refer to figure 3-20 and the flow diagrams of the
It passes through the pump boost element,
model 104 and model 139 fuel systems. The fuel
through the LP filter, and into the HP elements.
pump is an engine-driven, dual-element pump. It
From the pump's HP elements, the fuel passes
is mounted on the aft right side of the AGB. The
through the HP filter and into the LFV. Metered
boost element has an impeller-type centrifugal
fuel from the LFV passes through the fuel shutoff
pump and bypass valve. The HP element has a
valve. It then flows through the flow divider and
into the fuel manifolds and fuel nozzles. It is
dual-element (primary and secondary) gear-type
discharged into the combustion chambers by the
pump.
In operation, fuel enters the boost pump and
fuel nozzles. The fuel pump delivers more fuel
then flows externally through the LP filter. It then
than is required. So the LFV bypasses the excess
returns to the HP elements through passages in
fuel back to the inlet side of the pump's HP
the HP filter assembly. (It is not actually going
elements.
through the filter element at this point.) The
Model 139 Fuel System Flow Path
bypass valve (not shown) opens only if the boost
pump fails. This allows fuel to flow directly to
Fuel system operation of the model 139 is
the HP filters through the LP filter. Fuel normally
similar to the model 104 (refer to fig. 3-19). After
flows in series through the primary and secondary
the fuel shutoff valve, the fuel goes to the fuel
elements of the two HP elements. However, the
divider. Some of the fuel goes directly to the pilot
two HP elements. However, the two elements are
manifold. At 150 psig, fuel is also diverted to the
placed in parallel from about 2200 to 8400 rpm
main fuel manifold. Two manifold drain valves
by a solenoid-operated paralleling valve. The valve
are also used to drain both manifolds at shut-
is located in the fuel pump assembly. From the
down. Remember, some model 104 units have
HP element of the pump, fuel flows through
been modified to use this flow divider and dual-
internal passages to the HP filter.
entry fuel nozzles.
Low-Pressure Filter
Fuel Pump
The LP filter is a paper cartridge type. It is
To help you understand the following dis-
located in the fuel line between the boost pump
cussion of the various fuel system components,
Figure 3-20.--Fuel system components-bottom view of engine.
3-24
