Aircraft Wiring Installation

+ - External power plugExternal power solenoid Starter solenoid Battery (master) solenoid Starter motor Master switch To split bus - + S B R L OFF +V To main bus prevent any transient voltage spikes produced by the starter from entering the sensitive avionics. [Figure 9-93] The circuit employs a normally closed (NC) solenoid that connects the avionics bus to the main power bus. The electromagnet of the solenoid is activated whenever the starter is engaged. Current is sent from the starter switch through diode D1, causing the solenoid to open and depower the avionics bus. At that time, all electronics connected to the avionics bus will lose power. The avionics contactor is also activated whenever external power is connected to the aircraft. In this case, current travels through diodes D2 and D3 to the avionics bus contactor.

A separate avionics power switch may also be used to disconnect the entire avionics bus. A typical avionics power switch is shown wired in series with the avionics power bus. In some cases, this switch is combined with a circuit breaker and performs two functions (called a circuit breaker switch). It should also be noted that the avionics contactor is often referred to as a split bus relay, since the contactor separates (splits) the avionics bus from the main bus. Landing Gear Circuit Another common circuit found on light aircraft operates the retractable landing gear systems on high-performance + - External power plugExternal power solenoid (NO) Avionics contactor (NC) (a.k.a. split bus relay) To starter contactor S B R L OFF To main bus A V I O N I C S B U S M A I N B U S Ignition switch D3 D2 D1 Avionics master switch 9-50 light aircraft. These airplanes typically employ a hydraulic system to move the gear. After takeoff, the pilot moves the gear position switch to the retract position, starting an electric motor. The motor operates a hydraulic pump, and the hydraulic system moves the landing gear. To ensure correct operation of the system, the landing gear electrical system is relatively complex. The electrical system must detect the position of each gear (right, left, nose) and determine when each reaches full up or down; the motor is then controlled accordingly. There are safety systems to help prevent accidental actuation of the gear.

A series of limit switches are needed to monitor the position of each gear during the operation of the system. (A limit switch is simply a spring-loaded, momentary contact switch that is activated when a gear reaches its limit of travel.) Typically, there are six limit switches located in the landing gear wheel wells. The three up-limit switches are used to detect when the gear reaches the full retract (UP) position. Three down-limit switches are used to detect when the gear reaches the full extended (DOWN) position. Each of these switches is mechanically activated by a component of the landing gear assembly when the appropriate gear reaches a given limit.

The landing gear system must also provide an indication to the pilot that the gear is in a safe position for landing. Many aircraft employ a series of three green lights when all three gears are down and locked in the landing position. These three lights are activated by the up- and down-limit switches found in the gear wheel well. A typical instrument panel showing the landing gear position switch and the three gears down indicators is shown in Figure 9-94. The hydraulic motor/pump assembly located in the upper left corner of Figure 9-95 is powered through either the UP or DOWN solenoids (top left). The solenoids are controlled by the gear selector switch (bottom left) and the six landing gear limit switches (located in the center of Figure 9-95). The three gear DOWN indicators are individual green lights (center of As each gear reaches its DOWN position, the limit switch moves to the DOWN position, and the light is illuminated.

position. It is always important to know gear position when reading landing gear electrical diagrams. Knowing gear position helps the technician to analyze the diagram and understand correct operation of the circuits. Another important concept is that more than one circuit is used to operate the landing gear. On this system, there is a low current control circuit fused at 5 amps (CB2, top right of 9-51 Terminal #3 Squat switch Gear selector switch Throttle switch Gear unsafe light (red) Terminal #2 Terminal #1 Control current Down motor, high current UP DOWN Hydraulic pump motor assembly AdvancedRetarded Gear horn FLT POSGND POS Landing gear motor (25 Amp) Landing gear control (5 Amp) 30A C.B.1 5A C.B.2 Not UP UP Not DN DN Not UP UP Not DN DN Not UP UP Not DN DN UP limit DN limit UP limit DN limit UP limit DN limit Right green down light Nose green down light Left green down light LEFT GEAR NOSE GEAR RIGHT GEAR UP DN the control of the gear motor contactors. There is a separate circuit to power the gear motor fused at 30 amps (CB3, top The following paragraphs describe current flow through right of Figure 9-95). Since this circuit carries a large current the landing gear circuit as the system moves the gear up flow, the wires would be as short as practical and carefully and down. Be sure to refer to Figure 9-96 often during the 9-52 following discussions. Figure 9-96 shows current flow when the gear is traveling to the extend (DOWN) position. Current flow is highlighted in red for each description.

