PRE SB 72-032
 - discharge pressure: 1000 psig (6895 kPa gage) (maximum)

 - speed rating: 6250 RPM

 - Fuel Flow (maximum): 59 USgal.mm (223 l.min).

 POST SB 72-032

 - discharge pressure: 1145 psig (7895 kPa gage) (maximum)

 - speed rating: 6250 RPM

 - Fuel Flow (maximum): 68 USgal.mm (257 l.min).

 (b) Functional Description (Ref. Fig. 007) The HP fuel pump consists of 2 gearshafts. Each gearshaft is supported by 2 plain bearings ; one is stationary and the other self-aligning.
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 E E The filtered fuel enters the HP stage and, after pressurization, is delivered to the HMU. Should an overpressure occur (discharge pressure higher or equal to 1050 psi (7240 kPa), a valve bypasses the fuel toward the HP stage inlet.

 (c) HP stage lubrication system The fuel used to lubricate the LP stage items, lubricates also the HP stage rotating parts. The lubricated elements are:
 - the HP stage bearings, and

 - the thrust ring.
 The lubricating fuel then flows back to the filter inlet.

 (4) Fuel filter

 (a) General This fuel filter is located between the main oil/fuel heat exchanger and fuel pump HP stage. It protects the HMU from particles in suspension in the fuel. The filter disposable cartridge is designed to retain foreign material. The cartridge is periodically replaced and removed. The cartridge is discarded.

 (b) Functional description The fuel filter consists of a filter cartridge and a pressure relief valve. The filter cartridge is installed in a cavity on the pump body. The fuel circulates from the outside to the inside of the filter cartridge. The cartridge has a filtering capability of 32 microns absolute (PRE SB 72-032) or 38 microns absolute (POST SB 72-032). In case of a clogged filter, a pressure relief valve bypasses the fuel to the HP stage. Tappings on pump housing enable transmission of the differential pressure across the fuel filter to the fuel differential pressure switch. This pressure switch supplies a clogging indication to the cockpit (Ref. 73-30-00).

 (5) Wash flow filter

 (a) General The wash flow filter is located at the HP stage discharge. It catches the particles in suspension remaining in the fuel supplying the HMU servos. Foreign materials retained by the filter are removed by the fuel flow supplying the HMU metering system.
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 (b) Functional description The wash flow filter is incorporated with the pump housing. It consists of a filtering element and a pressure relief valve. Should the filter clog, a pressure relief valve bypasses the fuel to the HMU servos fuel supply. The wash flow filter has a filtering capability of 65 microns absolute.
 4. Servo Fuel Heater
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 A. General
 (Ref. Fig. 008)

 (1) The servo fuel heater raises the temperature. This prevents ice from entering the control servos inside the hydromechanical fuel unit (HMU).

 (2) The servo fuel heater is mounted on the aft section of the main oil/fuel heat exchanger located on the accessory gearbox (AGB) aft face, between the oil tank and the fuel pump/HMU package.

 B. Description and Operation
 (Ref. Fig. 009)
 The servo fuel heater consists of :

 - a housing with a heat exchanger core inside

 - a cover.
 The fuel enters and leaves through the cover.
 The oil :

 - enters the heater through one flange of the housing

 - circulates around the core tubes, and

 - leaves through the opposite flange of the housing.
 The oil-out flange has 2 orifices :
 - one corresponding to the normal circuit

 - the other is used if clogging of the main oil/fuel heat exchanger

 restricts oil circulation beyond acceptable limits. The servo fuel heater is a heat exchanger using oil as its heat source. Heat exchange between oil and fuel occurs by conduction and convection inside the unit. The 2 fluids circulate in the servo fuel heater through separate flowpaths. (Ref. Fig. 010) The oil initially lubricates and cools the engine sumps. Then it enters the servo fuel heater, giving off heat to the fuel from the wash filter. The heated fuel then flows to the internal HMU servos.
 NOTE : There is no fuel anti-clogging valve in the servo fuel heater
　
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