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Aviation History
1994
1994 - 0075.PDF
PW901 APU DESCRIBED The gas-generator section is basically the JT15D core — a single-stage centrifu gal high-pressure (HP) compressor, re verse-flow annular combustor and a single-stage HP turbine. As its name suggests, the gas-generator section gener ates the hot gas which drives the load- compressor spool. PROVISION OF POWER The load-compressor section supplies electrical and pneumatic power to the aircraft. The spool comprises a single- stage power turbine driving a single-stage centrifugal load compressor, which sup plies compressed air to meet aircraft pneumatic requirements. A stub-shaft bolted to the load-compressor impeller transmits torque to the load gearbox, which has drive pads for two 90kVA electrical generators and a cooling fan. A radial plenum provides a common air inlet for both the gas-generator and load- compressor sections. Air entering the plenum is diverted either aft, through the HP-compressor impeller, or forward, through the load-compressor impeller. The PW901A installation uses the inlet and exhaust ducts developed for the earlier Garrett-equipped 747s. Installed in the 747-400 tailcone, the PW901A provides compressed air for main-engine starting and for cooling or heating the aircraft on the ground. While engine starting requires high pressure, air-conditioning requires high flow: the PW901A provides a minimum 165kg/min at 4bar (581b/in2) for engine starting and 250kg/min at 3.7bar for air conditioning. The P&WC design requires that the load-compressor impeller be run at a constant 100% rpm, regardless of aircraft need for compressed-air supply, to pro vide uniform electrical generation. To achieve this, the flow of air, both into and out of the load compressor, is modulated in response to demands from the aircraft environmental-control system (ECS) to maintain impeller speed at 21.625RPM. Airflow into the load compressor is controlled by variable-geometry inlet guide vanes (IGVs), while delivery flow from the impeller is regulated using a diverter valve, which dumps excess com pressed air into the APU exhaust. The IGVs and diverter valve are controlled by the PW901A's electronic control unit (ECU) to optimise efficiency. The Hamilton Standard ECU provides unattended automatic operation of the APU from start-up to shut-down. The only cockpit control is a switch with three positions — OFF, ON and START. Turn ing the switch momentarily to START initiates an automatic sequence which begins with the opening of the air-inlet door in the 747 tailcone and ends with the APU running at its no-load speed. Turning the APU selector switch to OFF initiates an automatic sequence which begins with shut-down of the pneumatic supply and cancellation of the electrical load. The ECU then decelerates the engine to low speed for cooling and, after lmin, shuts down the APU. Between start-up and shut-down, APU operation is unattended. Parameters mon itored by the ECU are transmitted via AR1NC 429 digital databus to the 747- 400's engine-indication and crew-alerting system for display to the crew, and are stored in electronically erasable read-only memory for access, again via ARINC 429, by the 747-400's central maintenance- computer system. In addition to start and stop sequenc ing, communicating with the aircraft, and recording maintenance information, the PW901A ECU provides speed governing, speed and temperature limiting and inde pendent overspeed diverter-valve and IGV control. The ECU manages the APU through the fuel-metering unit, IGV actu ator and diverter-valve control. If, after establishing IGV position based on inlet temperature and ambient pres sure, the ECU receives a pneumatic- demand signal from the aircraft ECS, it commands the single fuel-powered pres sure actuator to re-position the IGVs to provide the required airflow. The ECU always maintains sufficient IGV travel to cope with a sudden electrical demand by closing the IGVs to maintain load- compressor speed. The position of the butterfly diverter valve is controlled by the ECU, via a fuel-powered pressure actuator, as a func tion of IGV angle, inlet temperature and ambient pressure. This allows the APU to operate close to the compressor surge line at all times, increasing its efficiency. The ECU uses several sensors, includ ing magnetic speed sensors on the load compressor and gas generator which also provide independent overspeed detection on both spools. Speed is controlled by the ECU through the fuel-metering unit. SENSING CHANGES Sensors on the airframe-mounted trans formers associated with each generator allow the ECU to sense rapid changes in generator load and so minimise the drop in engine speed. Exhaust-gas temperature sensors allow the ECU to limit APU temperature. The PW901A is shut down automatically in an emergency such as a severe overspeed or overtemperature. The PW901A's accessory gearbox is mounted beneath the air-inlet case and driven, via a tower shaft and bevel gears, by a stub-shaft bolted to the HP- compressor impeller. The gearbox has drives for the fuel and oil pumps and for the battery-powered starter, which turns the gas-generator spool via the tower shaft. One of the advantages of the two-spool January, 1994 25
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