Motive-flow pressure varies with engine rpm.
Consequently, jet pump discharge pressure
also varies with engine rpm. At idle, discharge
pressure is approximately 10 psi, while at full-
power settings, discharge pressure is approx-
imately 12 psi.
There are four jet pumps—one in each wing
tank adjacent to the standby boost pump and
one in each tip tank. The wing tank jet pumps
draw fuel from the wing tanks and supply low-
pressure fuel to the engine-driven, high-
pressure fuel pumps. Wing jet pump output can
be supplemented by the wing standby pump to
ensure positive pressure to an engine. The tip
tank jet pumps draw fuel from the tip tanks and
deliver it directly to the cavities where the
standby pumps and jet pumps are located.
Jet pumps require no electrical power and
have no moving parts. They are controlled by
two jet pump switches which electrically open
and close the motive-flow control valves. The
amber indicator lights next to the switches il-
luminate when the motive-flow valves are in
transit or are not in the position selected on the
switch. Each jet pump switch (and motive-
flow control valve) controls both jet pumps
(wing and tip) on that side.
NOTE
Should operational requirements
make it necessary to turn off a jet
pump switch, the standby pump on
the same side must first be turned on
to ensure against loss of pressure to
the engine-driven pump.
NOTE
With an engine shut down, both the
wing and the tip tank jet pumps are
inoperative on that side.
NOTE
If a jet pump indicator light remains
illuminated after a selection is made,
the motive-flow valve is not in the po-
sition selected by the switch.
FILTERS AND
PRESSURE SWITCHES
A fuel filter is installed in each engine feed
line. Should the filters become clogged from
ice or contaminants, the fuel is allowed to by-
pass the filters. A differential pressure switch
installed in each filter assembly signals the
cockpit when one or both filters are bypass-
ing fuel. One amber FUEL FILTER annunci-
a t o r i l l u m i n a t e s w h e n t h i s o c c u r s ( s e e
“Annunciator Panel” section).
MAIN FUEL SHUTOFF VALVES
The 28-VDC motorized fuel shutoff valves
are used to isolate fuel from the engines. They
are two-position (open-closed). The valves
are controlled by the left and right FIRE
switches on airplanes with glareshield warn-
ing lights and by the red guarded FIREWALL
SHUTOFF switches in airplanes without
glareshield warning lights.
To close the valves on airplanes equipped with
glareshield warning lights, raise the plastic
guard and push on the FIRE light. Adjacent to
the FIRE light is a red shutoff valve warning
light hot wired to the battery bus, which in-
dicates the position of the fuel shutoff valve
switch, not necessarily the valve position. If
the FIRE light has been depressed for the
closed position, the red shutoff valve light
will be on. To reopen the fuel shutoff valve,
depress the FIRE light again and the red shut-
off valve light extinguishes.
NOTE
On all Learjets except the 23 models,
when the fuel shutoff valves are closed,
both fire extinguishers are armed.
LOW FUEL PRESSURE
WARNING LIGHTS
A low fuel pressure switch is located between
the fuel shutoff valve and the engine-driven
f u e l p u m p i n e a c h e n g i n e f e e d l i n e . T h e
switches cause illumination of the appropri-
ate red L or R FUEL PRES annunciator light
5A-10
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
