Aircraft Electrical Systems Study Guide

 

Aircraft Electrical Systems Study Guide

This study guide is designed to help you review and consolidate your understanding of the electrical systems as described in the provided "ELECTRIC.pdf" excerpts for the A320/A321 aircraft.

I. Electrical System Overview

• Primary Power: Understand the two main phases of the electrical power system (three-phase 115/200 V 400 Hz constant-frequency AC system and a 28 V DC system).
• Secondary Priority: Recognize that electrical transients are acceptable for equipment and commercial supply has secondary priority.
• Generation Components: Identify the key components responsible for generating AC and DC power:
• AC Generators (Engine-driven and APU Generator)
• Transformer Rectifiers (TRs)
• Static Inverter
• Batteries
• Emergency Generator
• Power Distribution:
• AC power is supplied to AC electrical bars.
• DC power is generated from AC power by TRs for certain applications.
• Emergency power sources provide AC power when normal AC power is unavailable, and DC power from batteries into AC power.

II. Generation of Electrical Power

• AC Generators:
• Engine-Driven Generators (GEN 1, GEN 2):
• Supply three-phase 115/200 V 400 Hz power.
• Capacity: 90 kVA.
• Generator Control Units (GCU): Functions include controlling frequency, voltage, and protecting the network.
• APU Generator (APU GEN):
• Driven by the APU.
• Can replace either or both main engine generators.
• Ground Power Control Unit (GPCU): Protects the network.
• Ground and Auxiliary Power Unit Control Unit (GAPCU): Regulates frequency and voltage of the APU generator and protects the network.
• Emergency Generator:
• Supplies blue hydraulic system and AC power in emergency situations.
• Capacity: 5 KVA of three-phase 115 V and 200 V 400 Hz power.
• Generator Control Unit (GCU): Keeps emergency generator at a constant speed, controls output voltage, protects network, and controls emergency generator start-up.
• Static Inverter:
• Transforms DC power from Battery 1 into 1 kVA of single-phase 115 V 400 Hz AC power.
• Supplies AC essential bus when AC aircraft speed is above 50 kt or battery pushbuttons are ON.
• DC Generation:
• Transformer Rectifiers (TRs):
• Convert AC current to 200 A of DC current.
• TR 1 and TR 2: Supply the aircraft's electrical system.
• ESS TR: Supplies DC circuit from the emergency generator or when engine/APU generators fail or TR1/TR2 fail. Also powers DC ESS BUS.
• TR Entertainment (TR Ent): Powers the DC Entertainment bus bar for In-Flight Entertainment (IFE).
• Batteries (BAT 1, BAT 2):
• Main batteries: 2 x 23 Ah capacity.
• Battery Charger Limiter (BCL): Monitors charging current and controls battery contactor.

III. Circuit Breakers (C/Bs)

• Types: Monitored (green) and Non-monitored (black).
• Monitored C/Bs: Trigger a C/B TRIPPED warning on the ECAM if tripped for more than 1 min.
• Wing Tip Brake (WTB) C/Bs: Have red caps to prevent accidental reset.
• ECAM C/B TRIPPED Caution Clearance: Pressing the EMER CANC pushbutton clears and inhibits the ECAM C/B TRIPPED caution. The CLR pb clears the caution for the remainder of the flight. If the C/B remains pulled, additional tripped C/Bs will be detected, and the ECAM will trigger the caution again.

IV. Operations (Normal Configuration)

• General: GEN 1 and 2 generators have priority over the APU generator, which has priority over external power. Two generators are needed to supply the DC BUS Entertainment, except on ground when the APU generator (if not overloaded) or external power is sufficient.
• In Flight:
• Each engine-driven generator (GEN 1, GEN 2) supplies its associated AC BUS (1 and 2).
• AC BUS 1 supplies AC ESS BUS via a contactor.
• TR 1 supplies DC BUS 1, DC BAT BUS, and DC ESS BUS.
• TR 2 supplies DC BUS 2.
• Batteries are connected to DC BAT BUS; they need charging. Battery charge current limiter disconnects them when fully charged.
• On Ground:
• APU generator or external power supplies the complete system.
• When ground services are required, external power can supply the AC and DC GND/FLT BUSes directly without supplying the entire aircraft network. This is selected with the MAINT BUS switch.

