automatically and accurately regulate the pump
the range of operation. This characteristic requires
pressure and delivery to the engine.
the use of a pressure relief valve for disposing of
excess fuel. A constant-displacement gear-type
Where variable-displacement pumps are
pump is illustrated in figure 4-26.
installed, two similar pumps are provided,
connected in parallel. Either pump can carry the
load if the other fails during normal parallel
Variable-Displacement Pump
operations. At times, one pump is not enough to
meet power requirements. Pump duplication
The variable-displacement pump system dif-
increases safety in operation, especially in takeoffs
fers from the constant-displacement pump system.
and landings.
Pump displacement is changed to meet varying
fuel flow requirements; that is, the amount of fuel
The positive-displacement, variable-stroke
that is discharged from the pump can be made
type of pump incorporates a rotor, a piston, a
to vary at any one speed. With a pump of variable
maximum speed governor, and a relief valve
flow, the applicable fuel control unit can
Figure 4-26.-Cutaway view of a dual-element gear-type pump.
4-29