The Boeing 777’s Auxiliary Power Unit (APU) is a crucial component, providing both electrical and pneumatic power to the aircraft whether on the ground or in flight. The APU, an AlliedSignal Engines 331-500 model located in the tail cone of the aircraft is controlled and monitored by the APU controller (APUC).
The APU functions as a pneumatic power source up to an altitude of 22,000 feet, and as an electrical power source up to the aircraft’s service ceiling, with a 120 kVA generator. It features a dual starting system, comprising an electric and an air turbine starter, which facilitates the APU’s start when there is pressure in the pneumatic system. Uniquely, the APU utilizes an eductor air/oil cooling system instead of the typical mechanical fan for cooling.
An innovative aspect of the Boeing 777 APU is its autostart feature, which automatically starts the APU if the airplane is in the air and loses power in both the left and right transfer buses. It operates under a full authority digital electronic control system, employing microprocessor electronics to provide automatic control for all operating conditions. The APU can operate in two modes: attended and unattended. In the attended mode, only safety-related faults cause automatic shutdowns, whereas in the unattended mode, any fault that could damage the APU triggers a shutdown.
The APU’s design allows it to operate normally during refuelling operations and features a cluster component design for ease of maintenance. These subsystem components, like fuel, lubrication, and ignition, are organized into functional clusters that are line replaceable units and the exhaust muffler exhaust duct reduces noise.
In terms of operation data, the APU’s data memory module records various operational metrics such as the number of starts, operating hours, and average generator load. This information is crucial for maintenance and operational analysis.
The APU fuel system, sourced from the left main tank, includes components like fuel clusters and manifolds, ensuring the efficient supply and regulation of fuel for combustion. The APUC plays a key role in controlling fuel flow during different phases of operation and adjusting the APU generator speed for seamless power transfers.
The pneumatic system of the APU, controlled by the APUC and the electrical load management system (ELMS), is responsible for various functions including environmental control, hydraulic pumps operation, main engine start, and wing anti-ice. Maintenance convenience is enhanced by clustering some pneumatic components.
The ignition and starting system includes components like the air turbine starter, electric starter, and ignition unit with dual ignitors, ensuring reliable APU operation. The lubrication system, with its lube cluster, oil filters, and eductor oil cooler, is critical in removing heat and lubricating key APU components.
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