Parallel Systems Multiengine aircraft, such as the Boeing 727, MD-11, and  the early Boeing 747, employ a parallel power distribution  system. During normal flight conditions, all engine-driven  generators connect together and power the AC loads. In this  configuration, the generators are operated in parallel; hence  the name parallel power distribution system. In a parallel  system, all generator output current must be phase regulated.  Before generators are connected to the same bus, their output  frequency must be adjusted to ensure the AC output reaches  the positive and negative peaks simultaneously. During the  flight, generators must maintain this in-phase condition for  proper operation.  flight crew through the electrical control panel and used to  connect all necessary buses. GBs are used to connect and  disconnect the generators. Figure 9-105  shows a simplified parallel power distribution  system. This aircraft employs three main-engine driven  generators and one APU generator. The APU (bottom right)  is not operational in flight and cannot provide backup power.  The APU generator is for ground operations only. The three  main generators (bottom of  Figure 9-105 ) are connected to  their respective AC bus through GBs one, two, and three.  The AC buses are connected to the sync bus through three  BTBs. In this manner, all three generators share the entire AC  electrical loads. Keep in mind, all generators connected to  the sync bus must be in phase. If a generator fails, the flight  crew would simply isolate the defective generator and the  flight would continue without interruption.  One advantage of parallel systems is that in the event of a  generator failure, the buses are already connected and the  defective generator need only be isolated from the system.  A paralleling bus, or synchronizing bus, is used to connect  the generators during flight. The synchronizing bus is often  referred to as the sync bus. Most of these systems are less  automated and require that flight crew monitor systems and  manually control bus contactors. BTBs are operated by the  The number one and two DC buses ( Figure 9-105  top left)  are used to feed the DC electrical loads of the aircraft. DC  bus 1 receives power form AC bus 1 though TR1. DC bus  2 is fed in a similar manner from AC bus 2. The DC buses  Figure 9-105.  Parallel power distribution system. 9-63 DC BUS 2 HOT BAT BUS DC BUS 1 SYNC BUS ESS AC BUS GEN 1 GEN 2 GEN 3 APU GEN GB GB GB BTB BTB BTB GB AC BUS 1 AC BUS 2 AC BUS 3 ESS TR TR 1 TR 2 115V 28V DC ESS BUS BAT BUS R52 R56 EXT PWR RECP1 R52