On airplanes SNs 23-070 through 23-099, 24,
24B, and 25 models, when the float switch de-
tects a full fuselage tank, the transfer and cross-
flow valves are closed, both standby pumps are
turned off, and the FULL light is illuminated.
Fuselage Tank Low-
Pressure Switch
The fuselage tank low-pressure switch, lo-
cated in the fuselage tank transfer line, closes
a n d t u r n s o n t h e E M P T Y l i g h t ( s e e
“Annunciator Panel” section) when pressure
drops below 2.75 psi. The switch does not au-
tomatically close the transfer valve or shut
down the fuselage tank pump. The EMPTY
light indicates:
• The tank is empty.
• The fuselage transfer pump has failed.
Tip Tank Low-Pressure Switch
On model 23 airplanes with electric pumps, a
low-pressure switch in the output line from
each tip tank fuel pump turns on the EMPTY
warning light (see Figure 5A-1) to indicate that
the tank is empty or that the pump has failed.
Fuel Temperature
Warning Switch
On model 23 airplanes with a fuel temperature
warning system installed, a thermal switch in
each wing tank sump turns on a HOT FUEL light
located on the fuel control panel. The light will
be on if fuel temperature increases to 82°F and
will go off when fuel temperature drops to
75°F. The airplane shall not be flown at altitudes
above 25,000 feet if the HOT FUEL light is on.
FUSELAGE FUEL
FILL OPERATION
Model 23 without Jet Pumps
The fuselage tank refuel valve on airplanes SNs
23-003 through 23-0069 is open only when the
fuselage tank is being filled. This valve is lo-
cated in the refuel line between the left wing
tank and the fuselage tank. The fuselage
refuel valve is controlled by the FUS FILL
switch (see Figure 5A-1).
Fuel may be pumped from the wings to the
fuselage tank using the FUS FILL switch set
to FILL. This turns on the left wing electric
standby boost pump (23-003 through 23-069)
or the left and right wing electric standby
boost pumps (SNs 23-070 through 23-099). In
addition, the CROSS FLOW switch must be set
to OPEN with the battery switch on to allow
fuel to be taken from the right wing.
Model 24, 24B, 25, and
Model 23 with Jet Pumps
On these airplanes, the left and right wing
electric standby boost pumps fill the fuselage
tank when the fuselage tank XFER–FILL
switch is set to FILL. In addition, the cross-
flow valve and fuselage transfer valves open
automatically and the fuselage float switch is
armed. This allows fuel to be taken from both
wings to fill the fuselage tank.
If the tank is to be filled to capacity, the float
switch actuation automatically:
• Deenergizes the wing standby pumps
• Closes the crossflow and transfer valves
• I l l u m i n a t e s t h e g r e e n F U L L l i g h t ,
which will remain lighted until the
XFER–FILL switch is turned off
The filling process may be terminated at any
point by turning the XFER–FILL switch off.
TIP TANK FUEL JETTISON
SYSTEM (OPTIONAL)
This system is an option on SNs 25-030 and
subsequent. Two 28-VDC shutoff valves are
provided for dumping tip tank fuel in case of
an emergency. Two guarded DUMP switches
(fuel jettison) are installed. When the DUMP
switch(es) is placed to the DUMP or ON po-
sition, the shutoff valves are opened and an
amber light comes on to indicate that the
valves are open. Dumping may be started or
stopped at any time.
5A-13
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
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