V. Operations (Abnormal Configurations)

• Failure of One Engine Generator (e.g., GEN 1):
• System automatically replaces the failed generator with the APU GEN (if available) or other engine generator.
• The galley load and DC BUS Entertainment are automatically shed.
• The Galley Load Automatic Shedding sheds secondary galley loads automatically.
• Failure of AC BUS 1:
• AC BUS 2 can supply AC ESS BUS and ESS TR can supply DC ESS BUS via AC ESS FEED pb.
• DC BUS 2 supplies DC BUS 1 and DC BAT BUS automatically after 5 seconds.
• Failure of One TR (e.g., TR 1):
• Contactor opens automatically due to overheat or minimum current.
• Other TR automatically replaces the faulty one.
• ESS TR supplies the DC ESS BUS.
• Failure of TR 1 and TR 2:
• DC BUS 1, DC BUS 2, and DC BAT BUS are lost.
• DC ESS BUS is supplied by the ESS TR.
• Emergency Generation after Loss of All Main Generators:
• If both AC BUS 1 and AC BUS 2 are lost and aircraft speed > 100 kt, the RAT extends automatically.
• RAT drives blue hydraulic system, which powers emergency generator via a hydraulic motor.
• Emergency generator supplies AC ESS BUS and DC ESS BUS via ESS TR.
• When RAT stalls (speed < 100 kt), emergency generator no longer supplies AC ESS BUS and DC ESS BUS.
• Emergency generation transfers to batteries and static inverter. AC SHED ESS and DC SHED ESS buses are automatically shed.
• On ground: DC BAT BUS automatically connected to batteries.
• Below 50 kt: AC ESS BUS is automatically shed, leading to loss of all display units.
• Notes: RAT extension and emergency generator coupling (8s). On ground, only batteries are powering emergency generation until APU start or speed > 100 kt.
• EMER GEN Running: Specific configuration where the emergency generator is providing power.
• Flight With Batteries Only: Depicts the electrical schematic when only batteries are supplying power.
• On Ground, Batteries Only (Speed < 50 kt): Shows the electrical schematic when on the ground with only batteries supplying power, at low speed.
• Smoke Configuration: Main busbars are shed. Electrical distribution similar to emergency electrical configuration (loss of main generators). Fuel pumps are connected upstream of GEN 1 line contactor. 75% of electrical equipment is shed. Remaining equipment powered via circuit breakers on overhead panel.

