Airbus Flight Instructor
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DC Bus Faults

The DC side of the network raises four bus-fault cautions, and they sit at wildly different procedural weights: ELEC DC BUS 1 FAULT, ELEC DC BUS 2 FAULT, ELEC DC BAT BUS FAULT, and ELEC DC BUS 1+2 FAULT. One is pure crew awareness with no action step; another runs a four-line drill and ends with the aircraft on gravity gear extension and single-side fuel feed. This article is the procedural face of the bus map drawn in DC Network Transfer — it takes each DC bus, names what its loss costs, and reads the ECAM STATUS/INOP consequences the way a crew meets them.

One corrective finding shapes the whole chapter and is worth stating up front. The heaviest engine-side damage — both EIVMUs lost, giving ENG HI IDLE, auto-start only, loss of autothrust and loss of the reversers — does not hang on DC BUS 1+2. It hangs on the DC BAT BUS (3PP). So when a DC BUS 1+2 failure produces those engine symptoms, it is a collateral effect: both main TRs are gone, the battery bridge holds 3PP for seven seconds and then lets it go (see Transformer-Rectifiers and Batteries and the BCL). Nail the carrier to 3PP and the four cautions stop looking arbitrary.

1. Four cautions, four very different bills

By the end of this article you should be able to answer five questions:

  1. Why do the four cautions differ so much in procedural weight — DC BUS 1 is bare crew awareness, while DC BUS 2 runs a long INOP list?
  2. What is the mechanism behind the SEC 2 KEEP ON note, and where is its red line?
  3. What exactly is the engine bill that follows a loss of 3PP?
  4. What is the fuel-handling logic on DC BUS 1+2 (cross-feed and the single-pump seesaw)?
  5. Why does DC BUS 1+2 demand a gravity gear extension?

The short answer to the first question — the one that organises everything below — is that each DC bus is the dormitory of a different equipment family. DC BUS 1, once its TR substitution has silently reconfigured, is left only with an account of paper losses. DC BUS 2 houses the flight-control and landing-gear computers. The DC BAT BUS carries the engine-interface units. DC BUS 1+2 is the union of all three plus the new failures that only emerge when both halves go together.

2. ELEC DC BUS 1 FAULT — the lightest of the four

Procedure: crew awareness — there is no action step. By the time the caution is read, the TR cross-substitution has already restored the bus (TR2 picks up DC BUS 1 and the DC BAT BUS automatically — the silent reconfiguration described in Automatic Reconfiguration). Nothing is left for the crew to switch.

What remains is the STATUS/INOP account:

That last item is the diagnostic one. The CBMU rides on this line: lose DC BUS 1 and circuit-breaker monitoring goes with it. This is the cross-check of the 101PP = 1PP sub-segment relationship from Circuit Breakers and the CBMU — a NORMAL-looking absence of C/B reporting after a DC BUS 1 loss is consistent, not contradictory.

Per FCOM PRO-ABN-ELEC, p.3371–3372.

3. ELEC DC BUS 2 FAULT — the flight-control tenement

This is the heavy one of the pair. Four procedure steps and one golden note:

AIR DATA SWTG (IF ADR 3 AVAIL) ............... F/O ON 3
FM SOURCE .................................... BOTH ON 1
SEC 2 ........................................ KEEP ON
T.TANK MODE (if CG > 32 % and > 270 kt, not climbing) .. FWD

The note that sits on the SEC 2 KEEP ON step is the heart of this caution:

"SEC 2 is normally supplied by DC BUS 2. In case of a DC BUS 2 failure, the DC ESS BUS supplies SEC 2. However, in this case, the SEC 2 FAULT light, on the overhead panel, comes on. Do not attempt a reset: Selecting OFF would result in a loss of the SEC 2 backup power supply."

Per FCOM PRO-ABN-ELEC, p.3373.

[!warning]- The FAULT-lit SEC 2 is alive — do not reset it

This is another instance of the "the light cannot be trusted" pattern (the same family as the AP2 trap met in AC ESS Bus Fault and Shed): a SEC 2 with its FAULT legend on is in fact running perfectly well, drawing its backup supply from the DC ESS BUS. The legend lights only because the supply chain changed, not because the function failed. Selecting OFF to "reset" it would cut the backup supply and genuinely kill SEC 2. The lesson generalises: the supply chain of a light is not the supply chain of the function behind it.

The SECONDARY FAILURES line points to *F/CTL, and the STATUS page spells out why DC BUS 2 is heavy:

Read the INOP list and the conclusion writes itself: DC BUS 2 is the dormitory of the flight-control and landing-gear computers. Losing it does not threaten the supply of power so much as it removes whole computers — two of the three PRIMs, a SEC's normal feed, an LGCIU, an SFCC, an FCDC, an FCMC — which is why the procedure is long where DC BUS 1 was empty.

Per FCOM PRO-ABN-ELEC, p.3373–3376.

