Figure 5-8.-Engine start control system.
the pressure to a minimum value. Fuel passes
Ignition Exciter
through the low-pressure filters before going to
the secondary element. There is a differential
The ignition exciter (discussed earlier) is a dual
pressure switch connected across the inlet and
electronic ignition unit that uses 28-volt dc from
outlet of the filters. If the pressure differential
exceeds 7.5 PSI, the switch closes and completes
voltage to a proper level for firing the igniter
a circuit to a filter light at the flight deck. Fuel
plugs. The exciter unit contains two identical
then flows to the primary element and through
circuits, each one independently capable of firing
the high-pressure filter assembly before entering
its own igniter plug. The speed-sensitive control
the fuel control. Both the low- and high-pressure
energizes the ignition relay so the exciter is in
filters have bypass valves that open if the filters
operation between 16 and 65 percent of engine
become clogged.
RPM.
The capacity of the pump's primary element
is 10 percent greater than that of the secondary
Engine Fuel Pump and Filter
element. If the primary element were to fail, the
secondary element would provide enough flow to
The fuel pump and high-pressure filter
operate the engine.
assembly mount on the rear of the accessories
During engine starting, the elements operate
case. This assembly consists of a centrifugal boost
in parallel to provide enough fuel flow at low
pump, two gear-type pressure elements, and a
RPM; above 65 percent, they operate in series.
high-pressure filter.
Parallel operation occurs during starting when
Fuel entering the pump assembly passes
engine speed is between 16 percent and 65 percent
through the centrifugal boost pump, which raises
