GEN and IDG Failures

This article closes out the generation-side cautions of ATA 24. The mechanism — how an IDG turns a varying engine speed into a steady 400 Hz, how the GCU protects the channel, and how a disconnect releases a sick unit — was built in Integrated Drive Generator and GCU and AC Generation Control. Here the focus shifts from what the system is to what you do when a generation channel annunciates: the exact procedure steps, the STATUS bill that follows, and the dispatch picture.

Five cautions live on this page — ELEC GEN 1(2) FAULT, ELEC GEN 1(2) OFF, ELEC GEN 1(2)/APU GEN/EXT PWR OVERLOAD, ELEC IDG 1(2) DISCONNECTED, and ELEC IDG 1(2) OIL LO PR/OVHT — plus the ground-only ELEC IDG 1(2) OIL SYS FAULT. They share one structural fact: losing a single main generation channel is never an emergency configuration (the opposite GEN and the APU GEN cover it — see Automatic Reconfiguration), so every procedure here is a calm, bounded sequence ending in a STATUS read, not a scramble.

By the end you should be able to answer five things: (1) what the two GEN FAULT triggers are and why the procedure offers only one "OFF THEN ON" retry; (2) what an OVERLOAD caution tells you about where the automatic chain has already got to; (3) the precise way to press the IDG pushbutton for a disconnect; (4) the oil-temperature behaviour to expect after a disconnect; and (5) the common STATUS bill these failures hand you (CONSIDER APU GEN USE, PART GALLEY, the CAT 3 downgrade) and how to read it.

All procedure references are to FCOM PRO-ABN-ELEC (the ELEC abnormal/emergency unit); ECAM-field references are to FCOM DSC-24-20.

1. ELEC GEN 1(2) FAULT — the protection-trip caution

This is the caution you see when the GCU has tripped its generator off the bus for cause. The triggering conditions are two:

"This alert triggers when: ‐ The protection trip is initiated by the associated GCU, or ‐ The line contactor is open with GEN pb-sw set to ON."

Per FCOM PRO-ABN-ELEC. The two triggers map directly onto the two real-world events covered in the GCU article: a protection trip (one of the GCU's protective functions de-excites the generator and opens its GLC), or the line contactor open while you are still commanding the generator on (a GLC failure such as GLCF, where the contactor will not hold). The procedure body is short:

GEN (AFFECTED) ............ OFF THEN ON     ◄ one reset attempt for the GCU
   IF UNSUCCESSFUL:
GEN (AFFECTED) ............ OFF             ◄ accept the loss, stop trying

Why only one attempt. The single "OFF THEN ON" is deliberate, not a suggestion to keep cycling. The GCU's reset logic honours only a limited number of reset cycles for its latching protections (the reset-twice family worked through in the GCU article); the procedure spends one of those cycles and, if the channel does not come back, has you select OFF and accept the loss. The procedure is built so it never invites you toward a third attempt — a faulted generator is dropped, not coaxed.

STATUS. Per FCOM PRO-ABN-ELEC the GEN FAULT STATUS reads:

• CONSIDER APU GEN USE — the headline action: start the APU and bring its generator on line to rebuild the lost N-1 redundancy (the footnote notes the APU may be started when available).
• CAT 3 SINGLE ONLY — only if the APU GEN is not in line. Running on a single source downgrades the autoland capability from CAT 3 DUAL to CAT 3 SINGLE.
• INOP SYS: GEN 1(2); PART GALLEY (only if the APU GEN is not in line; not applicable to the A330-200F); CAT 3 DUAL (only if the APU GEN is not in line).

The thread through the STATUS is the same one driving the action: bring the APU GEN on line and the galley, the autoland category, and the redundancy all come back. Leave it off and you run single-source with the galley partly shed and CAT 3 down to SINGLE.

2. ELEC GEN 1(2) OFF — the inadvertent-selection caution

This caution is not a fault at all. It triggers on a switch position:

"This alert triggers when the GEN 1(2) pb-sw is abnormally set to OFF."

Per FCOM PRO-ABN-ELEC. In other words the generator is healthy but its pushbutton has been left or knocked to OFF (a mis-selection, or a checklist step not restored). The procedure is a single line:

"Switch affected GEN on."

