King Air C90GTi, Pilot Training Manual

Page: 161 / 557

ENGINE FUEL SYSTEM
The fuel control system for PT6A engines
is essentially a fuel gover nor that increases
or decreases fuel flow to the engine to main-
tain selected engine operating speeds. At
f irst glance, the system may appear quite
complicated. The engine fuel control sys-
tem consists of the main components shown
in the block diag ram (Figure 7-16). They
are the electric low-pressure boost pump,
oil-to-fuel heat exchanger, high-pressure
f u e l p u m p , f u e l c o n t r o l u n i t , f u e l c u t o ff
valve, flow divider, and dual fuel manifold
with 14 simplex nozzles.
The PT6A-135A engine uses an electric low-
pressure boost pump to supply a 30-psi head
pressure to the high-pressure engine-driven
fuel pump. This head pressure prevents fuel
cavitation at the high-pressure pump. The
fuel is also used for cooling and lubricating
the pump. The oil-to-fuel heat exchanger
uses war m engine oil to maintain a desired
fuel temperature at the fuel pump inlet to
prevent icing at the pump f ilter. This is done
with automatic temperature sensors and re-
quires no action by the pilot.
Fuel enters the engine fuel system through
the oil-to-fuel heat exchanger, and then flows
into the high-pressure engine-driven fuel
pump and on into the fuel control unit (FCU).
The high-pressure fuel pump is an engine-
d r ive n g e a r- t y p e p u m p w i t h a n i n l e t a n d
outlet f ilter. Flow rates and pressures will
var y with gas generator (N
1
) r pm. Its pri-
mar y pur pose is to provide suff icient pres-
KING AIR C90GTi/C90GTx PILOT TRAINING MANUAL
7-16
FOR TRAINING PURPOSES ONLY
7
POWERPLANT
ELECTRIC
BOOST
PUMP
FUEL FLOW
TRANSMITTER
ENGINE
DRIVEN
FUEL
PUMP
(800 PSI)
POWER
&
CONDITION
LEVERS
FUEL
CONTROL
UNIT
FUEL
TOPPING
GOVERNOR
MINIMUM
FLOW
VALVE
FUEL
CUTOFF
VALVE
FLOW
DIVIDER
FUEL DRAIN
PURGE
PURGE LINE TO
FUEL
TANK
P
3
AIR
P
3
AIR
N
1
GOVERNOR
OIL–TO–
FUEL
HEAT
EXCHANGER
LEGEND
VENT PRESSURE
MINIMUM FUEL FLOW
INDUCTION AIR
P
3
COMPRESSION DISCHARGE AIR
Figure 7-16. Simplified Fuel System Diagram