Integrated Drive Generator (IDG)

The two IDGs are the main generators of the whole electrical network — one per engine, 115 kVA each. The overview established that "AC is the source, DC is what the TRs make from it." The IDGs are the source of that source. This article dives into the mechanical and oil side of the unit: how an engine that never stops changing speed produces a rock-steady 400 Hz, why the oil is simultaneously a coolant and a "gearbox fluid", and why the disconnect lever is a one-way ticket.

The component splits cleanly into two halves — a constant-speed drive (the gearbox) and a generator (the electrical machine) — and so does its study. This article owns the drive, the oil system, the disconnect mechanism, the IDG's own monitoring (oil temperature/pressure/level, filter clogging), the ECAM IDG fields, and the dispatch case. The generator's electrical end — excitation, the PMG as a power source, voltage regulation at the Point of Regulation, the GCU's protective trips, and the GLC — belongs to GCU and AC Generation Control, as do the generator parameters a pilot reads on the ECAM (GEN load > 108 %, voltage 110–120, frequency 390–410).

1. Scope and boundaries

FCOM sets the overview-level tone for the IDG in one sentence, the same one the overview carries:

"Two three-phase AC generators (GEN1, GEN2), driven by the engine through an integrated drive, supply aircraft electrical power. Each generator can supply up to 115 KVA of three phase 115/200 V 400 Hz power."

Per FCOM DSC-24-10-20-10. By the end of this article you should be able to answer five questions:

1. The engine speed swings continuously from idle to take-off thrust — so why is the generator frequency pinned to 400 Hz? (the constant-speed-drive summing mechanism)
2. What two conditions light the IDG FAULT legend, which one is inhibited and under what condition, and what is the colour logic of the IDG label versus its number on the ECAM?
3. Before and after the IDG pushbutton is pressed, what can — and cannot — be done by the crew and by maintenance, and what is the 50-hour red line?
4. What jobs does the IDG oil do on one trip round the unit, and which pumps (including the inversion pump) does it pass through? Why does the air-cooled oil cooler's cold-oil bypass valve exist?
5. To dispatch with one IDG failed under the MEL, what are the core conditions, why must the APU run for the whole flight, and what does the APU burn at altitude?

A boundary worth fixing now: the accessory gearbox that drives the IDG is taken off the HP rotor and is an ATA 70 topic. But note one clarification carried through this article — the IDG's air-cooled oil cooler (ACOC) is cooled by LP-compressor air, yet the only verbatim source for its cooling mechanism is AMM 24-21-00 §6.F itself (see §5). The ATA 70 library mentions the IDG ACOC only in a maintenance sense ("do not damage the ACOC when fitting the engine") and carries no description of the IDG oil-cooling mechanism, so do not dangle a pointer to ATA 70 for it.

2. Architecture — a gearbox and a generator in one case

2.1 The two halves

"The IDG can be split into two parts: the drive: Constant Speed Drive (CSD), and the generator. A hydromechanical constant speed drive associated with a servo-valve drives the AC generator at constant speed."

Per AMM 24-21-00. The drive is where the engineering interest lives; the generator is conventional. The power source is stated in the same description:

"Each engine (HP rotor) drives its associated IDG through the accessory gearbox. The drive speed varies according to the engine rating. The IDG provides a 115/200 VAC, 3-phase, 400 Hz AC supply at the Point of Regulation (POR)."

Per AMM 24-21-00. The electrical-power overview chapter strings drive, disconnect and reset together in one higher-level sentence (note the wording: here it says HP compressor, while 24-21 says HP rotor — the same fact in two phrasings, the gearbox take-off being on the HP shaft):

"The engine HP compressor drives each main generator via the accessory gearbox and via an integrated hydromechanical speed regulator which transforms the engine variable speed into constant speed for the generator. If there is a mechanical failure, the IDG pushbutton switch protected by a guard … serves to disconnect the IDG. Reset of the disconnection mechanism is possible on the ground only with the engine shut down."

Per AMM 24-00-00.

Installation. The IDG (FIN 4000XU) mounts on the engine accessory-gearbox pad (zone 413), held by a QAD (Quick Attach Detach) adapter and a tension bolt (per AMM 24-21-00). Replacing an IDG does not mean part-stripping the engine — slacken one tension bolt and the whole unit lifts off as a single piece (the QAD section drawing shows the tension-bolt ring around the adapter).

2.2 The single-channel power chain

Engine HP rotor ─ accessory gearbox (speed varies with thrust) ─► IDG input shaft  4500–9120 rpm
   │                                                ┌─ GCU 1(2)  (avionics bay, zone 120)
   ▼                                                │   · watches PMG frequency vs 400 Hz reference
┌──────────────── IDG  4000XU ─────────────────┐    │   · error → biases the servo valve
│ CSD: differential (1:2) speed summing        │    │   · regulates V @POR + speed + protection (art. 02)
│  ◄ hydraulic trim (variable pump ⇄ fixed motor,│◄──┘   servo-valve command
│     servo-valve controlled)                   │
│  ▼ constant 24 000 rpm                        │
│ AC generator (PMG → exciter → rotating        │
│   rectifier → main generator)                 │
└───────────┬───────────────────────────────────┘
            ▼ 115/200 VAC 3-phase 400 Hz · 115 kVA  @POR
            │ power feeders  4001/4002/4003XU
            ▼ GLC 1(2) ─► AC BUS 1(2)  (art. 02)

2.3 Leading particulars

Per AMM 24-21-00 §6.G. The generator's own leading particulars — voltage-regulation range, the PMG, and so on — are referred by §6.G to AMM 24-22-00, i.e. to article 02.