Per FCOM PRO-ABN-ELEC. Because no real fault has occurred, the STATUS is lighter than the FAULT case: per FCOM PRO-ABN-ELEC it carries INOP SYS: GEN 1(2), PART GALLEY (only if the APU GEN is not in line; not applicable to the A330-200F), CAT 3 DUAL (only if the APU GEN is not in line) — but no CONSIDER APU GEN USE action and no CAT 3 SINGLE ONLY line. The judgement embedded here is that an inadvertent OFF is recoverable by simply putting the switch back, so the procedure does not push you to spin up the APU; the redundancy bill (PART GALLEY, CAT 3 DUAL lost) only stands while the generator is genuinely off the bus and the APU GEN is not already carrying the gap.

3. ELEC GEN/APU GEN/EXT PWR OVERLOAD — the galley as the relief valve

"This alert triggers when the load of one generator is above 100% of rated output."

Per FCOM PRO-ABN-ELEC. The procedure is one action — GALLEY OFF — and the caution carries a note that is the whole point of the article-15 load-shedding story:

"This warning is only displayed if galley automatic shedding has failed."

Per FCOM PRO-ABN-ELEC.

[!warning]- Counter-intuitive: an OVERLOAD caution means the automatic shed has already failed, not that it is about to start

The instinct is to read OVERLOAD as "the automatic system is about to shed the galleys." The opposite is true. The system always tries to shed the galleys automatically first; the caution is only raised after that automatic shed has failed to bring the load back under 100 %. So by the time you read OVERLOAD, the relief valve has already failed, and your GALLEY OFF is the manual back-up — pulling the two galley main contactors (2XA1/2XA2) by hand (see Galley and Commercial Loads). This is the manual fourth step of the same shed chain you met in the GCU and galley articles.

Once the galley load is removed, the surviving generator falls back below 100 % of its rated output and the caution clears. To restore the galleys you must first cure the source of the overload (typically bring the APU GEN on line to add capacity), then re-cycle the GALLEY pushbutton under the conditions in Galley and Commercial Loads. The STATUS is simply INOP SYS: GALLEY.

One inhibition is worth holding: per FCOM PRO-ABN-ELEC, "Alert is inhibited in flight phase 6 for EXT PWR OVERLOAD only." An external-power overload caution is meaningless once airborne (there is no external power in the cruise), so the EXT PWR branch of the caution is suppressed in flight phase 6.

4. ELEC IDG 1(2) OIL LO PR/OVHT — the disconnect procedure

This is the principal procedure of the article — the one that ends in pulling the disconnect lever. The two triggers:

"The alert ELEC IDG 1(2) OIL LO PR triggers when the IDG oil pressure is low. The alert is inhibited if N2 < 14%. The alert ELEC IDG 1(2) OIL OVHT triggers when the IDG outlet oil temperature is above 185 °C."

Per FCOM PRO-ABN-ELEC. These are the procedural face of the two thresholds laid down in the IDG article: LO PR watches the charge-oil pressure (inhibited below N2 14 %, because a near-stationary rotor cannot make oil pressure), and OVHT watches the outlet oil temperature against the 185 °C overheat line (the 151 °C value is an advisory handled earlier, by load reduction — see article 01). The same triggers are echoed by the IDG FAULT legend logic on the panel:

"FAULT lt: Comes on amber associated with an ECAM caution in case of: • IDG oil outlet overheat (above 185 °C), or • IDG oil pressure low. Inhibited when the engine is stopped or below idle. It goes off when the IDG is disconnected."

Per FCOM DSC-24-20. The procedure leads to the disconnect step:

"If the associated engine is running, the IDG must be disconnected from the engine at or above idle to prevent the damage to the disconnect mechanism. Note: The IDG disconnection is inhibited, when the engine is stopped or below idle. Press the IDG pb-sw until the GEN FAULT light comes on. However, do not press for more than 3 s, to avoid damage to the disconnect solenoid. The IDG FAULT light goes off, when the IDG is disconnected."

