AC BUS 1(2) FAULT

Losing one main AC busbar is a half-network failure — the overview and Automatic Reconfiguration already established that the two distribution networks are not symmetric, the left carrying the whole essential family while the right is a single clean bus. This article lays the two procedures — ELEC AC BUS 1 FAULT and ELEC AC BUS 2 FAULT — side by side and reads the asymmetry off them.

The two faults fail in opposite registers. Losing AC BUS 1 is dramatic in the handling itself — the failure can blank out the very ECAM you would use to handle it, so the first move is to recover a display, not to read a procedure. Losing AC BUS 2 is dramatic in the landing capability — it drops the aircraft to CAT 1 ONLY, worse than losing AC BUS 1. One thread runs through both: the same caution may in fact be only a sub-bus failure, in which case only a part of the listed systems is actually lost.

By the end of this article you should be able to answer five things: (1) what display crisis can open an AC BUS 1 FAULT and how to escape it; (2) the three-step escalation logic of ALTN → MAN ON inside the procedure; (3) why the landing-capability STATUS differs between the two sides, and which side is worse; (4) what each side's INOP list hits hardest; and (5) how to read the "sub BUS failure" note.

1. ELEC AC BUS 1 FAULT — when the procedure can blank its own tools

Trigger: AC BUS 1 (1XP) is de-energised. The left network — AC BUS 1 and the essential family it carries — is now in question. Per the overview, the AC ESS BUS normally hangs off AC BUS 1, so its survival depends on the AC ESS feed transfer to AC BUS 2 (AC ESS Feed and Transfer) firing cleanly.

1.1 The opening crisis — recover a display first

The procedure opens with a warning that is unique among the electrical faults, because it can remove the tools you need to work it:

"If the automatic transfer of AC ESS BUS is inoperative, the ECAM's E/WD and SD DUs are simultaneously lost. The ECAM/ND SEL must be used to recover the E/WD on the NDU, in order to apply the ECAM procedure."

Per FCOM PRO-ABN-ELEC. The mechanism is in the display architecture. Per FCOM DSC-31-05-50, in normal operation the three Display Management Computers split the screens:

"In normal operation : ‐ DMC 1 supplies data to the Captain's PFD and ND. ‐ DMC 2 supplies data to the First Officer's PFD and ND. ‐ DMC 3 supplies data to the upper and lower ECAM DU. ... In case of DMC 3 failure, DMC 1 automatically takes over and supplies the ECAM DUs, provided the ECAM SWITCHING selector is in the AUTO position."

So DMC 3 is the normal source of the ECAM display units (E/WD + SD), with DMC 1 as its automatic backup. The AC BUS 1 FAULT INOP list flags both of these by number — DMC 3 and DMC 1 (EFIS DMC 3, ECAM DMC 1) — on the left/essential side; only DMC 2, which drives the First Officer's EFIS from the surviving AC BUS 2, is untouched. So when AC BUS 1 is lost and the AC ESS network fails to transfer to AC BUS 2, the ECAM loses both its normal source and its automatic backup at once, and the E/WD and SD displays go dark together — the screens you would read the procedure on are the first casualty. The ECAM/ND SEL then recovers the E/WD onto a surviving (DMC 2-driven) ND.

[!warning]- Counter-intuitive: losing one main AC bus can take the ECAM with it

It is tempting to think "lose a bus, lose the systems on the INOP list, but ECAM itself keeps working." Not necessarily. With the AC ESS auto-transfer inoperative, the E/WD and SD DUs are lost simultaneously — the ECAM-driving DMCs are on the failed side. The escape is the ECAM/ND SEL switch, which recovers the E/WD onto the (F/O) ND display so the procedure can be applied. Recover the picture before you try to work the failure.

1.2 The escalation spine — auto → ALTN → MAN ON

The body of the procedure is a three-step escalation built around the AC ESS supply. Each manual step appears only if the step before it failed — the L2 condition lines spell this out:

AC ESS BUS auto-transfer to AC BUS 2 ── after 3 s relay delay (silent, see art. 09)
   │
   ▼  if the auto-transfer is inoperative, the procedure shows:
AC ESS FEED ........................ ALTN     ◄ "displayed, if automatic AC ESS transfer has failed"
   │
   ▼  if the ALTN (manual) selection ALSO fails:
EMER ELEC PWR ...................... MAN ON   ◄ "displayed, if manual AC ESS transfer has failed"
   │
   ▼
VENT EXTRACT ....................... OVRD
[certain engine variants] ENG 1 N1 DEGRADED MODE / ENG 1 AND 2 N1 MODE ... ON
   ├─ if A/THR unavailable: MAN THR ... ADJUST
   └─ if A/THR available:   A/THR ..... AS RQRD
[if AC ESS BUS is lost] AVOID ADVERSE WEATHER / ATC COM VOICE ONLY

The 3-second delay is the relay-timed automatic switch confirmed in the AMM:

"In case of AC BUS 1 loss, the AC ESS BUS, AC ESS SHED BUS and the ESS TR supply is automatically transferred from AC BUS 1 busbar to AC BUS 2 busbar via the contactor 3XC-B. On the ECAM system: - the ELEC AC page is displayed showing the AC BUS 1 indication in amber, - and after 3 seconds (time delay of the relay 11XC), the connection of the essential network to the AC BUS 2."

