配電システム

概要

The power distribution system is the electrical backbone of the aircraft, routing power from generators and batteries to thousands of individual consumers through a hierarchy of buses, contactors, and circuit protection devices. On a modern widebody commercial aircraft, this network encompasses dozens of named buses operating at multiple voltage levels, hundreds of electromechanical or solid-state contactors, and thousands of conventional or solid-state circuit breakers. The architecture must guarantee that a single fault anywhere in the distribution system cannot deprive flight-critical consumers of power, a requirement addressed through redundant buses, automatic transfer switching, and carefully designed load priorities.

The distribution system has evolved from simple single-bus designs on early jets to highly sophisticated load management systems that continuously monitor bus voltages, generator loads, and fault conditions, automatically reconfiguring the network in milliseconds when anomalies are detected. Advanced aircraft such as the Boeing 777 and 787 use dedicated electrical load management computers that supervise all contactors and provide maintenance crews with detailed fault logging.

動作原理

AC power from engine generators enters the distribution system at the Main AC Buses, one per generator on most aircraft. Bus Tie Contactors (BTCs) connect the main buses together through a split bus bar arrangement. Under normal operation the BTCs are open, isolating each generator on its own bus segment to prevent a generator fault from propagating across the system. If one generator fails, its BTC closes automatically to cross-connect the remaining generator to both bus segments, maintaining power to all loads at the cost of increased loading on the operating generator.

Transformer Rectifier Units (TRUs) convert 115V AC from the main buses to 28V DC for the DC distribution buses. The DC buses power loads that require direct current, including engine ignition, fuel pump controls, some avionics, and battery charging circuits. Essential buses sit at a higher priority tier, connected to both normal AC/DC sources and emergency sources so they remain powered across the full range of failure scenarios. Standby buses represent the final tier, kept live by batteries alone when all generated power is unavailable.

主要コンポーネント

• Main AC Bus: Primary 115V 400 Hz three-phase distribution point, one per generator channel, feeding the bulk of aircraft AC loads and all TRUs.
• AC Essential Bus: Reduced-load bus protected by automatic transfer to backup sources, powering primary flight instruments, FMC, and autopilot computers.
• Main DC Bus: 28V DC bus supplied by TRUs, distributing direct current to engine controls, fuel management, and secondary avionics.
• DC Essential Bus: Battery-backed 28V DC bus for minimum essential loads including radios, fire detection, and engine shutdown controls.
• Hot Battery Bus: Permanently energised, directly connected to battery terminals, powering smoke detectors and crash recorders without any switching.
• Load Control Centre (LCC): Panel housing all contactors and circuit breakers for a bus segment, providing both protection and switching functions.

航空機への適用

The Boeing 737-800 uses a split AC bus system with two main AC buses tied through a pair of bus tie contactors. Ground power or the APU connects to the system via the ground power bus. The AC transfer bus is automatically switched to the standby power system when both main generators fail. The Airbus A320-200 employs a similar split architecture with added sophistication: the Electrical Flight Control System (ELCS) is supplied by two independent AC essential buses, each from a different source, ensuring fly-by-wire actuation under any single failure. The Boeing 777 introduced the first generation of computerised electrical load management, with two Electrical Load Management Units (ELMUs) supervising all contactors and automatically shedding loads to prevent generator overload. The Boeing 787 extended this further with its Power Management Remote Data Concentrators distributed throughout the airframe, replacing miles of centralised wiring with local power management modules.

利点と制限事項

Hierarchical bus architectures with automatic transfer switching provide high reliability with minimal crew intervention during electrical failures. Computerised load management reduces the pilot workload associated with electrical system monitoring and enables predictive load shedding before generator overload occurs. The primary design challenge is the sheer complexity of the system: each added bus, contactor, and control function introduces potential failure modes, and the interaction of automatic switching logic during cascading failures can produce unexpected system states. Simulator training for electrical emergencies must therefore cover not only the prescribed procedures but also the non-normal states that automatic systems can create. Maintenance access to the distribution system, often spread across multiple avionics bays and overhead panels, represents a significant cost driver throughout the aircraft's service life.