Composite construction enabled the manufacturer to design a relatively stiff 10:1 aspect ratio wing for better lift-to-drag performance. Cabin pressurization also could be higher without incurring a significant weight penalty. Outside suppliers build large subassemblies that are shipped to Boeing facilities at Everett, Wash., and North Charleston, S.C., where they are joined using mechanical fasteners. Compared with conventional aluminum airframes, assembling the composite structure requires much less hand labor.
The 787 relies heavily on electrical power for functions that were powered by bleed air or hydraulics aboard the 777. Engine start, pressurization, horizontal stab trim, airframe ice protection and wheel brakes, for instance, are electrically powered. Each engine has two 250-kva starter generators in contrast to the single 120-kva integrated-drive generators on each engine of the 777. In addition, the auxiliary power unit (APU) has two 225-kva starter generators, thus there is a total 1.45 megawatts of power available. Each of the six starter generators produces three-phase, 235-volt alternating current (VAC) power. The 787 is also the first civil aircraft to have lithium-ion main batteries.
The starter generators are direct drive, so AC frequency varies with engine speed from about 360-800 hz. Approximately 40 systems, such as the wing anti-ice heaters, main fuel pumps, horizontal stab trim, alternate flaps and cargo bay heaters, are designed to use 235-volt variable-frequency AC power. Much of the 235-VAC power, though, is converted into 115-VAC 400-hz. three-phase, 28-volt DC and ±270-volt DC for use by other electrical systems.
High-amperage ±270-volt large-motor controllers—essentially liquid-cooled transformer rectifiers—supply power to variable-speed motors for hydraulic pumps, cabin air compressors, engine and APU starting, center tank fuel pumps and the fuel tank nitrogen inerting system, among a dozen such loads.
A network of 17 remote power distribution units supply 115Vac and 28Vdc for lighting, wheel brakes, avionics, windshield and air data probe heat, engine igniters, and cabin/galley services. The power supplies also provide secondary and back-up power for the digital fly-by-wire flight-control system.
In the cockpit, the 787 has electronic (“virtual”) rather than physical circuit breakers. These are monitored and controlled by means of point-and-click commands on a circuit breaker schematic displayed on the engine-indicating and crew-alerting system.
All fuel is stored in wet main and center wing tanks that are flooded with nitrogen to inhibit fuel vapor combustion. There are dual-AC fuel-boost pumps in each wing tank and center fuel tank. If no AC power is available, a stand-alone DC boost pump in the left main fuel tank supplies fuel for APU starting. If a fuel imbalance develops, the flight crew can balance the load simply by pressing a “balance” button in the overhead panel.
The aircraft has been upgraded with a 5,000-psi hydraulic system that uses smaller lines and actuators, thereby saving weight. Similar to the 777, the new aircraft has left and right engine-driven pumps. But, the left, right and center system pumps have variable-speed DC motors rather than constant-speed AC motors. The center system has two high-voltage DC pumps rather than two AC and two on-demand, bleed-air-powered pumps.
Hydraulic power is used for virtually the same functions as aboard the 777, but the normal and alternate wheel brakes are 28Vdc-powered.
The 787's digital fly-by-wire flight-control system architecture is similar to the 777 and it uses the same C*U (pronounced “Sea Star U”) pitch control law. C* means that fore/aft yoke movement commands pitch rate on the ground, and g rate or vertical acceleration (Nz) in the air. U means that speed stability is built into the control laws, so the pilot has to manually trim pitch in flight with speed changes.