To run the gear DOWN motor, current must flow in the control circuit leaving CB2 through terminal 1 to the NOT DOWN contacts of the DOWN limit switches, through terminal 3, to the DOWN solenoid positive terminal (upper left). The negative side of the DOWN solenoid coil is connected to ground through the gear selector switch. Remember, the gear DOWN switches are wired in parallel and activated when the gear reach the full-DOWN position. All three gears must reach full-DOWN to shut off the gear DOWN motor. Also note that the gear selector switch controls the negative side of the gear solenoids. The selector switch has independent control of the gear UP and DOWN motors through control of the ground circuit to both the UP and DOWN solenoids.

When the landing gear control circuit is sending a positive voltage to the DOWN solenoid, and the gear selector switch is sending negative voltage, the solenoid magnet is energized. When the gear-DOWN solenoid is energized, the high-current gear motor circuit sends current from CB1 through the down solenoid contact points to the gear DOWN motor. When the motor runs, the hydraulic pump produces pressure and the gear begins to move. When all three gears reach the DOWN position, the gear-DOWN switches move to the DOWN position, the three green lights illuminate, and the gear motor turns off completing the gear-DOWN cycle.

the current flow path shown in red as the gear moves to the retract (UP) position. Starting in the top right corner of the diagram, current must flow through CB2 in the control circuit through terminal 1 to each of the three gear-UP switches. With the gear-UP switches in the not UP position, current flows to terminal 2 and eventually through the squat switch to the UP solenoid electromagnet coil. The UP solenoid coil receives negative voltage through the gear selector switch. With the UP solenoid coil activated, the UP solenoid closes and power travels through the motor circuit. To power the motor, current leaves the bus through CB1 to the terminal at the DOWN solenoid onward through the UP solenoid to the UP motor. (Remember, current cannot travel through the DOWN solenoid at this time since the DOWN solenoid is not activated.) As the UP motor runs, each gear travels to the retract position. As this occurs, the gear UP switches move from the NOT UP position to the UP position. When the last gear reaches up, the current no longer travels to terminal 2 and the gear motor turns off. It should be noted that similar to DOWN, the gear switches are wired in parallel, which means the gear motor continues to run until all three gear reach the required position.

During both the DOWN and UP cycles of the landing gear operation, current travels from the limit switches to terminal 2. From terminal 2, there is a current path through the gear selector switch to the gear unsafe light. If the gear selector disagrees with the current gear position (e.g., gear is DOWN and pilot has selected UP), the unsafe light is illuminated. The gear unsafe light is shown at the bottom of Figure 9-96. The squat switch (shown mid left of Figure 9-96) is used to determine if the aircraft is on the GROUND or in FLIGHT. This switch is located on a landing gear strut. When the weight of the aircraft compresses the strut, the switch is activated and moved to the GROUND position. When the switch is in the GROUND position, the gear cannot be retracted and a warning horn sounds if the pilot selects gear UP. The squat switch is sometimes referred to as the weight-on-wheels switch.

A throttle switch is also used in conjunction with landing gear circuits on most aircraft. If the throttle is retarded (closed) beyond a certain point, the aircraft descends and eventually lands. Therefore, many manufacturers activate a throttle switch whenever engine power is reduced. If engine power is reduced too low, a warning horn sounds telling the pilot to lower the landing gear. Of course, this horn need not sound if the gear is already DOWN or the pilot has selected the DOWN position on the gear switch. This same horn also sounds if the aircraft is on the ground, and the gear handle is moved to the UP position. Figure 9-96 shows the gear warning horn in the bottom left corner.