VI. Controls and Indicators

• Overhead Panel: Contains various pushbuttons and indicators.
• BAT 1 (2) ind: Shows battery voltage.
• BAT 1 (2) pb-sw: Controls battery charge limiter, connection/disconnection to DC BAT BUS.
• FAULT light: Indicates charge limiter fault.
• Note: Automatic cut-off logic prevents batteries from discharging completely when aircraft is on ground (parking).
• IDG 1 (2) pb-sw (GUARDED):
• CAUTION: Do not disconnect IDG for more than 3s. Disconnect only when engine is running/windmilling to avoid damage.
• FAULT light: Indicates IDG oil overheat, low pressure, or disconnection.
• GEN 1 (2) pb-sw: Energizes/de-energizes generator field and closes/opens line contactor.
• FAULT light: Associated GCU trip or generator circuit fault.
• APU GEN pb-sw: Energizes APU generator field, opens/closes line contactor and EXT PWR line contactor.
• FAULT light: APU speed too low, or APU GEN line contactor opens after EXT PWR or ENG GEN take over.
• BUS TIE pb: Automatically opens/closes Bus Tie Contactors (BTCs). One contactor is closed when: one engine generator supplies associated AC BUS, APU generator, or external power supplies other side. Both are closed during single-engine operation or operation on APU/external power.
• AC ESS FEED pb: AC ESS BUS is supplied by AC BUS 1 (default) or AC BUS 2 (manual selection).
• FAULT/ALTN light: Total loss of main generators, AC ESS BUS supplied by emergency generator or static inverter.
• GALLEY pb OR GALY & CAB pb: Main galley, secondary galley, in-seat power, and IFE systems.
• FAULT/OFF light: When only one generator is operating, or on ground when one engine generator is operating (all galleys available when APU GEN or EXT PWR supplying power).
• Note: Galley Load Automatic Shedding sheds secondary galley.
• EXT PWR pb: Indicates external power availability. Closes/opens external power line contactor.
• AVAIL light: External power available, voltage normal.
• COMMERCIAL pb: Switches off commercial electrical loads (cabin/cargo lights, water/toilet, drain mast ice protection, galley, passenger entertainment, semi-automatic cargo loading).
• EMER ELEC PWR (Overhead Panel):
• MAN ON pb (guarded): Selects manual RAT extension when AC BUS 1/2 not supplied and aircraft speed > 100 kt.
• FAULT light: Emergency generator not supplying power to AC BUS 1 and AC BUS 2.
• EMER GEN TEST pb (guarded): Pressed and held for testing emergency generator functions (AC NORMAL BUSES supplied, EMER GEN driven hydraulically, DC ESS BUS, AC ESS BUS, DC ESS SHED, AC ESS SHED connected to emergency generator, ECAM display on ground, static inverter powers AC ESS BUS).
• GEN 1 LINE pb-sw: AC BUS 1 channel supplied from GEN 2 through bus tie contactors for smoke.
• External Power Panel:
• EXT PWR NOT IN USE: White light indicates ground power unit not supplying the aircraft network.
• EXT PWR AVAIL: Amber light indicates external power is available and voltage is correct.
• Forward Cabin:
• MAINT BUS SW: Energizes electrical circuits for ground servicing without energizing entire aircraft electrical system.
• ON/AVAIL light: External power connected, normal.
• Energizes AC and DC GND/FLT buses.
• Loads supplied: passenger compartment lighting, galley lighting, entrance area lights, lavatory lighting and service, vacuum cleaner sockets, flight compartment service outlets, hydraulic pump (yellow system), flight compartment flood lighting, fuel quantity indications, refueling, cargo hold lighting, main and nose landing gear compartment lighting, belly fairing panel service outlets, ground call, equipment compartment lights and service outlets, navigation lights, passenger address, interphone.
• Switch trips when external source removed. AC and DC GND/FLT buses connected to AC BUS 2 and DC BUS 2.
• ECAM ELEC Page: Displays detailed information and indications for the electrical system.
• Battery indications: Voltage and current. Amber indicates below 25V or above 31V for voltage, above 5A for discharge current.
• DC BAT indication: Normal (green), or below 25V (amber).
• Bus bar indication: Normal (green), not powered (amber), or SHED (white).
• TR 1(2) indication: Powered (green), amber for voltage/current issues.
• ESS TR indication: Normal (green), contactor open (white).
• EMER GEN indication: Normal (green), amber for voltage/frequency issues.
• GEN 1(2) indications: Running (number displayed), not running (blank). Normal operation (green legend), amber for load, voltage, or frequency issues. Load above 100% (amber 110%).
• APU GEN indications: Displayed when APU MASTER SW is ON and APU GEN pb-sw is ON. Similar to GEN 1(2) indications.
• EXT PWR indications: Available (green legend, voltage, frequency), not available (blank). Amber for voltage/frequency issues.
• STAT INV indications: Voltage and frequency. Amber for issues.
• IDG indications: Normal (green), amber for oil temperature overheat, low oil pressure, or disconnection. DISCONNECTED (white) and LO PR (white) if disconnected.
• GALLEY SHED indication: Appears when GALLEY pb-sw is OFF, main galleys shed, or only one generator operating (in flight) or only one generator supplying (on ground).
• RISE indication: Difference between IDG inlet and outlet temperatures.
• Memo Display: EMER GEN memo appears green when emergency generator is running.

