The fuel system
supplies clean, metered fuel to the combustion chambers during all conditions
of engine operation.
Boost pumps send fuel from the aircraft tanks to the fuel pump inlet.
The inlet fuel pressure is typically in the range of 15 - 25 psig. It
must be 5 psi higher than the pressure of the fuel but no more than
50 psi. The fuel flows thorough the (impeller) boost stage which increases
its pressure by approximately 10 to 60 psi from start to takeoff power.
If there is a boost stage failure the delta P across the impeller bypass
valve increases. The bypass valve opens permitting fuel to pass directly
to the pumps gear stage (if this occurs fuel does not pass through the
heater and main fuel filter). After passing through the pump it goes
to the fuel heater. The fuel is heated as it passes the heater providing
the fuel heat is on. If the pressure drop across the fuel heater is
large the fuel can bypass the heater. From the heater the fuel flows
to a fuel filter in the fuel pump. If this filter becomes clogged by
dirt or ice particles the delta P across the filter increases. If delta
P increases the differential pressure switch sends a signal which turns
on the icing light in the flight deck. If delta P increases further
the filter bypass opens. This permits the fuel from the heater to go
directly to the gear stage. (Fuel that goes from the boost stage through
the fuel heater and filter is called interstage fuel).
Fuel goes from the filter to the pump gear stage which increases it's
pressure. Approx. 150 psi when the engine is motored, approx. 900 psi
at takeoff power (1,000 psi for the -17/R/AR engines). If the output
pressure of the gear stage increases to more than 950 psi (1,050 psi
-17/R/AR) the high pressure relief valve opens. Returning some fuel
back to the gear stage inlet.
From the fuel pump gear stage fuel passes to the fuel control unit.
The FCU has inputs to it from some of the engine parameters and the
power level requested by the flight crew. It schedules the correct fuel
flow rate and pressure that is necessary for combustion. The metered
fuel then passes to the fuel flow transmitter, unneeded fuel is sent
back to the gear stage. (Fuel that goes from the FCU thorough the FF
Transmitter, fuel / oil cooler, P&D valve to the nozzles is called
metered fuel). The metered fuel goes through the fuel flow transmitter
which passes flow rate information to the flight deck. After the transmitter
fuel passes through the fuel / oil cooler which reduces the oil temperature.
Fuel now passes to the P & D valve (Pressurising & Dump). When
it enters the P & D valve it goes through a filter. This filter
has a bypass facility. The P&D valve divides fuel into primary and
secondary fuel flows and send it to the fuel manifolds. Above idle (higher
power settings) it also sends fuel to the secondary manifolds. Fuel
from the primary and secondary manifolds passes through the nozzle supports
and into nine fuel nozzles Here it is atomised and sent to the combustion
chambers.
COMPONENT LOCATIONS
Fuel / Oil Cooler
Located between
flanges B and D at the 8:00 position, attached to flange B Eng
LH side
Fuel Flow Transmitter
Located under
the fuel / oil cooler, attached to the fuel / oil cooler. Eng
LH side
Fuel Heater
Located between
flanges C and E at the 6:00 position, attached to the left side
of the fuel pump. Note that the tubing with 13th stage air for
the fuel heater passes on the right side of the engine. Eng underside
LH side"ish".
Tt2 Probe
Located at
the 7:00 position in the inlet case, connected to the FCU which
is on the RH side. Eng Inlet.
Pressurising
and Dump Valve (P&D)
Located between
flanges H and J at the 7:30 position, attached to the fan discharge
diffuser outer duct. Eng LH side.
Fuel Pump
Located between
flanges C and F at the 5:30 position, attached to the front of
the accessory gearbox. Eng RH side
Fuel Control
Unit (FCU)
Located between
flanges B and D at the 5:30 position, attached to the front of
the fuel pump. Eng RH side.
Moisture Trap
(Condensation Trap)
Located on
the front of the FCU
Fuel De-icing
Air Shutoff Valve and Actuator
Located between
flanges D and F at the 1:30 position, attached flange E. Eng RH
side.
External Tubing
Two tubes
with 13th stage air come out of the fan discharge diffuser outer
duct at the 1:00 and 11:00 positions. They connect to one tube
at the top of the engine, and goes to the fuel heater. A tube
with Ps4 air comes out of the fan discharge combustion outer duct
at the 12:30 position. It goes to the FCU (Routing varies on JT8
models)
Fuel Manifolds
Two short
fuel manifold tubes come out of the P&D valve and are divided
into two pairs of tubes. One pair of tubes goes into the diffuser
outer fan duct at the 5:00 position. The other pair of tubes goes
into the diffuser outer fan duct at the 7:00 position. The two
pairs of fuel manifold tubes go from the diffuser outer fan duct
to the diffuser case. They are connected to the fuel manifolds
which are installed on the diffuser case.
Fuel Nozzles
and Supports
Nine fuel
nozzles and support assemblies, located internally on both sides
of the engine.
The following components
are all LRU's. Fuel Pump, Fuel Control Unit, Fuel De-icing Air Shutoff
Valve and Actuator, External Fuel Manifolds.