AC Supply Many modern light aircraft employ a low-power AC electrical system. Commonly, the AC system is used to power certain instruments and some lighting that operate only using AC. The electroluminescent panel has become a popular lighting system for aircraft instrument panels and requires AC. Electroluminescent lighting is very efficient and lightweight; therefore, excellent for aircraft installations. The electroluminescent material is a paste-like substance that glows when supplied with a voltage. This material is typically molded into a plastic panel and used for lighting. A device called an inverter is used to supply AC when needed for light aircraft. Simply put, the inverter changes DC into AC. Two types of inverters may be found on aircraft: rotary inverters and static inverters. Rotary inverters are found only on older aircraft due to its poor reliability, excess weight, and inefficiency. The rotary inverters employee a DC motor that spins an AC generator. The unit is typically one unit and contains a voltage regulator circuit to ensure voltage stability.

Most aircraft have a modern static inverter instead of a rotary inverter. Static inverters, as the name implies, contain no moving parts and use electronic circuitry to convert DC to 9-53 Terminal #3 Current flow Squat switch Gear selector switch Throttle switch Gear unsafe light (red) Terminal #2 Terminal #1 Control current Down motor, high current UP DOWN Hydraulic pump motor assembly AdvancedRetarded Gear horn FLT POSGND POS Landing gear motor 25 Amp Landing gear control 5 Amp 30A C.B.1 5A C.B.2 Not UP UP Not DN DN Not UP UP Not DN DN Not UP UP Not DN DN UP limit DN limit UP limit DN limit UP limit DN limit Right green down light Nose green down light Left green down light LEFT GEAR NOSE GEAR RIGHT GEAR DOWNUP AC. Figure 9-98 shows a static inverter. Whenever AC is used on light aircraft, a distribution circuit separated from the DC system must be employed. [Figure 9-99] Some aircraft use an inverter power switch to control AC power. Many aircraft simply power the inverter whenever the DC bus is powered and no inverter power switch is needed.

On complex aircraft, more than one inverter may be used to provide a backup AC power source. Many inverters also offer more than one voltage output. Two common voltages found on aircraft inverters are 26V AC and 115V AC. 9-54 Terminal #3 Squat switch Gear selector switch Throttle switch Gear unsafe light (red) Terminal #2 Terminal #1 Control current Down motor, high current UP DOWN Hydraulic pump motor assembly AdvancedRetarded Gear horn FLT POSGND POS Landing gear motor (25 Amp) Landing gear control (5 Amp) 30A C.B.1 5A C.B.2 Not UP UP Not DN DN Not UP UP Not DN DN Not UP UP Not DN DN UP limit DN limit UP limit DN limit UP limit DN limit Right green down light Nose green down light Left green down light LEFT GEAR NOSE GEAR RIGHT GEAR DOWN UP Current flow Light Multiengine Aircraft light single-engine aircraft. With two engines, these aircraft Multiengine aircraft typically fly faster, higher, and farther can drive two alternators (or generators) that supply current than single engine aircraft. Multiengine aircraft are designed to the various loads of the aircraft. The electrical distribution for added safety and redundancy and, therefore, often contain bus system is also divided into two or more systems. These a more complex power distribution system when compared to bus systems are typically connected through a series of circuit 9-55 protectors, diodes, and relays. The bus system is designed to create a power distribution system that is extremely reliable by supplying current to most loads through more than one source.

Paralleling Alternators or Generators Since two alternators (or generators) are used on twin engine aircraft, it becomes vital to ensure both alternators share the electrical load equally. This process of equalizing alternator outputs is often called paralleling. In general, paralleling is a simple process when dealing with DC power systems found on light aircraft. If both alternators are connected to the same load bus and both alternators produce the same output voltage, the alternators share the load equally. Therefore, the paralleling systems must ensure both power producers maintain system voltage within a few tenths of a volt. For most twin-engine aircraft, the voltage would be between 26.5-volt and 28-volt DC with the alternators operating. A simple vibrating point system used for paralleling alternators is found in Figure 9-100.