VII. Distribution Table

• Understand how different components (AC BUS 1, AC BUS 2, AC ESS BUS, AC ESS SHED, STAT INV, TR 1, TR 2, ESS TR, TR Ent, DC 1, DC 2, DC BUS, DC BAT BUS, DC ESS BUS, DC ESS SHED, HOT BUS 1, HOT BUS 2) are supplied under various normal and abnormal configurations.
• Pay attention to which buses are active and which generators/sources are supplying them in scenarios like:
• Normal Configuration
• One GEN Inoperative (APU X, or APU)
• EMER CONF (before EMER GEN availability)
• EMER GEN RUNNING
• TR 1 FAULT, TR 2 FAULT, TR 1+2 FAULT
• On Ground (BAT only, speed > 100 kt, RAT stall or 50 kt < speed ≤ 100 kt, speed ≤ 50 kt)

Quiz: Aircraft Electrical Systems

Instructions: Answer each question in 2-3 sentences.

1. What are the two main phases of the aircraft's electrical power system, and what is the secondary priority of the electrical system?
2. Describe the primary function of the Generator Control Units (GCUs) for the engine-driven generators.
3. Explain the role of the Static Inverter in the aircraft's electrical system, and when does it supply power to the AC essential bus?
4. What happens to the Galley Load and the DC BUS Entertainment in the event of one engine generator failure?
5. How does the system respond when AC BUS 1 fails, and what bus takes over the supply to AC ESS BUS?
6. Describe the conditions under which the Ram Air Turbine (RAT) extends automatically during an electrical emergency.
7. What is the significance of "Monitored" circuit breakers (C/Bs), and how does the flight crew clear an ECAM C/B TRIPPED caution?
8. Explain the function of the MAINT BUS SW on the forward cabin panel and list two types of loads it can energize.
9. What are the normal voltage and frequency for the EMER GEN, and what do amber indications for these parameters signify on the ECAM?
10. Describe the conditions that lead to the "GALLEY SHED" indication appearing on the ECAM ELEC page.

Quiz Answer Key

1. The two main phases are a three-phase 115/200 V 400 Hz constant-frequency AC system and a 28 V DC system. Electrical transients are acceptable for equipment, and commercial supply has secondary priority.
2. The Generator Control Units (GCUs) are responsible for controlling the frequency and voltage of the generator output. They also protect the network by controlling the associated generator line contactor.
3. The Static Inverter transforms DC power from Battery 1 into 1 kVA of single-phase 115 V 400 Hz AC power. It is designed to supply the AC essential bus when the aircraft speed is above 50 kt or when the battery pushbuttons are selected ON.
4. In the event of one engine generator failure, the system automatically replaces the failed generator with the APU GEN or another engine generator if available. The galley load and the DC BUS Entertainment are automatically shed to reduce demand.
5. When AC BUS 1 fails, AC BUS 2 can supply the AC ESS BUS and the ESS TR can supply the DC ESS BUS. This is typically achieved via the AC ESS FEED pushbutton.
6. The RAT extends automatically if both AC BUS 1 and AC BUS 2 are lost and the aircraft speed is above 100 kt. It uses the blue hydraulic system to drive the emergency generator.
7. Monitored C/Bs trigger an ECAM C/B TRIPPED warning if they remain tripped for more than one minute. The flight crew can clear this caution by pressing the EMER CANC pushbutton, which inhibits it, or the CLR pb which clears it for the remainder of the flight.
8. The MAINT BUS SW allows personnel to energize specific electrical circuits for ground servicing without powering the entire aircraft electrical system. Two loads it can energize include passenger compartment lighting and lavatory lighting.
9. The normal voltage for the EMER GEN is 116 V and the normal frequency is 400 Hz. Amber indications for these parameters signify that the voltage is below 110 V or above 120 V, or the frequency is below 390 Hz or above 410 Hz, respectively.
10. The "GALLEY SHED" indication appears on the ECAM ELEC page when the GALLEY pb-sw is OFF, when the main galleys are shed, or when only one generator is operating in flight. On the ground, it indicates that only one generator is supplying power.