Per AMM 24-25-00. So in the normal case the essential family simply moves to AC BUS 2 after 3 seconds and there is no procedure line at all — the manual AC ESS FEED ALTN step (AC ESS Feed and Transfer) only appears when that automatic transfer has failed.

The second step deserves a pause. EMER ELEC PWR MAN ON is the command that forces the emergency generator (CSM/G) on line (Emergency Generator). In other words, with only half the main network lost, but both ways of feeding AC ESS from AC BUS 2 having failed, the procedure tells you to reach down into the emergency layer to keep the essential family alive. That is the design priority made visible: the AC ESS family ranks above everything, so the system would rather spin up the emergency generator than leave AC ESS dark.

The ENG 1 N1 DEGRADED MODE branch (on aircraft with the applicable engine fit) is the cross-system tail: losing AC BUS 1 takes out the left engine's EPR-measurement chain, so that engine reverts to N1 mode (the degraded-mode logic belongs to ATA 70). Under N1 mode the thrust is set by hand or by autothrust depending on availability — MAN THR ADJUST if A/THR is unavailable, A/THR AS RQRD if it is available.

1.3 STATUS and the INOP list — the left-network casualties

Landing capability. When AC ESS is lost, the STATUS carries AVOID ADVERSE WEATHER and LDG DIST PROC APPLY; the autoland capability degrades but is not wiped out:

"If A/THR is not available: CAT 2 ONLY ... If A/THR is available: CAT 1 WITH A/THR / CAT 2 WITH MAN THR."

Per FCOM PRO-ABN-ELEC. CAT 3 is lost (it appears in the INOP list), but a CAT 2 or CAT 1 capability survives — better than the right-side fault, as §2 shows.

INOP families (left network). The list is long; read it by family rather than line by line:

Note the dual GPWS + TCAS loss: with both terrain and traffic surveillance gone, the procedure's AVOID ADVERSE WEATHER is not advisory padding — it is the direct consequence of losing those sensors. A worthwhile footnote sits under the air-data line:

"If AIR DATA SWTG is set to CAPT ON 3, STBY PITOT is supplied by the AC ESS BUS, CAPT PITOT is no longer supplied."

Per FCOM PRO-ABN-ELEC — a reminder that the air-data source switching re-plumbs which pitot survives.

The DC side recovers itself. TR 1 appears in the INOP list because it draws from AC BUS 1, but the DC network does not need manual intervention: TR 2 automatically restores DC BUS 1 and the DC BAT BUS (the symmetric TR recovery — Transformer-Rectifiers and DC Network Transfer). One TR is lost, but the DC layer is held automatically.

2. ELEC AC BUS 2 FAULT — symmetric trigger, asymmetric sting

Trigger: AC BUS 2 (2XP) is de-energised. The trigger mirrors AC BUS 1, but the content does not. There is no display crisis to open with (the ECAM-driving DMC 1 is on the left side, untouched), and by default no ALTN/MAN ON ladder — the essential family lives on the left network, which is still powered.

2.1 The one place AC ESS still bites — the ALTN-feed exception

It is tempting to say "AC BUS 2 has nothing to do with AC ESS." That is almost true, and the exception matters:

[!warning]- "AC BUS 2 is unrelated to AC ESS" — only by default

In the normal feed configuration the AC ESS family is fed from AC BUS 1, so losing AC BUS 2 does not touch it. But if the crew had previously selected AC ESS FEED ALTN (manually feeding AC ESS from AC BUS 2), then losing AC BUS 2 would also drop AC ESS. That is exactly why the AC BUS 2 procedure and STATUS still retain the conditional branch "If AC ESS BUS is lost: AVOID ADVERSE WEATHER" — it covers this ALTN-feed case, not the default one.

So the procedure-specific lines on the right side are ATC/XPDR SYS 1 (transponder 2 lost, switch to system 1), the symmetric ENG 2 N1 DEGRADED MODE branch (on aircraft with the applicable engine fit, with the same MAN THR ADJUST / A/THR AS RQRD split as ENG 1), and the conditional If AC ESS BUS is lost: AVOID ADVERSE WEATHER.

2.2 Why CAT 1 ONLY — the autoland redundancy lives on the right

The STATUS reversal is the headline:

"CAT 1 ONLY."

