B2-13F Aircraft Electrical Systems

• The study of aircraft electrical systems involves understanding basic components, systems, and safety during maintenance.

Core Topics

• Electrical Power (ATA 24)
• Equipment and Furnishings (ATA-25)
• Lights (ATA-33)
• Fire Protection (ATA-26)
• Ice and Rain Protection (ATA-30)

Electrical Power (ATA 24)

• Explain aircraft battery systems and installation procedures
• Identify components and system operations for DC power generation
• Identify components and system operations for AC power generation
• Identify components and system operations for emergency power generation
• Identify components and operation of voltage regulation systems
• Identify components and operation of power distribution systems
• Identify the purpose and operation of rectifiers, inverters and transformers
• Identify circuit protection methods and their operation
• Identify components of external/ground power systems and their operation
• Describe the safety precautions for working with electrical power and during battery maintenance.

Equipment and Furnishings (ATA-25)

• Focus on identifying electronic emergency equipment, requirements, and operation.
• Concentrate on describing cabin entertainment equipment operation.

Lights (ATA-33)

• Focus on external lights such as navigation, anti-collision, landing, taxiing and ice lights
• Internal lights include: cabin, cockpit, and cargo lights
• Emergency lights
• Strobe lights warrant specific safety precautions during maintenance

Fire Protection (ATA-26)

• Fire detection
• Smoke detection
• Fire protection warning
• Fire extinguishing;
• Aircraft fire protection systems testing
• Observe safety precautions when performing maintenance on fire protection systems
• Portable fire extinguishers and their application for different onboard fire classes

Ice and Rain Protection (ATA-30)

• Focus on understanding ice formation types
• Ice detection components: Identify and explain
• Anti-icing systems: Electrical, hot air, and chemical components and operation
• De-icing systems: Electrical, hot air, and chemical components and operation
• Rain repellent and rain removal systems: Understand operation
• Probe and Drain Heating: Understand methods

Aircraft Electrical Power

• Manuals must comply with Air Transport Association (ATA) Specifications Reference under chapters ATA 20 (Standard Practices) and ATA 24 (Electrical Power)

Aircraft Electrical Power Key Areas

• Reference electrical power information in relevant aircraft system manuals like Aircraft Maintenance Manual, Wiring Diagrams Manual, Illustrated Parts Catalogue, and component vendor manuals
• A typical electrical power distribution system includes a generator drive, AC/DC generation, external power input, standby power and AC electrical load distribution
• Emergency power is supplied by APU generator, starter-generator and/or external power.
• Turbine-powered aircraft use engine-driven generators via a Constant Speed Drive (CSD)
• Newer aircraft like the Boeing 727 use an Integrated Drive Generator (IDG)
• AC generators supply 3-phase power at 115/200 volts and 400 Hz
• Systems often prevent two sources from powering the same load simultaneously
• Normal DC power is from Transformer Rectifier Units (TRUs) converting 115 V AC to 28 V DC
• Batteries serve as backup DC sources controlled by a Standby Power Control Unit (SPCU), which is crucial if other sources fail

Aircraft Power Systems

• Switches control the electrical power by energizing or de-energizing contactors (relays)
• Instrument panels with lights and digital displays to monitor the power system status
• Generating source outputs connect to low resistance bus bars made of copper or aluminum
• Aircraft contain several bus bars with specific tasks for power distribution and load sharing

Bus Hierarchy

• Hot Battery Bus supplies power to critical loads like fire extinguishers and fire shut-off valves
• Essential DC Bus is connected to the hot battery bus via a contactor.
• AC Bus is fed from the main generator, such as generator one feeding AC bus 1
• Essential AC Busses are connected to AC bus systems
• Non-Essential Busses supply galley power and non-essential lighting, which can be isolated during emergencies and may be called shed busses or isolation busses.
• Non-essential systems Load shedding is used prevent overloading of generators by isolating non-essential systems during generator failure

Basic DC Bus Systems

• Consist of a simple electrical power system with one generator
• Generator output is at a higher voltage than the battery, battery is charged from the bus
• Inverter provides AC voltage for aircraft instruments.

