The A320 Electrical System: A Foundation Series for Oral Exam Preparation
Start here for the architecture. The spoke pages go where the exam does.
Most A320 pilots can name the electrical sources. Fewer can walk the bus structure without hesitating, and fewer still can state the conditions the manual attaches to the answers, which is usually where an oral exam goes next.
This is the hub for a three-spoke series on the A320 electrical system, written for pilots preparing for a type rating or recurrent oral. Every claim across the series is anchored to the Flight Crew Operating Manual and has been verified against the manual text rather than summarized from memory or inherited from a study guide.
Start here for the architecture. The spoke pages go deeper into the specific questions examiners actually ask.
The architecture, in brief
In normal configuration the A320 electrical power system is constituted of two engine generators and one APU generator. On the ground, either the APU generator or external power may supply the complete system. If normal AC power is not available, an emergency generator can provide AC power. If all AC power is not available, DC power from the batteries can be inverted into AC power.
That is the manual's own opening description, and it is worth learning in that shape because it moves from normal to degraded in the same order the system does.
Frequently Asked Questions
What are the main AC and DC buses on the A320?
On the AC side, AC BUS 1 is normally supplied by GEN 1 through generator line contactor 1, and AC BUS 2 is normally supplied by GEN 2 through generator line contactor 2. The AC ESS BUS is normally supplied by AC BUS 1.
On the DC side, TR 1 normally supplies DC BUS 1, the DC BAT BUS, and the DC ESS BUS. TR 2 normally supplies DC BUS 2.
HOT BUS 1 and HOT BUS 2 are permanently connected to BAT 1 and BAT 2 respectively.
A useful way to hold it: GEN 1 to AC BUS 1 to AC ESS BUS, and through TR 1 to DC BUS 1 plus DC BAT BUS plus DC ESS BUS. GEN 2 to AC BUS 2, and through TR 2 to DC BUS 2.
How many transformer rectifiers does the A320 have?
There are two main transformer rectifier units, TR 1 and TR 2, each with a maximum continuous load of 200 A per TRU. A third identical transformer rectifier, the ESS TR, supplies the DC ESS BUS. Following loss of all main generators, the emergency generator supplies the AC ESS BUS and the ESS TR supplies the DC ESS BUS. Following failure of one main TR, the remaining main TR automatically replaces the failed TR and the ESS TR supplies the DC ESS BUS. Where fitted, the FCOM also lists a fourth transformer rectifier, TR Entertainment, which powers the DC Entertainment busbar for the IFE system.
Answering only TR 1 and TR 2 is incomplete. For oral-exam precision: TR 1 and TR 2 are the two main TRs; ESS TR is the essential TR; and where fitted, TR Entertainment powers the DC Entertainment busbar. Do not give three TRs as a universal aircraft-total answer.
Is the A320 DC distribution symmetric?
In normal configuration it is not. TR 1 normally supplies three buses, DC BUS 1 and the DC BAT BUS and the DC ESS BUS, while TR 2 normally supplies DC BUS 2 only.
The recovery, however, is symmetric. In a single transformer rectifier failure the other TR automatically replaces the faulty one, and the ESS TR supplies the DC ESS BUS. That holds for a TR 1 fault and for a TR 2 fault alike.
So the normal picture is lopsided and the failure picture is balanced. A candidate who has learned only the normal allocation tends to assume the consequences of losing TR 1 and TR 2 are proportionally different. They are not, because the reconfiguration is generic.
When are the A320 batteries connected to the DC BAT BUS?
With the BAT pushbuttons in AUTO and the battery charge limiters operative, each BCL automatically controls connection and disconnection of its battery to the DC BAT BUS. Automatic connection occurs in the following published cases.
APU starting: APU MASTER SW pushbutton ON and APU N below 95 percent. If the emergency generator is running, the connection is limited to 3 min.
Battery charge: battery voltage below 26.5 V. The charging cycle ends when battery charge current goes below 4 A, on ground immediately and in flight after a 30 min time delay.
Loss of AC BUS 1 and AC BUS 2 below 100 kt when the emergency generator is not supplying.
Separately, the FCOM publishes a note that, in normal configuration, the batteries are disconnected most of the time.