As can be seen in Figure 9-100, both left and right voltage regulators contain a paralleling coil connected to the output of each alternator. This paralleling coil works in conjunction with the voltage coil of the regulator to ensure proper alternator output. The paralleling coils are wired in series between the output terminals of both alternators. Therefore, if the two alternators provide equal voltages, the paralleling coil has no effect. If one alternator has a higher voltage output, the paralleling coils create the appropriate magnetic force to open/close the contact points, controlling field current and control alternator output.

Today’s aircraft employ solid-state control circuits to ensure proper paralleling of the alternators. Older aircraft use vibrating point voltage regulators or carbon-pile regulators to monitor and control alternator output. For the most part, all carbon-pile regulators have been replaced except on historic aircraft. Many aircraft still maintain a vibrating point system, although these systems are no longer being used on contemporary aircraft. The different types of voltage regulators were described earlier in this chapter. Power Distribution on Multiengine Aircraft The power distribution systems found on modern multiengine aircraft contain several distribution points (buses) and a variety of control and protection components to ensure the reliability of electrical power. As aircraft employ more electronics to perform various tasks, the electrical power systems becomes more complex and more reliable. One Inverter power switch (optional) DC bus A C L O A D S Inverter Various DC loads 115 VAC (optional) AC loads Power from aircraft battery or alternator DC AC 9-56 Alternator output to bus Alternator output to bus Left alternator A+ F Generator control A+ F Right alternator Paralleling switch To voltage regulator and circuit breaker To voltage regulator and circuit breaker Paralleling coil Paralleling coil Voltage coil Voltage coil Left voltage regulator and alternator Right voltage regulator and alternator means to increase reliability is to ensure more than one power source can be used to power any given load. Another important design concept is to supply critical electrical loads from more than one bus. Twin-engine aircraft, such as a typical corporate jet or commuter aircraft, have two DC generators; they also have multiple distribution buses fed from each generator. Figure 9-101 shows a simplified diagram of the power distribution system for a twin-engine turboprop aircraft.

This aircraft contains two starter generator units used to start the engines and generate DC electrical power. The system is typically defined as a split-bus power distribution system since there is a left and right generator bus that splits (shares) the electrical loads by connecting to each sub-bus through a diode and current limiter. The generators are operated in parallel and equally carry the loads. The primary power supplied for this aircraft is DC, although small amounts of AC are supplied by two inverters. The aircraft diagram shows the AC power distribution at the top and mid left side of the diagram. One inverter is used for main AC power and the second is operated in standby and ready as a backup. Both inverters produce 26-volt AC and 115-volt AC. There is an inverter select relay operated by a pilot controlled switch used to choose which inverter is active.

The hot battery bus (right side of Figure 9-101) shows a direct connection to the aircraft battery. This bus is always hot if there is a charged battery in the aircraft. Items powered by this bus may include some basics like the entry door lighting and the aircraft clock, which should always have power available. Other items on this bus would be critical to flight safety, such as fire extinguishers, fuel shutoffs, and fuel pumps. During a massive system failure, the hot battery bus is the last bus on the aircraft that should fail. If the battery switch is closed and the battery relay activated, battery power is connected to the main battery bus and the isolation bus. The main battery bus carries current for engine starts and external power. So the main battery bus must be large enough to carry the heaviest current loads of the aircraft. It is logical to place this bus as close as practical to the battery and starters and to ensure the bus is well protected from shorts to ground.

The isolation bus connects to the left and right buses and receives power whenever the main battery bus is energized. The isolation bus connects output of the left and right generators in parallel. The output of the two generators is then sent to the loads through additional buses. The generator buses are connected to the isolation bus through a fuse known as a current limiter. Current limiters are high amperage fuses that isolate buses if a short circuit occurs. There are several current limiters used in this system for protection 9-57 INV No. 1 115 V AC Right gen control + − Left gen control Left starter gen Right starter gen + − INV No. 2 Left gen bus Right gen bus 26 V AC 115 V AC Relay panel To AC loads (26V AC) To AC loads (115V AC) 26V AC 115V AC No. 4 DUAL FED BUS No. 3 DUAL FED BUS No. 2 DUAL FED BUS No. 1 DUAL FED BUS ISOLATION BUS To miscellaneous DC loads Current limiter Current limiter MAIN BATTERY BUS SUB BUS Right start relay Left start relay EXT power connection HOT BATTERY BUS EXT power relay Loads powered by hot battery bus Battery Battery sw Avionics bus No. 2 power relay To avionics master control CB Avionics bus No. 1 OFF ON Avionics No. 1 power relay Avionics bus No. 3 power relay (optional) 50 A 60A 50A 60A between buses. As can be seen in Figure 9-101, a current that caused the excess current draw is repaired.