Essay Format Questions

1. Compare and contrast the roles and operational priorities of the Engine-Driven Generators, the APU Generator, and External Power in supplying the aircraft's electrical system during normal operations, both in flight and on the ground.
2. Describe the various abnormal electrical configurations that can occur (e.g., loss of a generator, loss of a TR, total loss of main generators) and explain, in detail, the system's automatic responses and alternative power sources for each scenario.
3. Analyze the safety and control mechanisms implemented in the aircraft's electrical system, focusing on the functions of Generator Control Units (GCUs), Battery Charger Limiters (BCLs), and the indications provided on the ECAM ELEC page.
4. Discuss the importance of the Circuit Breakers (C/Bs) and their monitoring system. Explain how a pilot would identify and manage a tripped C/B, specifically addressing the differences between monitored and non-monitored C/Bs and the ECAM caution procedure.
5. Examine the concept of electrical load shedding in the A320/A321. Provide specific examples of when and what loads are shed under different abnormal electrical configurations (e.g., generator failure, emergency generation) and explain the rationale behind these shedding priorities.

Glossary of Key Terms

• AC (Alternating Current): Electrical current that periodically reverses direction. In this aircraft, it's typically three-phase 115/200 V 400 Hz.
• AC BUS: A main distribution point for AC electrical power to various aircraft systems.
• AC ESS BUS (AC Essential Bus): A critical AC power distribution point that supplies essential aircraft systems.
• AC ESS FEED pb (AC Essential Feed Pushbutton): A control that allows manual selection of AC BUS 1 or AC BUS 2 to supply the AC ESS BUS.
• APU GEN (Auxiliary Power Unit Generator): An AC generator driven by the Auxiliary Power Unit, capable of supplying electrical power to the aircraft.
• BAT 1 (2) ind (Battery 1/2 Indicator): An indicator on the overhead panel that displays the voltage of the respective battery.
• BAT 1 (2) pb-sw (Battery 1/2 Pushbutton Switch): A guarded pushbutton switch that controls the operation of the corresponding battery charge limiter and its connection to the DC BAT BUS.
• Battery Charger Limiter (BCL): A unit that monitors and controls the charging current of the aircraft batteries, and opens/closes the battery contactor.
• BUS TIE pb (Bus Tie Pushbutton): A control that commands the Bus Tie Contactors (BTCs) to automatically open or close to maintain power supply to both AC BUS 1 and 2.
• Bus Tie Contactor (BTC): An electrical switch that connects or disconnects AC BUS 1 and AC BUS 2, or other bus sections.
• C/B (Circuit Breaker): An automatic electrical safety device that opens a circuit when overloaded. Can be monitored (green) or non-monitored (black).
• CLR pb (Clear Pushbutton): A pushbutton used to clear an ECAM caution.
• COMMERCIAL pb (Commercial Pushbutton): A control to switch off non-essential commercial electrical loads in the aircraft.
• DC (Direct Current): Electrical current that flows in only one direction. In this aircraft, it's typically 28 V.
• DC BUS: A main distribution point for DC electrical power to various aircraft systems.
• DC BAT BUS (DC Battery Bus): A DC power distribution point primarily supplied by the aircraft batteries and TRs.
• DC ESS BUS (DC Essential Bus): A critical DC power distribution point that supplies essential aircraft systems.
• ECAM (Electronic Centralized Aircraft Monitoring): An integrated system that displays aircraft system information and warnings to the flight crew.
• ELEC page (Electrical Page): A display on the ECAM showing the status and configuration of the aircraft's electrical system.
