposite side, in the event of an emergency.
The fuel cells are of the bladder type. The inboard cells (main) and the outboard cells (auxiliary) are
installed in cavities in the wings. Each inboard cell has a capacity of 56 U.S. gallons and each outboard has a
capacity of 40 U.S. gallons. There are (optional) nacelle fuel cells (PA-31-325 only), each cell holds 27 U.S.
gallons.
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, 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 the normal passage. The fuel in the nacelle fuel cells ( PA-31-325 only) is transferred to the inboard
fuel cells by gravity feed.
Drains are provided for each fuel cell, filter bowl and the cross feed line. The cell drains are visible on the
underside of each wing at the inboard end of the cells. The filter bowl drains are accessible through an access
door on the panel that is located between the underside of each wing and the fuselage. The cross feed is located
on the left panel, aft of the filter bowl access door.
The fuel valves are operated through controls located in a panel just ahead of the main spar, between the
pilot seats. Fuel gauges will indicate the quantity of fuel in each cell that fuel is being drawn from except when
the (optional) nacelle fuel cell is installed (PA-31-325 only) then the nacelle and (main) inboard fuel quantity is
read off of the same gauge.
Airplanes equipped with 325 HP engines or those airplanes which have Piper Kit No. 760 887L or
764 036L have two extra fuel boost pumps placed in the fuel system. Each pump is an electric, continuous
duty, inline type pump located between the fuel filter and the emergency 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 controlled by a separate circuit breaker located in the circuit breaker 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 wind-
shield divider panel, provide a visual indication of an inoperative pump. Each warning light is controlled by a
sensor switch located above the firewall shutoff valve, forward of the cross feed 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 below 2 thru 4 PSIG. ( Refer to Figure 9-1c.) Figures 11-119 and 11-120 are the electrical schematic for
the fuel boost pumps and the fuel flow warning circuit.
Revised: 2/18/94
FUEL SYSTEM
3A10