Parallel DC Bus Systems

• Outputs of generators are paralleled, "Tied Bus System"
• Under normal operations, the Bus Tie Breaker (BTB) is closed where both generators supply DC buses and share the load
• Generator voltage regulators are interconnected to ensure equal load sharing between generators
• Certain system issues will cause the BTB to open and isolate electrical systems

Split Bus Systems

• Each generator feeds its own bus while the bus tie contactor is open
• Bus Tie Contactor closes to tie the buses when there is only one generator available if there is a generator failure or engine failure
• Split AC bus systems are also used

AC Bus Systems

• AC bus systems can be parallel where generators must synchronize, and the outputs of the generators can be tied
• The generators must synchronize for equal frequency and output
• The generators must synchronize for being “in phase” and rotation
• A split System Breaker (SSB) is normally closed when both generators are online
• SSB opens and isolates systems when a defect is detected such as issues of voltage outside of limits, or incorrect phase rotation or sequence

Multiengine Split AC Bus System

• When operating with external power and no engines running, all loads are supplied by the external power source via an external power relay and the BTB
• Both GCB's will be tripped
• When one aircraft engine is started, and its generator operates within limits, its GCB will energize close, and then the external power relay will open to remove external power from system
• The BTB will remain closed
• When the second aircraft engine is started and its generator is operating within system limits, its GCB will close
• The power to the BTB’s close coil is now redirected through GCB to BTB's trip coil, and thus that BTB will trip and cause the operating generators to supply their respective busbars instead
• Inoperative generators cause power via BTB through tripped GCB
• Normal, Essential and Emergency busses are separated, in relation to flight safety
• Two inoperative (failed or switched OFF) generators mean both GCBs will trip
• Battery then closes BTB: no power available to operate non-essential loads so changeover relay between generator bus and essential AC bus will de-energize and connect inverter
• The loads will be supplied with 115 AC power as non-essential AC loads are isolated

Relays, Breakers and Contactors

• Relays are electromagnets with a fixed core and pivoting mechanical linkage
• Relays are typically used for low-current switching applications
• Solenoids are similar
• The armature is the part of the relay that is attracted by the electromagnet
• Many aircraft manufacturers have substituted the term contactor, relay, or breaker for electrical switching solenoids

Bus Tie Breakers (BTB or Contactors)

• Contain three heavy duty contacts which connect three phase power to generator bus bars and distribute to aircraft AC loads
• BTBs can also contain various auxiliary contacts that connect control and indication circuits within the AC distribution system
• Close coils cause main contacts to close; auxiliary contacts change, close and become mechanically locked, thus power can be removed and breaker remains closed
• Trip coil releases the mechanical lock while spring tension forces contacts open
• BTBs are same units, but perform different function
• Generator Circuit Breakers (GCBs) are used are used and controlled by the Generator Control Unit (GCU)

Generator Circuit Breakers

• To connect AC generator output to respective bus, given serviceable, GCB is used
• The outputs are connected when generator output is connected to generator bus, to ensure loads receive power from only source, GCB is used
• GCB is used to isolate the generator output from aircraft if fault detected
• Interlocking prevents parallel connections
• On-coming switches turn off present
• Ground power supplies are de-energized by APU breaker

Aircraft Battery Systems

• Aircraft use negative-ground, single wire earth (ground) typically
• Negative terminals connect directly to metallic aircraft structure placing structure at negative potential
• Electrical components connected to structure forms return path to battery to close circuit, this is called grounding or earthing
• Benefits of grounding in this manner include the reduced chance of developing RFI and reduction in weight,
• For non-conductive composite built aircraft, a two-wire system must be used
• A typical battery circuit is a light aircraft with a positive terminal to battery solenoid, a heavy-duty switch
• Solenoids can also be remotely controlled via small conductor and switch
• When switch closed, it grounds path to energize battery solenoid coil to then close main power connection
• A freewheeling diode is installed across coil by opening master switch to eliminate spikes due to collapsing coil and induced Voltage

Hot Battery Bus

• The aircraft battery is normally directly connected to the hot battery bus
• Supplies power to critical loads such as fire extinguishers, and hydraulic fire shut-offs
• Hot Battery is connected only one battery switch, which controls the battery contactor
• Usually on during operation to allow battery to charge

Battery Temperature Monitor Systems

• Most Nicad batteries use battery monitor systems
• They provide a continuous temperature indication
• Provides action for isolation in case damage resulting from temperature arises
• Thermistors in the batteries form one independent temperatures

Battery Ventilation

• Lead acid batteries, when fully charged, release hydrogen/oxygen gas in high amounts
• Ventilation removes dangerous gas safely
• Aircraft Airstream can create positive ventilation in case or compartment via pickup or outflow
• Air pressure is often used if aircraft is pressured, via a non-return valve and overboard
• Some sumps provide neutralizing agent by passing fumes over pad which prevents damage to skin
• Ni-Cads do not need ventilation due to very little gas production with assistance which is often used to keep the temperatures the same

Battery Connection

• Light aircraft batteries connected are bolted bolt, be sure to grease with Vaseline to end corrosion
• For enclosed batteries quick disconnect are use with a positive locking device or lock wire
• All terminals must be greased with white Vaseline to remove corrosion

Battery Installation

• Aircra