Per FCOM PRO-ABN-ELEC. Losing AC BUS 2 is harsher on landing capability than losing AC BUS 1, and the reason is in the INOP list: ILS 2, LS 2 and CAT 2 all hang on the right network. The autoland redundancy that buys a CAT 2/CAT 3 capability lives on side 2, so losing the right bus drops the aeroplane straight to CAT 1 ONLY. The STATUS also carries FWD CRG VENT REDUCED (forward cargo ventilation degraded) — the line to count when carrying live animals (AVI) or temperature-sensitive cargo.

[!warning]- LDG DIST PROC belongs to AC BUS 1, not AC BUS 2

Do not transplant the LDG DIST PROC APPLY STATUS line onto an AC BUS 2 FAULT — it appears only in the AC BUS 1 STATUS (under "if AC ESS BUS is lost"). The AC BUS 2 STATUS has no LDG DIST PROC line. The landing concern on the right side is the capability downgrade (CAT 1 ONLY), not a landing-distance procedure.

2.3 INOP families (right network)

(Second-order and maintenance-layer entries — the cargo-ventilation temperature cluster FWD CRG VENT / TEMP / BULK VENT, R CAB VENT, DRAIN HEAT, PRESSURIZED WATER SYS, ground/load items — also appear on the list but are not expanded here.)

2.4 Reading the two sides together

The summary line: lose AC 1 = captain side + AC ESS risk + both electric hydraulic pumps, the drama in the process; lose AC 2 = F/O side + recording/monitoring + autoland redundancy, the drama in the landing. On either side one TR follows the bus down, and on either side the DC network self-heals through the symmetric 1PC recovery (Automatic Reconfiguration).

3. The sub-bus note — the same caution, discounted

Both procedures end with the same footnote, and it changes how you read the whole INOP list:

"The warning may be caused by a sub BUS failure. Consequently, only a part of the above-listed systems may be lost."

Per FCOM PRO-ABN-ELEC. This is the procedure-side face of the three-level bus model in AC Distribution and Busbars: a main busbar has no separate "sub-bus caution", so an RCCB-level (sub-bus) failure borrows the same AC BUS FAULT warning. The practical reading:

• If you see AC BUS FAULT but the ELEC page still shows the busbar green, and the actual INOP entries are far fewer than the printed list — treat it as the sub-bus case. Handle the systems you have genuinely lost; do not frighten yourself with the full list.
• Cross-check the C/B page for a TRIP (Circuit Breakers and the CBMU) to localise the sub-bus fault.

4. Flight-deck scenarios

1. Cruise: AC BUS 1 FAULT with both ECAM screens blank. First action is the ECAM/ND SEL — recover the E/WD onto the ND so you can see the procedure, then work it. Step the AC ESS supply: AC ESS FEED ALTN; if that does not restore it, EMER ELEC PWR MAN ON. Restore the ability to read the procedure before reading it.
2. AC BUS 2 FAULT, preparing the approach. Reset expectations to CAT 1 ONLY — even if the weather happens to sit exactly at CAT 2 minima, the capability is gone. The transponder has already moved to SYS 1. And remember the LDG DIST PROC line belongs to AC BUS 1, not here — do not apply a left-side landing-distance procedure to a right-side fault.
3. AC BUS 1 FAULT but all screens are alive and the INOP list is sparse. This is the sub-bus case — the busbar is still green, only a part of the listed systems is lost. Handle the real INOP set and check the C/B page; do not pre-plan for the worst case.

[!warning]- Common misconceptions — predict, then check

Read each statement, decide true or false, then check the truth in brackets.

1. "Losing AC 1 is the bigger event — the captain side goes dark — so it must be more serious than losing AC 2." — Half true. The drama of losing AC 1 is in the handling (screens, the AC ESS ladder), but the landing capability is worse on AC 2: that side is CAT 1 ONLY, because ILS 2 / LS 2 / CAT 2 — the autoland redundancy — all hang on the right network (§2.2).
2. "Lose one main AC bus and you simply lose the systems on the list — ECAM itself keeps working." — False. With the AC ESS auto-transfer inoperative, the E/WD and SD DUs are lost simultaneously (the ECAM-driving DMCs are on the failed side). The escape is the ECAM/ND SEL recovering the E/WD onto the ND (§1.1).
3. "The emergency generator only comes out when both engines have stopped." — False. In this half-network failure, if both ways of feeding AC ESS from AC BUS 2 fail (auto transfer and the manual ALTN), the procedure tells you to command EMER ELEC PWR MAN ON and bring up the emergency generator — half the main network is gone, yet the emergency layer is used, because the AC ESS family ranks first (§1.2).
4. "See AC BUS FAULT, plan for the whole INOP list, worst case." — Conditional. The same caution may be only a sub-bus failure — busbar still green, INOP far shorter than the list — in which case handle only what you have actually lost (§3).
5. "Lose either AC bus and a TR goes with it, so you must transfer the DC side by hand." — Half true. One TR does follow the bus down, but the DC network recovers automatically through the symmetric 1PC chain — no crew action (§1.3, §2.4).