limiter symbol looks like two triangles pointed toward each other. The current limiter between the isolation bus and the The left and right DC generators are connected to their main generator buses are rated at 325 amps and can only be respective main generator buses. Each generator feeds its replaced on the ground. Most current limiters are designed respective bus, and since the buses are connected under for ground replacement only and only after the malfunction normal circumstances, the generators operate in parallel. 9-58 Both generators feed all loads together. If one generator fails or a current limiter opens, the generators can operate independently. This design allows for redundancy in the event of failure and provides battery backup in the event of a dual generator failure.

In the center of Figure 9-101 are four dual-feed electrical buses. These buses are considered dual-feed since they receive power from both the left and right generator buses. If a fault occurs, either generator bus can power any or all loads on a dual-feed bus. During the design phase of the aircraft, the electrical loads must be evenly distributed between each of the dual-feed buses. It is also important to power redundant systems from different buses. For example, the pilot’s windshield heat would be powered by a different bus from the one that powers the copilot’s windshield heat. If one bus fails, at least one windshield heat continues to work properly, and the aircraft can be landed safely in icing conditions.

Notice that the dual-feed buses are connected to the main generator buses through both a current limiter and a diode. Remember, a diode allows current flow in only one direction. [Figure 9-102] The current can flow from the generator bus to the dual- feed bus, but the current cannot flow from the dual fed bus to the main generator bus. The diode is placed in the circuit so the main bus must be more positive than the sub bus for current flow. This circuit also contains a current limiter and a circuit breaker. The circuit breaker is located on the flight deck and can be reset by the pilot. The current limiter can only be replaced on the ground by a technician. The circuit breaker is rated at a slightly lower current value than the current limiter; therefore, the circuit breaker should open if a current overload exists. If the circuit breaker fails to open, the current limiter provides backup protection and disconnects the circuit.

Large Multiengine Aircraft Transport category aircraft typically carry hundreds of passengers and fly thousands of miles each trip. Therefore, large aircraft require extremely reliable power distribution systems that are computer controlled. These aircraft have multiple power sources (AC generators) and a variety of distribution buses. A typical airliner contains two or more main AC generators driven by the aircraft turbine engines, as well as more than one backup AC generator. DC systems are also employed on large aircraft and the ship’s battery is used to supply emergency power in case of a multiple failures.

The AC generator (sometimes called an alternator) produces three-phase 115-volt AC at 400 Hz. AC generators were discussed previously in this chapter. Since most modern transport category aircraft are designed with two engines, there are two main AC generators. The APU also drives an AC generator. This unit is available during flight if one of the main generators fails. The main and auxiliary generators are typically similar in output capacity and supply a maximum of 110 kilovolt amps (KV A). A fourth generator, driven by an emergency ram air turbine, is also available in the event the two main generators and one auxiliary generator fail.

The emergency generator is typically smaller and produces less power. With four AC generators available on modern aircraft, it is highly unlikely that a complete power failure occurs. However, if all AC generators are lost, the aircraft battery will continue to supply DC electrical power to operate vital systems. AC Power Systems Transport category aircraft use large amounts of electrical power for a variety of systems. Passenger comfort requires power for lighting, audio visual systems, and galley power for food warmers and beverage coolers. A variety of electrical systems are required to fly the aircraft, such as flight control systems, electronic engine controls, communication, and navigation systems. The output capacity of one engine-driven AC generator can typically power all necessary electrical systems. A second engine-driven generator is operated during flight to share the electrical loads and provide redundancy.