• EMER CANC pb (Emergency Cancel Pushbutton): A pushbutton used to clear and inhibit an ECAM caution, typically used for C/B TRIPPED warnings.
• EMER GEN (Emergency Generator): A generator that automatically supplies electrical power in the event of a total loss of main generators, often driven by the RAT.
• EMER GEN TEST pb (Emergency Generator Test Pushbutton): A guarded pushbutton used to test the emergency generator and associated electrical systems.
• ESS TR (Essential Transformer Rectifier): A Transformer Rectifier that supplies DC power to the DC ESS BUS, especially during abnormal electrical conditions.
• EXT PWR (External Power): Electrical power supplied to the aircraft from a ground source.
• EXT PWR AVAIL (External Power Available): An amber light indication that external power is connected and its voltage is correct.
• EXT PWR pb (External Power Pushbutton): A control to connect or disconnect external power to the aircraft's electrical system.
• FADEC (Full Authority Digital Engine Control): An electronic engine control system that manages engine performance.
• GALLEY pb OR GALY & CAB pb (Galley or Galley & Cabin Pushbutton): Controls the power supply to the main galley, secondary galley, in-seat power, and In-Flight Entertainment (IFE) systems.
• GALLEY SHED: A condition where power to certain galley loads is automatically removed to reduce electrical demand.
• GAPCU (Ground and Auxiliary Power Unit Control Unit): A control unit that regulates the frequency and voltage of the APU generator and protects its network.
• GCU (Generator Control Unit): A unit that controls the frequency and voltage of AC generators and protects the electrical network.
• GEN 1 (2) pb-sw (Generator 1/2 Pushbutton Switch): A control that energizes or de-energizes the generator field and opens or closes the generator line contactor for the respective engine generator.
• GPCU (Ground Power Control Unit): A unit that protects the network supplied by ground power.
• Hot Bus: An electrical bus that remains powered directly by the aircraft batteries regardless of the main electrical system configuration, providing power to essential systems.
• IDG (Integrated Drive Generator): A type of engine-driven AC generator that integrates the constant speed drive and generator into a single unit.
• IDG 1 (2) pb-sw (Integrated Drive Generator 1/2 Pushbutton Switch): A guarded pushbutton switch used to disconnect the respective IDG from its driveshaft.
• IFE (In-Flight Entertainment): Systems providing entertainment to passengers during flight.
• kVA (Kilovolt-ampere): A unit of apparent power, used to rate generators.
• LO PR (Low Pressure): An indication, typically associated with IDG, signifying low oil pressure.
• MAINT BUS SW (Maintenance Bus Switch): A switch on the forward cabin panel that allows electrical circuits for ground servicing to be energized without powering the entire aircraft electrical system.
• MAN ON pb (Manual On Pushbutton): A guarded pushbutton to manually extend the Ram Air Turbine (RAT).
• RAT (Ram Air Turbine): A small turbine deployed from the aircraft to generate hydraulic or electrical power from the airflow in an emergency.
• RISE: An indication on the ECAM ELEC page representing the temperature difference between the IDG inlet and outlet.
• Shed: The automatic or manual disconnection of non-essential electrical loads.
• STAT INV (Static Inverter): An electronic device that converts DC power to AC power.
• TR (Transformer Rectifier): A device that converts AC current to DC current.
• TR Ent (Transformer Rectifier Entertainment): A TR specifically for the In-Flight Entertainment (IFE) system.
• V (Volts): A unit of electrical potential difference.

• WTB C/Bs (Wing Tip Brake Circuit Breakers): Circuit breakers with red caps designed to prevent accidental reset.