Self-test

[!note]- Q1. What can open an AC BUS 1 FAULT, and what is the escape?

When the AC ESS auto-transfer to AC BUS 2 is inoperative, the AC ESS supply collapses and the E/WD and SD DUs are lost simultaneously — the ECAM's normal source DMC 3 and its automatic backup DMC 1 are both on the left/essential side, leaving only DMC 2 (the F/O EFIS source) on AC BUS 2. The escape is the ECAM/ND SEL switch, which recovers the E/WD onto the (F/O) ND so the ECAM procedure can be applied. Recover the display first, then work the failure.

[!note]- Q2. Walk through the three-step AC ESS escalation in the AC BUS 1 FAULT procedure.

(1) Automatic transfer of the essential network to AC BUS 2 after a 3-second relay delay — silent, no procedure line. (2) If that auto-transfer has failed, AC ESS FEED ALTN appears (manual selection to AC BUS 2). (3) If the manual ALTN transfer has also failed, EMER ELEC PWR MAN ON appears — forcing the emergency generator on line. The escalation reaches into the emergency layer even though only half the main network is lost, because the AC ESS family has top supply priority.

[!note]- Q3. Compare the landing capability of the two faults and explain the difference.

AC BUS 1 FAULT: CAT 2 ONLY (if A/THR unavailable) or CAT 1 WITH A/THR / CAT 2 WITH MAN THR — CAT 3 is lost but a CAT 1/2 capability survives. AC BUS 2 FAULT: CAT 1 ONLY — harsher. The reason is the INOP list: ILS 2, LS 2 and CAT 2 all hang on the right network, so losing AC BUS 2 strips the autoland redundancy and drops the aircraft straight to CAT 1.

[!note]- Q4. Name the worst-hit families on each side.

Left (AC BUS 1): captain-side displays (DMC 1 + DMC 3, HUD), air data (ADR 3, STBY pitot/AOA, CAPT TAT), both hydraulic electric pumps (Green + Yellow), TR 1, GPWS + TCAS, datalink (= ATC COM VOICE ONLY). Right (AC BUS 2): F/O-side displays (DMC 2, F/O PFD + ND), recording/monitoring (DFDR, SDAC 2, FWC 2, FDIU, CMC 2), the right navigation bank (ADR 2, ILS 2, GPS 2, RA 2, VOR/DME/ADF 2), APU TR + TR 2, the Blue electric pump, and the forward transfer pump.

[!note]- Q5. How do you use the sub-bus note, and where does LDG DIST PROC belong?

The sub-bus note warns that an AC BUS FAULT caution can be raised by a sub-bus (RCCB-level) failure, so only part of the listed systems may be lost. If the busbar is still green and the INOP set is short, handle the real losses and check the C/B page — do not plan for the full list. LDG DIST PROC APPLY belongs only to the AC BUS 1 STATUS (when AC ESS is lost); the AC BUS 2 STATUS has no such line.

Key takeaways

References

Per FCOM PRO-ABN-ELEC (ELEC AC BUS 1 FAULT — opening E/WD + SD display crisis and ECAM/ND SEL recovery, the AC ESS FEED ALTN / EMER ELEC PWR MAN ON / VENT EXTRACT OVRD escalation with its display conditions, the N1 degraded-mode branch, the STATUS CAT 2 ONLY / CAT 1 WITH A/THR capability and LDG DIST PROC, the left-network INOP list and the AIR DATA SWTG footnote; ELEC AC BUS 2 FAULT — ATC/XPDR SYS 1, the CAT 1 ONLY STATUS and FWD CRG VENT REDUCED, the right-network INOP list, and the "If AC ESS BUS is lost" ALTN-feed branch; the sub-bus note common to both); AMM 24-25-00 (AC essential generation switching — automatic transfer of AC ESS / AC SHED ESS / ESS TR from AC BUS 1 to AC BUS 2 via contactor 3XC-B after the 3-second relay 11XC delay); AMM 24-00-00 (symmetric TR recovery of the DC network). The "drama in the handling vs drama in the landing" reading, the DMC-chain explanation of the display crisis, and the left/right comparison are integrative syntheses of the above and contain no facts from outside the library. Mechanisms referenced are developed in AC ESS Feed and Transfer, Emergency Generator, AC Distribution and Busbars, Automatic Reconfiguration, AC ESS Bus Fault and Shed and Emergency Electrical Configuration.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, and QRH for operational use.