4. ELEC DC BAT BUS FAULT — the engine bill (the core finding)

The DC BAT BUS (3PP) feeds both EIVMUs, and that single fact produces the chapter's most surprising consequence:

"Due to the loss of the EIVMUs."

This is the cause line printed under ENG HI IDLE in the procedure. Per FCOM PRO-ABN-ELEC, p.3370.

Lose 3PP and the EIVMUs go with it, producing the engine bill:

The full mechanism lives in ATA 70 (the EIVMU is a five-module interface unit — the autothrust EPR target, the reverser command and the idle selection all pass through it). The electrical chapter's contribution is narrower but decisive: it nails the carrier. This engine bill follows 3PP, wherever 3PP goes. It appears on a standalone DC BAT BUS FAULT, and it appears again — exactly the same symptoms — when 3PP is lost collaterally seven seconds into a DC BUS 1+2 failure (the next section's STATUS page proves it).

[!warning]- Do not hunt for the engine bill in the DC BUS 1 / DC BUS 2 checklists

ENG HI IDLE, auto-start-only, loss of autothrust and loss of the reversers belong to 3PP, not to either main DC bus. On a standalone DC BAT BUS FAULT these engine symptoms arrive on their own, with neither DC BUS 1 nor DC BUS 2 lost. 3PP keeps its own registration — diagnose it as a DC BAT BUS event, not as a side-1 or side-2 bus loss.

5. ELEC DC BUS 1+2 FAULT — DC hemiplegia

When both main DC buses go, the procedure is led by fuel:

IF NO FUEL LEAK:
  WING X FEED ........ ON         (only L FUEL PUMP 2 is still powered)
  FM SOURCE .......... BOTH ON 1
  SEC 2 .............. KEEP ON
  FUEL IMBALANCE ..... MONITOR

Only L FUEL PUMP 2 still has power — it draws off the surviving DC ESS chain — so the wing is fed from one side, and the crew must watch the balance:

"Due to the fact that fuel is fed from one side only, the flight crew must monitor fuel balance. To maintain fuel balance, L PUMP 2 can be set to OFF. R PUMP 2 will then automatically start, and the corresponding FAULT light will go off."

Per FCOM PRO-ABN-ELEC, p.3377.

This is an elegant seesaw: the two No. 2 pumps share a single backup supply, so switching the left pump OFF hands the power to the right, which starts automatically — and the instant its FAULT light extinguishes proves the "switch one off, the other comes on" logic of the fuel chapter still holds under an electrical fault. The order matters: clear any leak first, cross-feed second — the first gate of the ATA 28 leak procedure.

The STATUS / APPR PROC page then lays out the consequences:

[!warning]- DC BUS 1+2 is not just "DC 1 plus DC 2" — new flight-critical items emerge

Several handling losses appear only when both halves go together; they are not in the DC BUS 1 list nor the DC BUS 2 list, but emerge from the superposition: F/CTL PROT (flight-control protections lost) · N/W STRG (nosewheel steering failed — ground-roll directional control is left to rudder plus differential braking, which compounds badly with "no reverser") · REAC W/S DET (reactive windshear detection lost) · AUTOLAND / GLS. Landing capability also drops from the CAT 3 SINGLE of either single-bus loss down to CAT 2. So the "union, plus extra" is a real extra — these are genuine flight-critical additions, not bookkeeping.

Per FCOM PRO-ABN-ELEC, p.3377–3384.

5.1 The mechanism behind it (synthesis)

The typical root cause of a simultaneous DC BUS 1+2 loss is the failure of both main TRs. With TR1 and TR2 gone, the normal DC chain has no source left to borrow from; the battery bridge holds the DC BAT BUS (3PP) for seven seconds and then gives it up to save charge (see Transformer-Rectifiers and Batteries and the BCL). But the DC ESS BUS survives — it is fed by the independent ESS TR, which draws from the AC side and sits outside the main-TR fault chain (this is the "both mains lost but DC ESS alive" case flagged in the overview). That one surviving line is exactly why SEC 2 and L FUEL PUMP 2 remain alive. The whole DC BUS 1+2 procedure is, in essence, the art of bringing the aircraft home on the single live line of the DC ESS BUS.

6. Flight-deck scenarios

  1. DC BUS 2 FAULT, then the SEC 2 FAULT light comes on. Keep your hand off it — SEC 2 is drawing its backup supply from the DC ESS BUS and is working; resetting it (OFF) kills it. This validates the article-24 pattern: the light's supply chain is not the function's supply chain.
  2. DC BUS 1+2. Fuel rhythm first: no leak → cross-feed ON → monitor imbalance, trimming it with the L/R PUMP 2 seesaw. Plan the gravity extension early — the 200 kt limit and the fact that it is an irreversible decision point both want forethought. Expect the landing to be flown with no reverser and the engines at HI IDLE, so run the LDG DIST PROC for the rollout distance.
  3. Standalone DC BAT BUS FAULT. The engine bill appears on its own — HI IDLE, auto-start only, autothrust lost, reversers lost — with neither main DC bus failed. Do not search the DC BUS 1 or DC BUS 2 checklists for a cause: 3PP has its own registration.

