General Discussion Of Engine Fuel Pumps

The primary fuel pump used for providing the boost and high-pressure fuel pumping function for an engine is called the "main fuel pump." It is engine gearbox-mounted and driven. In its most common form, it includes a positive displacement external spur gear high-pressure stage and an integral centrifugal boost stage. This type of pump is exclusively used on all modern commercial airliner engines and supplies all of the fuel flow requirements of the engine combustors. On commercial airliner engines, the main fuel pump is the sole engine fuel pump and therefore is a prime reliability piece of equipment. All engines, commercial and military, have a main fuel pump.

Positive displacement pumps have certain operating characteristics that make them very adaptable to aircraft gas turbine engine fuel systems. These characteristics include the ability to reprime from a completely dry condition and the ability to deliver useful fuel pressure over a wide speed range. Engine control systems utilize a portion of the main fuel pump discharge flow to power the actuation systems for fuel burn flow metering and engine variable geometry control for all operating conditions. Corner point operating conditions, such as altitude re-light and ground starting, occur at 8 to 10 percent of rated operating speed. The pressure required for actuator muscle and response characteristics at these conditions is generally a 250 lb/in2 (17.2 bar) pressure rise. At the maximum power take off condition at 100 percent speed, the pressure rise requirement ranges up to 1500 lb/in2 (103 bar). Positive displacement pumps have proven capable of meeting these needs and the external spur gear pump type has received the widest acceptance from the industry.

The key attribute of the external spur gear main fuel pumps relates to the safety of flight and prime reliability requirements. In over fifty years of experience, this type of

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Survival Treasure

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