NO-BLEED, MORE ELECTRIC SYSTEMS ARCHITECTURE
The Boeing 787 reflects a completely new approach to onboard systems. Virtually everything that has traditionally been powered by bleed-air from the engines has been transitioned to an electric architecture. The affected systems include:
- Engine start
- Auxiliary power unit (APU) start
- Wing ice protection
- Cabin pressurization
- Hydraulic pumps
|UNAFFECTED SYSTEMS:||PNEUMATIC COMPONENTS REMOVED FROM THE ENGINE AND APU:||PNEUMATIC COMPONENTS REMOVED FROM THE AIRFRAME:||AFFECTED SYSTEMS:|
|Engine anti-ice system||Precooler
APU load compressor
Overheat monitoring systems
Duct burst protection systems
Wing ice protection
The only remaining bleed system on the 787 is the anti-ice system for the engine inlets.
While much can be said regarding the efficiency gains achieved by changing the means of extracting power for airplane systems from the engines, the 787’s no-bleed architecture brings with it some significant maintenance cost and reliability advantages as well. By eliminating the pneumatic systems from the airplane, the 787 will realize a notable reduction in the mechanical complexity of airplane systems. Overall, the 787 will reduce mechanical systems complexity by more than
- Pneumatic engine and APU start motors
- APU load compressor
- Various ducts, valves, and air control systems
- Leak and overheat detection systems
Auxiliary power unit. The APU provides an excellent illustration of the benefits of the more electric architecture. One of the primary functions of a conventional APU is driving a large pneumatic load compressor. Replacing the pneumatic load compressor with starter generators results in significantly improved start reliability and power availability. The use of starter generators reduces maintenance requirements and increases reliability due to the simpler design and lower parts count. In terms of inflight start reliability, the 787 APU is expected to be approximately four times more reliable than conventional APU s with a pneumatic load compressor.
Electrical power generation. Another fundamental architectural change on the 787 is the use of variable frequency electrical power and the integration of the engine generator and starter functions into a single unit. This change enables elimination of the constant speed drive (also known as the integrated drive generator, IDG), greatly reducing the complexity of the generator. In addition, by using the engine generator as the starter motor (an approach used with great success on the Next-G eneration 737 APU), the 787 has been able to eliminate the pneumatic starter from the engine.
When compared to the more complex 767 IDG, the 787 starter generator is predicted to have a mean time between faults (MTBF) of 30,000 flight hours — a 300 percent reliability improvement compared to its in-service counterpart.
Brakes. One innovative application is the move from hydraulically actuated brakes to electric. Electric brakes significantly reduce the mechanical complexity of the braking system and eliminate the potential for delays associated with leaking brake hydraulic fluid, leaking valves, and other hydraulic failures.One innovative application of the more-electric systems architecture on the 787 is the move from hydraulically actuated brakes to electric. Electric brakes significantly reduce the mechanical complexity of the braking system and eliminate the potential for delays associated with leaking brake hydraulic fluid, leaking valves, and other hydraulic failures. Because its electric brake systems are modular (four independent brake actuators per wheel), the 787 will be able to dispatch with one electric brake actuator (EBA) inoperative per wheel and will have significantly reduced performance penalties compared with dispatch of a hydraulic brake system with a failure present. The EBA is line-replaceable enabling in-situ maintenance of the brakes.
In general, electric systems are much easier to monitor for health and system status than hydraulic or pneumatic systems; the brakes take full advantage of this. Continuous onboard monitoring of the brakes provides airlines with a number of advantages, such as:
- Fault detection and isolation
- Electrical monitoring of brake wear
- Ability to eliminate scheduled visual brake wear inspections
- Extended parking times
Because the 787 brakes can monitor the braking force applied even while parked, the electric brakes enable extended parking brake times by monitoring and automatically adjusting its parking brakes as the brakes cool.
At an airplane level, the reduction in systems parts by moving to a primarily electric architecture is significant. Overall, the 787 will reduce mechanical systems complexity by more than 50 percent compared to a 767; elimination of pneumatic systems is a major contributor. As a consequence of this reduction in complexity, airlines will experience reduced airplane-level maintenance costs and improved airplane-level dispatch reliability.
In fact, the move to electric systems is expected to cut about a third of the schedule interrupts compared to a 767 for the systems affected by the no-bleed/more-electric architecture. Other benefits include improved health monitoring, greater fault tolerance, and better potential for future technology improvements.