The start derichment valve is open during light-off and acceleration to a set Pc. The open derichment
valve vents Py pressure to atmosphere. Venting Py allows the governor bellows to move the metering
valve against the minimum flow stop. At minimum flow the metering valve provides the required lean
fuel schedule after light-off. As compressor rpm increases, the derich-ment valve is closed by Pc acting on
the derichment bellows. When the derichment valve is closed, control of the metering valve is returned to
the normal operating schedule.
During acceleration, the Px and Pv pressures are equal to the modified compressor discharge pressure
(Pc) up to the point where the speed enrichment orifice is opened by flyweight action. Opening the
speed enrichment orifice bleeds Px pressure while Py remains at a value equal to Pc. Under the influence
of the Py minus Px pressure drop across the governor bellows, the metering valve moves toward the
maximum flow stop where it increases fuel flow.
Gas producer speed is controlled by the gas producer fuel control governor. A set of flyweights operate
the governor lever which controls the governor bellows (Py ) bleed at the governing orifice. Flyweight
operation of the governor lever is opposed by a variable spring load. The spring force is established by the
throttle lever acting on a spring scheduling cam. Opening the governing orifice bleeds Py pressure and
allows Px pressure to control the governor bellows. The Px influence on the bellows moves the metering
valve toward minimum flow and at a position where metered flow is at steady state requirements.
The governor reset assembly in the gas producer fuel control limits or governs power turbine speed.
Control of the reset assembly is derived from the power turbine governor. The power turbine governor
also provides quick responding overspeed protection by bleeding governor servo (Py) pressure from the
gas producer fuel control.
1-16. Power Turbine Governor. Power turbine speed is scheduled by the power turbine governor lever
and the power turbine speed scheduling cam. The cam sets a governor spring load which opposes a fly-
weight output. As the desired speed is approached, the flyweights operating against the governor spring
move a link to open the power turbine governor orifice.
The flyweights also open the overspeed bleed
(Py) orifice but at a higher speed than the regular governor (Pg) orifice.
The governor orifice is downstream of a bleed supplied by a regulated air pressure (Pr). Opening the
orifice results in a reduced pressure downstream of the bleed (Pg) as an inverse function of increasing
speed. Regulated pressure (Pr) and governing pressure (Pg) are applied to opposite sides of a diaphragm in
the governor reset section of the gas producer fuel control. The force generated by Pr minus Pg across
the diaphragm acts on the gas producer power output link through the governor reset rod. This force
supplements the weight force in the gas producer fuel governor to reset (reduce) the gas producer speed.
Gas producer speed cannot exceed the gas producer fuel governor setting. The Pr minus Pg diaphragm is
preloaded for establishing the active Pr minus Pg range. Pr pressure is supplied from engine Pc pressure by
an air regulator valve.
The overspeed orifice bleeds Py pressure from the governing system of the gas producer fuel control.
Bleeding Py pressure at the power turbine governor gives the fuel control system a rapid response to
1-17. Fuel Pump and Filter Assembly. The fuel pump and filter assembly (fig. 1-8) has two gear-type
pumping elements arranged in tandem and driven by a common drive shaft. Fuel enters the engine fuel
system at the inlet port of the pump and passes through a low pressure filter before entering the gear
elements. The gear elements are in parallel. Each pumping element has sufficient capacity to permit
takeoff power operation if the other pumping element fails. Two discharge check valves are in the assem-
bly to prevent reverse flow if one gear pumping element fails. A bypass valve in the pump assembly by-
passes fuel if the filter element becomes clogged.