(3) The returning fuel bypassed from the HMU is tapped to feed the Integrated Drive Generator (IDG) oil cooler. There it cools the oil coming from the engine IDG. The fuel returns then to the pump upstream of the filter, between the LP and HP fuel pump stages. The fuel returns via a port located on the oil/fuel heat exchanger.
(4) If the filters or heat exchanger become clogged:
- a number of bypass valves are provided to prevent excessive pressure build-ups.
(5) The fuel pump and HMU are mounted as a unit. This unit is located on the accessory gearbox (AGB) (aft face on the left side of the horizontal drive shaft housing, aft looking forward).
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Location of Fuel Pump on Engine
Figure 003
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R Fuel Pump System Diagram Figure 004 (SHEET 1)
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R R Fuel Pump System Diagram Figure 004 (SHEET 2)
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R R Fuel Pump System Diagram Figure 004 (SHEET 3)
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Fuel Pump and Filter
Figure 005
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B. Functional Description
(1) Fuel pump drive system
(Ref. Fig. 006)
The fuel pump drive system consists of the following:
(a) A main drive shaft, driven by the AGB which, in turn, drives the HP stage drive spur gear. The main drive shaft has a shear neck to protect the AGB from any excessive torque in the fuel pump drive.
(b) A LP stage drive shaft, driven by the HP stage drive spur gear, providing drive for:
1 The LP stage via a hollow shaft.
_
2 The HMU via the HMU drive shaft. The LP pump drive shaft has a
_
shear neck:
- to protect the HMU from LP stage failures
- to maintain proper engine operation even in case of total failure (seizing) of the LP stage.
(2) Fuel pump LP stage
(a) General The LP stage of the fuel pump is of the centrifugal type. It delivers a boost pressure to the HP stage to avoid pump cavitation. The LP stage general characteristics at takeoff power are as follows :
1 Boost (LP) stage pressure rise: 165 psid (1138 kPa) (minimum).
_
2 Speed rating: 6250 RPM.
_
(b) Functional Description
(Ref. Fig. 007)
1 The fuel pump LP stage consists of:
_
- an impeller supported by 2 plain bearings provided with swirl ramps,
- an inducer having helical grooves and a scroll.
2 The fuel discharged from the boost pumps:
_
- enters the fuel pump LP stage,
- leaves the pump under pressure,
- flows to the main oil/fuel heat exchanger
- and controls the fuel return valve.
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Fuel Pump Driving Components
Figure 006
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Fuel Pump LP and HP Stages
Figure 007
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(c) LP stage lubrication system The fuel tapped from the scroll at the discharge of the LP stage lubricate the LP stage rotating parts. The lubricated elements are :
- the front and rear bearings
- the LP pump drive shaft splines
- the HMU drive shaft splines
(3) Fuel pump HP stage
(a) General
1 The fuel pump HP stage converts mechanical energy into
_
hydraulic power required to supply:
- the fuel nozzles
- the HMU servos
- the variable bleed valve (VBV) and variable stator vane (VSV) hydromechanical systems.
2 The HP stage hydraulic power is supplied by a positive
_ displacement (gear-type) pump. For a given number of revolutions, the pump delivers a constant fuel flow regardless of the discharge pressure. A pressure relief valve connected in parallel with the HP pump protects the pump.
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