The complexity of multiple generators and a variety of distribution buses requires several control units to maintain a constant supply of safe electrical power. The AC electrical system must maintain a constant output of 115 to 120 volts at Right main generator bus Circuit breaker Reverse polarity diode Current limiter Current limiterMain generator bus Dual fed bus # 1 Dual fed bus # 2 9-59 a frequency of 400 Hz (±10 percent). The system must ensure power limits are not exceeded. AC generators are connected to the appropriate distribution buses at the appropriate time, and generators are in phase when needed. There is also the need to monitor and control any external power supplied to the aircraft, as well as control of all DC electrical power.

Two electronic line replaceable units are used to control the electrical power on a typical large aircraft. The generator control unit (GCU) is used for control of AC generator functions, such as voltage regulation and frequency control. The bus power control unit (BPCU) is used to control the distribution of electrical power between the various distribution buses on the aircraft. The GCU and BPCU work together to control electrical power, detect faults, take corrective actions when needed, and report any defect to the pilots and the aircraft’s central maintenance system. There is typically one GCU for each AC generator and at least one BPCU to control bus connections. These LRUs are located in the aircraft’s electronics equipment bay and are designed for easy replacement.

When the pilot calls for generator power by activating the generator control switch on the flight deck, the GCU monitors the system to ensure correct operation. If all systems are operating within limits, the GCU energizes the appropriate generator circuits and provides voltage regulation for the system. The GCU also monitors AC output to ensure a constant 400-Hz frequency. If the generator output is within limits, the GCU then connects the electrical power to the main generator bus through an electrical contactor (solenoid). These contactors are often called generator breakers (GB) since they break (open) or make (close) the main generator circuit.

After generator power is available, the BPCU activates various contactors to distribute the electrical power. The BPCU monitors the complete electrical system and communicates with the GCU to ensure proper operation. The BPCU employs remote current sensors known as a current transformers (CT) to monitor the system. [Figure 9-103] A CT is an inductive unit that surrounds the main power cables of the electrical distribution system. As AC power flows through the main cables, the CT receives an induced voltage. The amount of CT voltage is directly related to the current flowing through the cable. The CT connects to the BPCU, which allows accurate current monitoring of the system. A typical aircraft employs several CTs throughout the electrical system.

The BPCU is a dedicated computer that controls the electrical connections between the various distribution buses found on the aircraft. The BPCU uses contactors (solenoids) called bus BPCU CT output Current transformer Main AC power cable tie breakers (BTB) for connection of various circuits. These BTBs open/close the connections between the buses as needed for system operation as called for by the pilots and the BPCU. This sounds like a simple task, yet to ensure proper operation under a variety of conditions, the bus system becomes very complex. There are three common types of distribution bus systems found on transport category aircraft: split bus, parallel bus, and split parallel.

Split-Bus Power Distribution Systems Modern twin-engine aircraft, such as the Boeing 737, 757, 777, Airbus A-300, A-320, and A-310, employ a split-bus power distribution system. During normal conditions, each engine-driven AC generator powers only one main AC bus. The buses are kept split from each other, and two generators can never power the same bus simultaneously. This is very important since the generator output current is not phase regulated. (If two out-of-phase generators were connected to the same bus, damage to the system would occur.) The split-bus system does allow both engine-driven generators to power any given bus, but not at the same time. Generators must remain isolated from each other to avoid damage. The GCUs and BPCU ensures proper generator operation and power distribution.

On all modern split bus systems, the APU can be started and operated during flight. This allows the APU generator to provide back-up power in the event of a main generator failure. A fourth emergency generator powered by the ram air turbine is also available if the other generators fail. The four AC generators are shown at the bottom of respective buses through the generator breakers. For example, generator 1 sends current through GB1 to AC bus 1. AC bus 1 feeds a variety of primary electrical loads, and also feeds sub-buses that in turn power additional loads. 9-60 With both generators operating and all systems normal, AC bus 1 and AC bus 2 are kept isolated. Typically during flight, the APB (bottom center of Figure 9-104) would be open and the APU generator off; the emergency generator (bottom right) would also be off and disconnected. If generator one should fail, the following happens: 1. The GB 1 is opened by the GCU to disconnect the failed generator.