Self-test

[!note]- Q1. Why do the four DC cautions differ so much in procedural weight?

Because each DC bus houses a different equipment family. DC BUS 1 is bare crew awareness — by the time it is read, the TR cross-substitution has already restored it, leaving only a paper account. DC BUS 2 is the dormitory of the flight-control and landing-gear computers (PRIM 2+3, a SEC's normal feed, LGCIU 2, SFCC 2, FCDC 2, FCMC 2, half the spoilers), so the procedure is long. DC BAT BUS (3PP) carries the EIVMUs, producing the engine bill. DC BUS 1+2 is the union of all three, plus the gravity extension, single-side fuel feed, and the flight-critical items that only emerge in superposition.

[!note]- Q2. What is the SEC 2 KEEP ON note, and where is its red line?

SEC 2 is normally fed from DC BUS 2; on a DC BUS 2 failure it transfers automatically to the DC ESS BUS backup supply. Because the supply chain changed, the overhead SEC 2 FAULT light comes on — but the function is intact. The red line: do not reset. Selecting OFF removes the backup supply and genuinely loses SEC 2. It is a "the light cannot be trusted" case — the light's supply chain is not the function's.

[!note]- Q3. What is the engine bill that follows a loss of 3PP?

The DC BAT BUS (3PP) feeds both EIVMUs; lose it and you get ENG HI IDLE (cause line: "Due to the loss of the EIVMUs"), ENG 1+2 AUTO START ONLY, and the loss of A/THR, REVERSERS, MAN CAB PR and APU LOOP B. The detailed mechanism is in ATA 70. This bill follows 3PP wherever it goes — appearing on a standalone DC BAT BUS FAULT, and again seven seconds into a DC BUS 1+2 failure when the battery bridge drops 3PP.

[!note]- Q4. What is the fuel logic on a DC BUS 1+2 failure?

Only L FUEL PUMP 2 still has power (off the DC ESS chain), so after clearing any leak the crew sets cross-feed ON to feed both engines from one side, and monitors imbalance. To rebalance, L PUMP 2 can be set to OFF — R PUMP 2 then starts automatically and its FAULT light goes out (the shared-backup seesaw). The right wing is annunciated R FUEL GRVTY FEED ONLY.

[!note]- Q5. Why does DC BUS 1+2 require a gravity gear extension?

The landing-gear electrical control chain (LGCIU and its paired resources) is dead with both main DC buses lost, so the APPR PROC calls L/G GRVTY EXTN, MAX 200 KT. The matching landing account: no reverser, engines at HI IDLE, SLATS FLAPS SLOW and CAT 2 — so the LDG DIST PROC is mandatory.

Key takeaways

# Point
1 DC BUS 1 = crew awareness only (silent TR reconfiguration done); its one diagnostic loss is C/B MONITOR (CBMU on 1PP).
2 DC BUS 2 = the flight-control/landing-gear computer tenement (PRIM 2+3, half spoilers, LGCIU 2, SFCC 2, FCDC 2, FCMC 2).
3 SEC 2 KEEP ON: a FAULT-lit SEC 2 is alive on DC ESS backup — resetting it (OFF) kills it. The light's supply chain ≠ the function's.
4 The engine bill (ENG HI IDLE / auto-start only / A.THR / reversers) belongs to 3PP, not to DC BUS 1+2 — it is collateral when 3PP drops after 7 s.
5 DC BUS 1+2 = both main TRs lost; survives on the single live DC ESS line — gravity gear (MAX 200 kt), single-side fuel seesaw, CAT 2, plus new losses (F/CTL PROT, N/W STRG, REAC W/S DET, AUTOLAND/GLS).

References

Per FCOM PRO-ABN-ELEC, p.3370–3384, rev 14 JAN 26 — the four DC bus-fault procedures in full: DC BUS 1 crew awareness (p.3371–3372); DC BUS 2 four-step drill with the SEC 2 backup-supply note and the full STATUS/INOP set (p.3373–3376); DC BAT BUS engine bill with the "Due to the loss of the EIVMUs" cause line (p.3370); DC BUS 1+2 fuel seesaw, gravity extension and STATUS/APPR PROC (p.3377–3384). The carrier-mapping (the engine bill follows 3PP), the both-main-TR-loss root cause, the seven-second battery-bridge drop, and the "single live DC ESS line" reading are integrative syntheses across Transformer-Rectifiers, DC Network Transfer, Batteries and the BCL and Circuit Breakers and the CBMU, and contain no facts from outside the library. The EIVMU mechanism is treated in ATA 70; the leak-before-cross-feed gate in ATA 28; the gravity-extension procedure detail in ATA 32; the SEC/PRIM/spoiler architecture in ATA 27. For the downstream degradations see DC ESS Bus Fault and Shed, Emergency Electrical Configuration and Battery-Only Flight.

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

ESC