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pressure to the fuselage vent system, and a separate set of underwing
scoops (outboard) admit pressure for the wing vent systems. The fuselage
vent lines are connected to a common sump that has a moisture
drain valve. Each wing tank vent system has a sump with a moisture
drain valve located next to the wing vent underwing scoops . Overpressurization
due to thermal expansion in the wing tanks is relieved
through the left and right expansion lines to the fuselage tank. Overpressurization
of the fuselage tank is relieved overboard through a pair
of pressure relief valves.
SINGLE-POINT PRESSURE REFUEL (SPPR) SYSTEM (OPTIONAL)
The single-point pressure refueling (SPPR) system (if installed) allows
the entire fuel system to be serviced through a fuel servicing adapter located
on the right side of the aircraft below the engine pylon. The SPPR
incorporates a precheck system which allows the operator to check the
operation of the system vent and shutoff valves before commencing refuel
operations . The major system components are the refuel adapter,
the control panel, a vent valve, a shutoff valve and pilot valve for each
tank (both wings and fuselage), solenoid valve for the fuselage tank,
two precheck valves, and associated plumbing and wiring. The refuel
adapter and control panel are located on the right side of the fuselage
below the engine pylon. Electrical power to operate the system indicator
lights and solenoid valve is 28 VDC supplied from the right battery
through the remote BATswitch on the refuel control panel.
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Pilot's Manual
Learjet 31A
The vent valve is installed to prevent system overpressurization in the
event of a shutoff valve failure. Operation of the valve is checked during
the precheck sequence . The valve automatically opens whenever
fuel pressure is applied to the system . When the valve reaches the full
open position, a switch in the valve completes a circuit to illuminate the
VENT OPEN light on the SPPR control panel.
Each shutoff valve is controlled by the associated pilot valve located at
the high point in each tank. When refueling pressure is applied to the
system through the refuel adapter, pressurized fuel is applied to each
shutoff valve. This pressure is applied to both sides ofthe valve poppet.
If the pilot valve is open (associated tank not full), some of the pressure
acting to hold the valve closed will be vented through the pilot valve
and the pressure acting to unseat the poppet will drive the valve open
against the spring tension. When the tank fills, the pilot valve will close,
fuel pressure on both sides of the shutoff valve poppet will equalize,
and spring tension will drive the valve closed .
The solenoid valve for the fuselage tank is located in the line between
the fuselage tank pilot valve and shutoff valve. This valve is normally
dosed and must be energized open in order to open the fuselage shutoff
valve for filling the tank. The valve is used to isolate the fuselage
tank if filling the fuselage tank is not desired.
WING AND FUSELAGE PRECHECK VALVES
The WING and FUSELAGE PRECHECK valves are used to check operation
of the system vent valve and individual shutoff valves before
full refueling procedures are commenced. System precheck is accomplished
with the Refuel Selector switch set to TOTAL in order to check
all shutoff valves . When the WING and FUSELAGE PRECHECK
valves are set to OPEN (grips vertical) and refuel pressure is applied to
the refuel adapter, fuel will be admitted to the precheck lines and to the
tank fill lines. The shutoff valves will open and fuel will flow into all
tanks. The fuel in the precheck lines will empty into a float basin at each
pilot valve. When the basin fills the pilot valve float will close the pilot
valve, which causes the associated shutoff valve to dose terminating
fuel flow. The vent valve should open when fuel flow is initiated. Fuel
flow should stop within 30 seconds.
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Learjet 31A
Pilot's Manual
REFUEL SELECTOR SWITCH
The Refuel Selector switch, on the SPPR fuel control panel, is used to
select the tank(s) to be filled during refueling. The switch has two positions
TOTALand PARTIAL.
The TOTAL position of the Refuel Selector switch is used to fill the
wing and fuselage tanks simultaneously. When TOTAL is selected
and refueling pressure is applied (vent valve opens), circuits are completed
to open the fuselage tank solenoid valve. When the solenoid
valve opens the fuselage tank shutoff valve will open to admit fuel
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