Per FCOM PRO-ABN-ELEC. This passage makes the article-01 "press briefly" precise. The disconnect is not a tap-and-release: you hold the pushbutton until the GEN FAULT light illuminates — that is the success marker, the moment the disconnect takes effect and the now-undriven generator trips off the bus — but you must not hold beyond 3 s, to protect the disconnect solenoid. Once disconnection has taken, the IDG FAULT light extinguishes — the confirmation marker. Two markers, one short press:

┌──────────────────────────────────────────────────────────┐
│  IDG disconnect — the press technique                     │
└──────────────────────────────────────────────────────────┘
   Engine at or above idle? ──No──► button inert
        │ Yes                       (GCU underspeed gate;
        ▼                            disconnect inhibited)
   Lift guard, press IDG pb-sw
        │
        ▼
   HOLD until GEN FAULT light ON ───◄ success marker
        │   (do NOT exceed 3 s — solenoid protection)
        ▼
   IDG FAULT light goes OFF ────────◄ confirmation marker
        │
        ▼
   ECAM ELEC AC page: IDG label & number amber, DISC shown

The disconnect is irreversible in flight — only maintenance can reconnect the unit on the ground with the engine shut down (article 01). There is no "press again to put it back."

The oil temperature after a disconnect. A new number lives in this procedure:

"When the IDG is disconnected before takeoff, the oil temperature of the affected IDG may reach a higher value than during normal operations, but without exceeding 150 °C."

Per FCOM PRO-ABN-ELEC. The mechanism is the one set up in article 01: after disconnection the input shaft is still dragged round by the gearbox while the normal oil circulation has stopped, so the oil warms above its usual running value. This is expected behaviour, not a new fault — the FCOM commits that it will not exceed 150 °C, which sits below the 151 °C advisory line, so a disconnected IDG warming up before take-off raises no caution.

STATUS. The OIL LO PR/OVHT STATUS carries the same single-source consequences as the GEN FAULT case (CONSIDER APU GEN USE, the CAT 3 downgrade and PART GALLEY when the APU GEN is not in line) — because the end state is identical: that channel's generation is gone and the network is running single-source.

5. ELEC IDG 1(2) DISCONNECTED and the ground-only OIL SYS FAULT

5.1 ELEC IDG 1(2) DISCONNECTED (ON GROUND)

"This alert triggers when the IDG is disconnected."

Per FCOM PRO-ABN-ELEC (annunciated on the ground — the caution carries the ON GROUND tag). The procedure is crew awareness, and the STATUS, per FCOM PRO-ABN-ELEC, is CONSIDER APU GEN USE; INOP SYS: GEN 1(2), PART GALLEY (only if the APU GEN is not in line, not applicable to the A330-200F) — the same single-source bill as a faulted generator, since a disconnected IDG is a lost generation channel. A configuration-specific note attaches:

"For A330 aircraft equipped with PW engines, when an IDG is disconnected, the oil temperature of the affected IDG may increase."

Per FCOM PRO-ABN-ELEC. This note applies only to PW-engined airframes; on Trent 700-powered aircraft it does not apply, and the note should not be read as a general rule for every A330.

5.2 ELEC IDG 1(2) OIL SYS FAULT (ON GROUND)

"This alert triggers when the oil system is failed."

Per FCOM PRO-ABN-ELEC (a ground-only caution; the exact applicability is MSN-dependent). The procedure is again crew awareness, and it comes with the note that explains its single most common false-trigger:

"During pushback, the ECAM may unduly trigger the ELEC IDG 1(2) OIL SYS FAULT due to IDG oil level variation caused by aircraft movement."

Per FCOM PRO-ABN-ELEC.

[!warning]- An OIL SYS FAULT during pushback is most likely the oil sloshing, not a real failure

The IDG oil-system fault detection reads oil level; when the aircraft is being pushed back, the oil sloshes in the unit and the level reading swings, which can unduly (spuriously) raise the caution. The first response to an OIL SYS FAULT during pushback is therefore to let the aircraft settle and see whether it self-clears, not to treat it as a confirmed failure. If it persists, it is routed through the operator MEL's true-vs-spurious split for this caution (a confirmed false warning is dispatched against a daily sight-glass oil check; a true failure is handled as a main-generation failure) — the dispatch detail is in the IDG article §9.

6. The dispatch picture — a single channel is Category C, not no-go

The in-flight STATUS tells you the immediate bill of losing a generator (CAT 3 down to SINGLE, the galley partly shed). The MEL answers the other half: can you depart on the ground carrying this failure? For a single failed main generation channel, the answer is yes, with conditions.

