Airbus A220 Electrical System

The Airbus A220 generates its electrical power with two variable frequency generators (VFGs), one on each engine, backed by a generator on the auxiliary power unit (APU), a generator driven by a ram air turbine (RAT) and two batteries. On the ground it takes 115 V, 400 Hz AC power from a ground power unit; there is no connection for DC ground power. Three transformer rectifier units (TRUs) supply the DC network, and the distribution includes three electrical power centres (EPCs) and two bus power control units (BPCUs).

The public documents used here, the Transport Canada MMEL, the Airbus Aircraft Characteristics and the engine type certificate, do not show which load sits on which bus. They do show how the system is built and what backs up what, which is what a type-rating candidate is asked. The general principles are covered in electrical power distribution, aircraft batteries and generator control and protection.

Power sources

The overhead panel carries the corresponding pushbutton annunciators: L GEN, R GEN and APU GEN, each with FAIL and OFF legends; EXT PWR with AVAIL and IN USE; RAT GEN with an ON light; the guarded L DISC and R DISC switches with OIL and DISC legends; and CABIN PWR. The L GEN FAIL, R GEN FAIL and APU GEN FAIL messages are cautions.

Variable frequency generators

The engine type certificate lists two aircraft accessory drive pads on each PW1500G, both geared to N2, the high-pressure spool: a hydraulic pump pad at 0.1835:1 and a generator pad at 0.8595:1, which the certificate labels for an integrated drive generator (IDG). The A220 MMEL calls the generators themselves VFGs; none of the public sources describes the generator drive further or gives the output frequency.

Each VFG has its own oil system. A remote oil level sensor (ROLS) on each generator monitors the oil level, and the L GEN OIL LO LEVEL and R GEN OIL LO LEVEL INFO messages report a low level. A VFG with low oil pressure or high oil temperature is treated as inoperative: before dispatch the MMEL requires it to be selected OFF, its oil drained and the generator disconnected.

That disconnection uses a remotely activated disconnect on each generator, controlled by the guarded L DISC and R DISC switches on the overhead panel. Their legends read OIL and DISC. A missing guard on one of these switches is acceptable for dispatch only if the opposite VFG works.

The MMEL notes link each engine generator with an AC bus. With a low oil level on the left VFG, the APU generator must be verified able to supply AC BUS 1 before each flight; with a low level on the right VFG, AC BUS 2.

APU generator and external power

The APU generator can stand in for an engine generator. Except for extended operations (ETOPS), the MMEL allows dispatch with one VFG system inoperative provided the failed VFG is selected OFF, the APU is started before departure and run throughout the flight, and all EPCs, all TRUs and the opposite VFG are verified working. The APU remains available as an electrical source when its bleed air valve is inoperative, and it can normally be started on battery power: the MMEL warns that battery APU start may be inoperative with BPCU 2 degraded.

External power is AC only. The Airbus Aircraft Characteristics set these requirements:

When a ground power unit is plugged in, AVAIL lights come on both on the cockpit electrical panel and on the electrical/towing service panel. Power reaches the aircraft only when the crew selects EXT PWR ON: BPCU 1 then energises the external line contactor, and it also protects the aircraft systems from poor ground power. External AC can power the whole AC network or only the buses that supply the passenger cabin.

Airbus warns against any interruption just after power-up. Critical systems such as fly-by-wire and electrical then run their power-up built-in test (PBIT), and a ground power unit dropping off line or delivering poor power can interrupt it and latch failures that need maintenance action, with possible delays or cancellations.

Distribution, TRUs and essential buses

The MMEL names three electrical power centres (EPC 1, EPC 2 and EPC 3), each with an EPC DEGRADED INFO message, bus tie contactors BTC 1, BTC 2 and BTC 3 and DC tie contactors (DTCs). Its notes reveal some of the paths: with EPC 1 degraded and BTC 1 inoperative, the APU generator cannot supply AC BUS 1, and external power can supply neither AC BUS 1 nor AC BUS 2.

Three transformer rectifier units, TRU 1, TRU 2 and TRU 3, convert AC to DC. TRU 1 and TRU 2 each connect through a TRU line contactor (TLC 1 and TLC 2). With TRU 1 or TRU 2 inoperative, dispatch requires TRU 3 and the remaining TRU working, both VFG systems and all EPCs.

Essential buses exist: TCCA's list of failures that degrade anti-icing names DC ESS BUS 2, and the landing gear system has GEAR FAULT messages for 28V ESS and 28V NORM redundancy loss. Each engine's electronic control has an aircraft 28 V DC backup supply to each of its two channels. The public sources do not say which loads each bus carries.

The MMEL mentions cockpit thermal circuit breakers (TCBs) with a status indication and INFO messages for the left and right CB panels, and the CB TRIP advisory reports a tripped breaker. Two electrical units named after the fly-by-wire system, the L and R FBW PC, are each associated with a permanent magnet generator (PMG). The MMEL also lists CDC units 1 to 5 with power supply modules: with a CDC 3 power supply module inoperative the left thrust reverser is considered inoperative, and with a CDC 4 module the right one.

Batteries

The A220 has two batteries. Battery system 1 comprises battery 1, its charger, heater and temperature sensor and battery line contactor BLC 1; a battery system 2 is also installed. EASA's certification basis includes special conditions on lithium battery installations. The electrical/towing service panel has a BATT annunciator light; the MMEL notes that without it the battery may run down if not selected OFF.

Battery-only operation on the ground is kept short. With battery system 1 inoperative, the MMEL asks crews to limit battery-only ground operation to keep battery 2 fully charged, and with the APU fire warning horn inoperative it limits battery-only power to 5 minutes.

Ram air turbine and emergency power

The ram air turbine sits in the electrical chapter of the MMEL. It drives a generator: the overhead RAT GEN pushbutton, under a guard, shows ON, and a sensor monitors whether the RAT is deployed. Two heaters, A and B, have their own INFO messages. None of the public sources lists a RAT-driven hydraulic pump; on the A320, by contrast, the RAT drives a hydraulic pump that in turn drives the emergency generator.

The sources also name an EMER PWR ONLY warning: Build 8.0A2 masks several air system advisories while it is displayed, and Build 8.0A3 suppresses the equipment bay smoke detection messages with it. Dispatch with EPC 3 degraded requires the RAT to be verified not deployed and the APU to run throughout the flight.

Failure cases and the MMEL

The main MMEL reliefs show what backs up what:

All these reliefs except the last exclude extended operations (ETOPS).

The MMEL asks for the APU to run throughout the flight in other cases too: with flight control computer PFCC 3 deactivated, for example, or with EPC 2 degraded in some configurations. In flight, a generator failure posts a GEN FAIL caution, and a DC ESS BUS 2 failure is one of those whose procedure gained an ice dispersal step with Build 8.0A2.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.