2. The BPCU closes BTB 1 and BTB 2. This supplies AC power to AC bus 1 from generator 2. 3. The pilots start the APU and connect the APU generator. At that time, the BPCU and GCUs move the appropriate BTBs to correctly configure the system so the APU powers bus 1 and generator 2 powers bus 2. Once again, two AC generators operate independently to power AC bus 1 and 2. If all generators fail, AC is also available through the static inverter (center of Figure 9-104). The inverter is powered from the hot battery bus and used for essential AC loads if all AC generators fail. Of course, the GCUs and BPCU take the appropriate actions to disconnect defective units and continue to feed essential AC loads using inverter power.

To produce DC power, AC bus 1 sends current to its transformer rectifier (TR), TR 1 (center left of Figure 9-104). The TR unit is used to change AC to DC. The TR contains a transformer to step down the voltage from 115-volt AC to 26-volt AC and a rectifier to change the 26-volt AC to 26- volt DC. The output of the TR is therefore compatible with the aircraft battery at 26-volt DC. Since DC power is not phase sensitive, the DC buses are connected during normal operation. In the event of a bus problem, the BPCU may isolate one or more DC buses to ensure correct distribution of DC power. This aircraft contains two batteries that are used to supply emergency DC power.

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. AC BUS 1 AC BUS 2 AC ESS AC ESS INV DC BUS 1 DC BUS 2BAT BUSDC HOT BUS 1 HOT BUS 2 DC ESS BUS STAT INV EXT PWR BAT 2BAT 1 TR 1 TR 2 BTB 1 BTB 2 GB 1 APB GB 2 EGB ESS TR GEN 1 APU GEN 2 EMER GEN 9-61 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.

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 flight crew through the electrical control panel and used to connect all necessary buses. GBs are used to connect and disconnect the generators.

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.

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 also connect to the battery bus and eventually to the battery. The essential DC bus (top left) can be fed from DC bus 1 or the essential TR. A diode prevents the essential DC bus from powering DC bus 1. The essential DC bus receives power from the essential TR, which receives power from the essential AC bus. This provides an extra layer of redundancy DC BUS 2 HOT BAT BUS DC BUS 1 SYNC BUSESS 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 9-62 since the essential AC bus can be isolated and fed from any main generator. Figure 9-105 shows generator 3 powering the essential AC bus.

Split-Parallel Systems A split-parallel bus basically employs the best of both split- bus and the parallel-bus systems. The split-parallel system is found on the Boeing 747-400 and contains four generators driven by the main engines and two APU-driven generators. The system can operate with all generators in parallel, or the generators can be operated independently as in a split- bus system. During a normal flight, all four engine-driven generators are operated in parallel. The system is operated in split-bus mode only under certain failure conditions or when using external power. The Boeing 747-400 split- parallel system is computer controlled using four GCUs and two BPCUs. There is one GCU controlling each generator; BPCU 1 controls the left side bus power distribution, and BPCU 2 controls the right side bus power. The GCUs and BPCUs operate similarly to those previously discussed under the split-bus system.

distribution system. The main generators (top of Figure 9-106) are driven by the main turbine engines. Each generator is connected to its load bus through a generator control breaker (GCB). The generator control unit closes the GCB when the pilot calls for generator power and all systems are operating normally. Each load bus is connected to various electrical systems and additional sub-buses. The BTBs are controlled by the BPCU and connect each load bus to the left and right sync bus. A split systems breaker (SSB) is used to connect the left and right sync buses and is closed during a normal flight. With the SSB, GCBs, and BTBs, in the closed position the generators operate in parallel. When operating in parallel, all generators must be in phase.

If the aircraft electrical system experiences a malfunction, the control units make the appropriate adjustments to ensure all necessary loads receive electrical power. For example, if generator 1 fails, GCU 1 detects the fault and commands GCB 1 to open. With GCB 1 open, load bus 1 now feeds from the sync bus and the three operating generators. In another example, if load bus 4 should short to ground, BPCU 4 opens the GCB 4 and BTB 4. This isolates the shorted bus (load bus 4). All loads on the shorted bus are no longer powered, and generator 4 is no longer available. However, with three remaining generators operational, the flight continues safely.