Per the operator MEL, one AC main generation system (the IDG, its GCU and its line contactor taken as one item) inoperative may be dispatched — Category C, 2 installed / 1 required (full conditions in the IDG article §9 and MEL Dispatch View). The defining condition is that the APU and its generation system be operating and used throughout the flight: dispatching with one main generator gone leaves the aircraft on a single main generator, so the MEL pre-restores the redundancy by keeping the APU GEN on line for the whole flight (lose one more and you would drop straight toward the emergency configuration of Emergency Electrical Configuration).

The routing depends on which caution you saw: a GEN OFF is pilot-selected, not a failed part, so it is not itself a dispatch item; a true IDG disconnect or an IDG OIL OVHT routes to the main-generation dispatch item, while a confirmed spurious IDG DISCONNECTED or OIL SYS FAULT has its own lighter branch (article 01 §9). The teaching point is the contrast: a lost main generation channel is "Category C go" — you can depart carrying it — which sits between the no-go floor of the Static Inverter and the Ground Service / Maintenance Bus, which is outside the MEL coordinate system altogether (a non-flight function). You may go, but you go degraded: single-source, CAT 3 down to SINGLE, galley partly shed, and an APU fuel-burn penalty to plan for.

7. Flight-deck scenarios

1. Cruise, ELEC GEN 2 FAULT. Apply the reconfiguration habit first: read the ELEC page (the network has already reconfigured — the BUS TIE closed, and the APU GEN or GEN 1 is carrying the right half). Then run the ECAM procedure — OFF THEN ON once; if unsuccessful, OFF. Finally read the STATUS: CONSIDER APU GEN USE — starting the APU restores the redundancy, brings the galley fully back, and returns CAT 3 DUAL.
2. ELEC IDG 1 OIL OVHT. The procedure reaches the IDG OFF step. Lift the guard and hold the IDG 1 pushbutton until GEN 1 FAULT illuminates, then release (≤ 3 s); confirm IDG FAULT out and an amber DISC on the ECAM ELEC page. Thereafter handle the GEN 1 FAULT per §1 — but note a reset is now pointless, since the generator has no drive at all.
3. OVERLOAD sounds. The automatic galley shed has already failed — GALLEY OFF pulls the main contactors by hand; the load drops back under 100 % and the caution clears. To get the galleys back, cure the source first (start the APU for more capacity), then re-cycle the GALLEY pushbutton under its restore conditions (article 15).
4. OIL SYS FAULT during pushback. Most probably the oil sloshing as the aircraft moves — let it settle and watch for a self-clear. If it persists, route it through the MEL spurious-vs-true split (false warning dispatched against the daily check; a true failure handled as a main-generation failure).

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

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

1. "If one GEN FAULT retry fails, cycle the switch a few more times — it might come back." — False. The procedure offers a single OFF THEN ON, then OFF to accept the loss; the GCU's reset logic only honours a limited number of cycles, and the procedure is built not to push you toward a third attempt (article 02).
2. "An OVERLOAD caution means the automatic galley shed is about to act." — False. The opposite: per FCOM, the warning "is only displayed if galley automatic shedding has failed." The caution means the automatic relief has already failed, and your GALLEY OFF is the manual back-up that pulls the 2XA1/2XA2 contactors (article 15).
3. "To disconnect the IDG, just tap the button and let go — don't hold it." — Half true. You must hold until the GEN FAULT light comes on (the success marker), then release — only do not exceed 3 s (solenoid protection). GEN FAULT on = success; IDG FAULT off = confirmation (FCOM PRO-ABN-ELEC).
4. "The engine is stopped on the ground — a good moment to disconnect the IDG precautionarily." — False. Per FCOM, disconnection is inhibited when the engine is stopped or below idle; it must be done at or above idle, or the disconnect mechanism may be damaged (the GCU underspeed gate of article 01).
5. "The IDG was disconnected before take-off and the oil temperature is climbing — that's a new fault." — False. With the input shaft still turning but no normal oil circulation, a temperature above normal is expected, and the FCOM commits it will not exceed 150 °C — below the 151 °C advisory line, so it raises no caution.
6. "An IDG OIL SYS FAULT during pushback — treat it as a real failure at once." — Half true. It is most likely the moving aircraft sloshing the oil and unduly triggering the caution (FCOM); let it settle and watch for a self-clear, and only route a persistent one through the MEL spurious-vs-true split (article 01 §9).

