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light but will not close the left or right transfer valves .
FUEL QUANTITY INDICATOR AND SELECTOR SWITCH
The fuel quantity indicator, on the fuel control panel, indicates fuel
quantity in pounds of fuel. The indicator face is graduated in 100-
pound increments from 0 to 5000 pounds on aircraft with standard fuselage
tank and 0 to 7000 pounds on aircraft with extended range fuselage
tank. Fuel quantity is sensed by four capacitance-type fuel
quantity probes in each wing tank and a capacitance-type fuel quantity
probe in the fuselage fuel tank. The left inboard fuel quantity probe
incorporates a fuel temperature compensater which compensates for
fuel density changes due to temperature. The selector switch permits
reading of each tank separately as well as total system quantity. The
usable fuel quantities for each position of the selector switch are
based upon a fuel density of 6.7 pounds per gallon . The fuel quantity
indicating system operates on 28 VDC supplied through the 2-amp
FUEL QTY circuit breaker on the copilot's circuit breaker panel. The
fuel quantity indicator is operative during EMER BUSmode.
FUEL COUNTER
The fuel counter, on the fuel control panel, indicates quantity of fuel
used in pounds. The resettable digital-type counter is operated by
pulsating DC signals from the fuel flow indicating system monitor.
PM-121
Z - 29
Change 1
Pilot's Manual
Learjet 31A
FUEL SYSTEM GLARESHIELD LIGHTS
FUEL PRESS LIGHTS
The red L FUEL PRESS and R FUEL PRESS warning lights in the
glareshield annunciator panel are installed to alert the pilot of a low
fuel pressure condition. The FUEL PRESS lights are energized by a
pressure switch installed in each engine fuel supply line between the
aircraft fuel filter and the engine-driven fuel pump. When fuel supply
pressure drops to 0.25 psi (1.72 kPa) orbelow, the pressure switch closes
to illuminate the respective light. At 1 .0 psi (6.9 kPa), the switch will reopen.
Should the light illuminate, the standby pumps should be used
to supply engine fuel.
FUEL FILTER LIGHT
Aircraft equipped with fuel heaters: Illumination of the amber FUEL
FILTER light on the glareshield annunciator panel indicates either an
engine fuel filter or an aircraft fuel system fuel filter is clogging. Engine
fuel filter annunciation is discussed in ENGINE INDICATING. An aircraft
fuel system filter is installed in each engine supply line. Each filter
installation includes a bypass valve with an integral pressure switch .
During ground operations (squat switch in ground mode) if a filter is
clogging, a pressure drop across the filter element will cause the corresponding
bypass valve to open and actuate the pressure switch completing
a ground circuit for the light. The light will illuminate if either
or both filters become clogged. Since the annunciation circuit for the
aircraft fuel filters is wired through the squat switches, illumination of
the FUEL FILTER light in flight indicates a clogged engine fuel filter.
(Refer to ENGINE INDICATING.)
Aircraft not equipped with fuel heaters: Illumination of the amber
FUEL FILTER light on the glareshield annunciator panel indicates an
aircraft fuel system fuel filter is clogging. An aircraft fuel system filter
is installed in each engine supply line. Each filter installation includes
a bypass valve with an integral pressure switch. If a filter is clogging, a
pressure drop across the filter element will cause the corresponding bypass
valve to open and actuate the pressure switch completing a
ground circuit for the light. The light will illuminate ifeither or both filters
become clogged.
2-30
PM-121
Change 3
Learjet31A
Pilot's Manual
LOW FUEL LIGHT
The amber LOW FUEL light in the glareshield annunciator panel will
illuminate when the fuel quantity in either wing tank decreases to approximately
350 pounds (158 kilograms) of fuel with the aircraft in a
level attitude. The light is operated by a low wing fuel float switch installed
in each wing tank. Either float switch may cause the light to illuminate.
In nose-up attitudes, the light may illuminate at a lesser
quantity. In nose-down attitudes the light may illuminate at a greater
quantity.
FUEL XFLO LIGHT
The amber FUELXFLOlight, on the glareshield annunciator panel, will
illuminate when the crossflow valve is in the open position . The crossflow
valve will be open during all fuel transfer procedures.
RAM AIR FUEL VENT SYSTEM
The fuel vent system provides ram air pressure to all interconnected
components of the fuel system to ensure positive pressure during all
flight conditions . Flush mounted underwing scoops (inboard) admit
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