The charging passage defines one normal battery connection and disconnection case. The note states the resulting normal-configuration state. The two statements are compatible, and they sit in different sections, but the FCOM does not state that the note is derived solely from the charging rule. Charging is only one of the three published connection cases. A candidate who can state both statements and cite each to its own section has the complete picture.
Most study material gives only a loose version of the charging case, which is why the published thresholds are worth carrying. Need charging and APU start are shorthand, not conditions.
Separately from all three cases, HOT BUS 1 and HOT BUS 2 remain permanently connected to BAT 1 and BAT 2 whether or not the battery line contactor is closed. The battery line contactor connects the batteries to the DC BAT BUS. It is not the path to the hot buses; that is a separate and permanent connection.
The DC BAT BUS itself is normally supplied by TR 1.
What does the BUS TIE do on the A320, and when do the contactors close?
In BUS TIE AUTO, bus tie contactor 1 and bus tie contactor 2 automatically open or close to maintain power supply to AC BUS 1 and AC BUS 2. In BUS TIE AUTO, they automatically open or close to maintain power supply to AC BUS 1 and AC BUS 2.
Both contactors are closed during single-engine operation, operation on the APU generator, or external power supply. In each of those cases a single source is feeding both AC buses through the tie bus.
One contactor is closed when one engine generator supplies its associated AC BUS and the APU generator or external power supplies the other side. That is a split-source configuration rather than a single source feeding both.
Selecting BUS TIE OFF opens both contactors.
A common incomplete answer is that the bus tie closes only if a generator fails. That misses the normal reconfiguration cases the manual explicitly describes, where a single source is supplying the aircraft on the ground or in single-engine operation.
What is the static inverter on the A320 and when does it operate?
The static inverter transforms DC power from Battery 1 into one kVA of single-phase 115 V 400 Hz AC power, supplied to part of the AC essential bus.
When only the batteries supply electrical power to the aircraft, the static inverter is automatically activated above 50 kt regardless of the BAT 1 and BAT 2 pushbutton positions. Below 50 kt, it is activated only when both BAT 1 and BAT 2 pushbuttons are on.
It is part of the complete electrical power architecture but it is not part of the normal configuration source list. Candidates who include it among the normal sources, or who exclude it from the complete architecture, are making the same mistake from opposite directions.
What are the A320 electrical power ratings?
Each engine generator is driven by an Integrated Drive Generator and produces 115/200 V three-phase AC at a constant 400 Hz, rated up to 90 kVA. The APU generator produces identical output ratings and can replace either or both engine generators.
The generators are never connected in parallel.
TR 1 and TR 2 are the two main transformer rectifier units, each with a maximum continuous load of 200 A per TRU. The static inverter produces one kVA. The two main batteries each have a normal capacity of 23 Ah.
The Series
Spoke 1: source priority and the external power indications
Which source powers the aircraft when engine generators, external power, and the APU generator are all available. What the AVAIL light means before you press the pushbutton and what the blue ON light does and does not tell you afterwards. What actually happens to the APU generator while external power is feeding the network.
Spoke 2: single source loss and automatic reconfiguration
What triggers a generator fault, whether the reset attempt has a published limit, and why the guarded IDG pushbutton is the wrong reach for a generator fault. The published constraints on the IDG disconnect, the oil temperature ladder, and what the aircraft reconfigures on its own before the crew touches anything.
Spoke 3: emergency electrical configuration
Loss of all main generation. The RAT, the emergency generator it drives, the static inverter, and battery-only operation. What is shed, at what speeds, and what remains, down through the two ground thresholds. Read the Spoke 3 page.
Why the conditions matter more than the facts
A pattern runs through this whole chapter. The manual attaches conditions to most of its statements, and the common failure on an oral is not forgetting the fact. It is giving a universal answer to a conditional question.
The APU generator replaces a failed engine generator if it is available. Galley shedding differs between flight and ground and between aircraft standards. Essential bus recovery after an AC BUS 1 fault is automatic on aircraft with one modification and a crew action on aircraft without it. Crew awareness sometimes means monitor and continue and sometimes follows a directed selection.
Learning the conditions is what separates a candidate who has read the manual from one who has read a summary of it.