Self-test

[!note]- Q1. What are the two GEN FAULT triggers, and why does the procedure offer only one retry?

Triggers (per FCOM PRO-ABN-ELEC): a protection trip initiated by the associated GCU, or the line contactor open while the GEN pushbutton is set to ON (a GLC failure). The procedure gives a single OFF THEN ON, and IF UNSUCCESSFUL: OFF — accepting the loss. The single attempt matches the GCU's limited-reset protection logic (article 02): it spends one reset cycle and is deliberately built not to invite a second or third attempt at a genuinely faulted channel.

[!note]- Q2. Where in the load chain does an OVERLOAD caution sit?

It triggers when one generator's load is above 100 % of rated output, and — crucially — it is "only displayed if galley automatic shedding has failed." So the caution does not announce the start of shedding; it announces that the automatic shed has already failed. Your GALLEY OFF is the manual back-up, pulling the 2XA1/2XA2 galley contactors by hand (article 15). STATUS: INOP SYS GALLEY. (EXT PWR OVERLOAD is inhibited in flight phase 6.)

[!note]- Q3. State the precise IDG disconnect technique.

Engine at or above idle (disconnection is inhibited stopped / below idle). Lift the guard and hold the IDG pushbutton until the GEN FAULT light illuminates (the success marker), but do not hold beyond 3 s (disconnect-solenoid protection). The IDG FAULT light extinguishing confirms the disconnect; the ECAM ELEC page shows the IDG label and number amber with DISC. Irreversible in flight.

[!note]- Q4. What oil-temperature behaviour is expected after a disconnect?

Higher than normal but not exceeding 150 °C (the FCOM commitment for an IDG disconnected before take-off). With the input shaft still dragged round by the gearbox but the normal oil circulation stopped, the warming is expected behaviour — it sits below the 151 °C advisory line and raises no new caution.

[!note]- Q5. What is the common STATUS bill of these generation-side failures, and how do you read it?

For GEN FAULT / IDG OIL LO PR-OVHT / IDG DISCONNECTED: CONSIDER APU GEN USE (rebuild the lost N-1), INOP SYS GEN 1(2), and — only while the APU GEN is not in line — PART GALLEY and the loss of CAT 3 DUAL (retaining CAT 3 SINGLE). The single action that clears most of the bill is to bring the APU GEN on line: it restores the redundancy, the full galley and CAT 3 DUAL. Running single-source also carries a fuel penalty to plan for. (GEN OFF is the lighter case: no CONSIDER APU GEN USE action.)

Key takeaways

References

Per FCOM PRO-ABN-ELEC (the ELEC abnormal/emergency unit: GEN FAULT triggers and OFF-THEN-ON procedure with STATUS; GEN OFF trigger and "Switch affected GEN on" with its lighter STATUS; OVERLOAD trigger above 100 % rated output, GALLEY OFF, the "only displayed if galley automatic shedding has failed" note, the EXT PWR phase-6 inhibition and INOP SYS GALLEY; IDG OIL LO PR/OVHT triggers — LO PR low oil pressure inhibited N2 < 14 %, OVHT above 185 °C — the disconnect procedure with "at or above idle / inhibited when stopped or below idle / press until GEN FAULT comes on / do not press for more than 3 s / IDG FAULT goes off when disconnected", and the "without exceeding 150 °C" before-take-off note; IDG DISCONNECTED trigger, crew awareness, CONSIDER APU GEN USE / PART GALLEY STATUS and the PW-engine oil-temperature note; OIL SYS FAULT trigger "when the oil system is failed" and the pushback "unduly trigger / oil level variation caused by aircraft movement" note); FCOM DSC-24-20 (IDG FAULT legend logic: amber for outlet overheat above 185 °C or oil low pressure, inhibited when stopped/below idle, extinguishing on disconnect; LO PR inhibited N2 < 14 %); and the operator MEL (single AC main generation system inoperative — Category C, 2 installed / 1 required, APU GEN on line throughout, and the spurious-vs-true dispatch branches). The reset-twice / single-source / "fuel penalty" readings and the dispatch-contrast framing are integrative syntheses of the above and of articles 01, 02, 15, 21 and 32, and contain no facts from outside the library.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.