which causes the jet pumps to function. The
fuel is routed through the motive-flow control
valves (one on each side) to the jet pumps,
where it sprays through a small orifice into a
venturi. The low pressure created in the ven-
turi draws fuel from the respective tank, result-
ing in a low-pressure, high-volume output
from the jet pump (Figure 5-6).
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 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 pres-
sure 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
position selected by the switch.
The motive-flow control valves close auto-
matically when the GEN–START switch is
energized, and reopen when the GEN–START
switch is moved to OFF.
FILTERS
A fuel filter is installed in each engine feed line
to filter the fuel before it enters the engine-
driven fuel pump. Should the filters become
clogged from ice or contaminants, the fuel by-
passes the primary element and is filtered
through a secondary element. A differential
pressure switch installed in each filter assem-
bly illuminates the amber FUEL FILTER light
when one or both filters are bypassing fuel.
MAIN FUEL SHUTOFF VALVES
The fuel shutoff valves are powered by essen-
tial bus DC power through the L and R FW
SOV circuit breakers and are controlled by
the FIRE switches on the glareshield. Pushing
either FIRE switch closes the associated
valves, illuminates the SOV light, and arms the
fire extinguisher switches. Pushing the FIRE
switch a second time will open the valve.
NOTE
When the FIRE switch is pushed,
the fire extinguisher is armed. Do
n o t a c c i d e n t a l l y d e p r e s s t h e
ARMED buttons.
LOW FUEL PRESSURE
WARNING LIGHTS
A low fuel pressure switch is located in each
engine feed line. The switches cause illumi-
nation of the appropriate red L or R FUEL
PRESS annunciator light when fuel pressure
drops below 0.25 psi. The light extinguishes
w h e n p r e s s u r e i n c r e a s e s a b o v e 1 . 0 p s i .
Illumination of a FUEL PRESS warning light
is an indication of loss of fuel pressure to the
5-9
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