As do all large aircraft, the Boeing 747-400 contains a DC power distribution system. The DC system is used for battery and emergency operations. The DC system is similar to those previously discussed, powered by TR units. The TRs are connected to the AC buses and convert AC into 26-volt DC. The DC power systems are the final backups in the event of a catastrophic electrical failure. The systems most critical to fly the aircraft can typically receive power from the battery. This aircraft also contains two static inverters to provide emergency AC power when needed. GEN 1 GEN 2 GEN 3 GEN 4 GCB 1 BTB 1 AC LOAD BUS 1 GCB 2 BTB 2 AC LOAD BUS 2 LEFT SYNC BUS GCB 2 BTB 2 AC LOAD BUS 2 GCB 3 BTB 3 AC LOAD BUS 3 GCB 4 BTB 4 AC LOAD BUS 4 LEFT SYNC BUS APU 1 EXP APB External power 1 RIGHT SYNC BUS APU 2 EXP APB External power 2 SSB 9-63 Wiring Installation Wiring Diagrams Electrical wiring diagrams are included in most aircraft service manuals and specify information, such as the size of the wire and type of terminals to be used for a particular application. Furthermore, wiring diagrams typically identify each component within a system by its part number and its serial number, including any changes that were made during the production run of an aircraft. Wiring diagrams are often used for troubleshooting electrical malfunctions.

Block Diagrams A block diagram is used as an aid for troubleshooting complex electrical and electronic systems. A block diagram consists of individual blocks that represent several components, such as a printed circuit board or some other type of replaceable module. Pictorial Diagrams In a pictorial diagram, pictures of components are used instead of the conventional electrical symbols found in schematic diagrams. A pictorial diagram helps the maintenance technician visualize the operation of a system. [Figure 9-108] Schematic Diagrams A schematic diagram is used to illustrate a principle of operation, and therefore does not show parts as they actually appear or function. [Figure 9-109] However, schematic diagrams do indicate the location of components with respect to each other. Schematic diagrams are best utilized for troubleshooting.

Wire Types The satisfactory performance of any modern aircraft depends to a very great degree on the continuing reliability of electrical systems and subsystems. Improperly or carelessly maintained wiring can be a source of both immediate and potential danger. The continued proper performance of electrical systems depends on the knowledge and techniques of the technician who installs, inspects, and maintains the electrical system wires and cables. Procedures and practices outlined in this section are general recommendations and are not intended to replace the manufacturer’s instructions and approved practices.

A wire is described as a single, solid conductor, or as a stranded conductor covered with an insulating material. Because of in-flight vibration and flexing, conductor round wire should be stranded to minimize fatigue breakage. The term “cable,” as used in aircraft electrical installations, includes: R ALT BUSL ALT BUS Bat 2 Bat 1 BAT BUS 2BAT BUS 1 R GEN TIE BusL GEN TIE Bus EMER TIE Bus ESS DC Bus MAIN DC Bus AC EXT PWR Equip AC BUS MAIN INV BUS SEC INV BUSINST INV BUS E R GEN EMER RLY E L GEN EMER RLY N NORM BAT RLY 2 N NORM BAT RLY 1 E EMER BAT RLY2A E EMER BAT RLY 1A E EMER BAT RLY 2 E EMER BAT RLY 1 N L FOR RLY CONT RLY 1 RCCO DC EXT PWR DIST RLY DC EXT PWR RLY N BUS TIE RLY 2 BUS TIE RLY 1 E EMER BUS RLY EMER FEEDER N RCCOR GEN R GEN R FEEDER L FEEDER L GEN PROTECTORS R FWD R AFT PROTECTORS L FWD L AFT TR RCCOEMER 200A TRANS RECT APU ALT L ALT R ALT CONT RLY CONT RLY CONT RLY CONT RLY CONT RLY INV B T/R INV C T/R INV A T/R R AC MON BUSL AC MON BUS DC EXT PWR INV C INV B INV A INV E SEC INV BUS CONT RLY MAIN INV BUS CONT RLY INST INV BUS CONT RLY DC EXT PWR TIR CONT RLY Windows & lights EXT PWR CONT RLY APU ALT CONT RLY L ALT CONT RLY 9-64