3. The constant-speed drive — a differential run backwards

This is the heart of the article. The AMM speed-summing passage:

"Speed summing is accomplished in the differential by adding or subtracting the trim speed of the trim ring gear to the meshing planet gear which is orbited as a function of input speed by the carrier shaft. The second or output planet is in mesh with the first planet and the output ring gear. The output gear is thereby made to rotate at a constant speed. Since the output ring gear is meshed with the generator drive gear, constant shaft speed (24,000 rpm) is converted directly into 400 Hz constant frequency AC power."

Per AMM 24-21-00 §6.C.

In plain terms: a car differential is "one shaft in, two shafts out" — it lets the two driven wheels turn at different speeds. The CSD runs it the other way round — "two shafts in, one shaft out". One input is the engine's varying speed; the other is the hydraulic unit's "trim speed". The two are added or subtracted inside the differential to make a constant output. When the engine speeds up, the hydraulic unit trims back a little; when it slows down, the unit trims forward. The phrase to hold on to is the two planets in series: the first (trim) planet receives the trim correction, and the second (output) planet meshes with both the first planet and the output ring gear — that is how the correction reaches the output ring. The 1:2 marked on the differential block is the gear ratio of those two planet stages.

Where does the trim speed come from? A pair of hydraulic units (§6.C). A variable-displacement unit (pump) is driven directly by the carrier shaft (so its speed follows the engine), hydraulically coupled to a fixed-displacement unit (motor) that is mechanically connected to the trim ring gear. Inside the variable unit is an adjustable swash-plate (wobbler): the plate angle sets the displacement, and the displacement sets the fixed unit's speed and direction. What controls the plate is a control piston driven by the servo valve — this is the "infinitely variable hydromechanical gearbox": the plate angle is continuously adjustable, the trim speed is continuously adjustable, and the output is held precisely at 24 000 rpm.

3.1 The frequency loop — the GCU uses the PMG as a tachometer

How far the swash-plate is driven is decided by an electronic closed loop. First, the GCU's three named functions for the IDG:

"The main functions for the regulation and protection of the IDG are: ‐ regulation of the generator voltage at Point Of Regulation (POR), ‐ regulation of the generator speed, ‐ monitoring and protection of the system."

Per AMM 24-21-00 §3.B. "Speed regulation" is exactly the loop below:

"The electronic control circuit monitors the generator PMG frequency and compares this frequency to a reference source. Based on the error signal obtained, the control circuit then biases the servo valve to increase or decrease the control piston pressure as necessary to return the generator frequency to 400 Hz."

Per AMM 24-21-00 §6.D. The PMG (Permanent Magnet Generator) is a small permanent-magnet machine on the generator shaft — its field is constant, so its output frequency is a faithful scale of shaft speed. The GCU reads PMG frequency as a tachometer, compares it with the 400 Hz reference, and feeds the error to the servo valve as a correction command. The loop closes: PMG senses speed → GCU compares → servo valve → control piston → swash-plate angle → trim speed → differential summing → constant output → PMG senses again. (The PMG has a second job — supplying the GCU's own excitation power — covered in article 02.)

What this means for you: frequency control is a collaboration of the IDG's internal hydraulics + the GCU electronic loop + the speed-regulation function. A frequency-drift fault can sit either on the IDG mechanical side (servo-valve / hydraulic-unit wear) or on the GCU side — which is why the MEL packages "IDG, GCU and line contactor" into a single dispatch item (24-22-01, "AC main generation system"): a fault in any of the three counts as that one item.

4. The GCU–IDG interface

The GCU's sensing of, and outputs to, the IDG — i.e. the data source behind every IDG field on the ECAM — in one table (per AMM 24-21-00 §5.A):

Three electrical connectors split the work (per AMM 24-21-00 §6.E): A = three-phase output ØA/B/C + three CTs + servo-valve position + disconnect solenoid and indication + charge-pressure switch; B = main exciter stator/rotor + rotating rectifier + main-generator field + oil inlet/outlet temperature bulbs + clogged-filter indication + PMG three-phase + input-speed sensor; C = oil-level sensor. Knowing this layering, the GCU protection in article 02 reads cleanly — "control signals on A, sensing signals on B".

5. The oil system — coolant and transmission fluid in one

The IDG carries 5.5 L in its own self-contained circuit, and that oil does two jobs at once (§6.F). The circuit, read off the oil-cooling-system figure (note the inversion pump on the scavenge side, easy to miss):

OIL IN PORT → OIL IN TEMP BULB → DEAERATOR → CHARGE PUMP
                                                │
   charge oil distributed to (job 1 = drive + cooling points):
   ├─ CHARGE PRESSURE SWITCH  ← this charge-oil pressure is what LO PR watches
   ├─ SERVO / HYDRAULIC UNITS / CFV  ← drive "motive flow"
   ├─ PMG STATOR (spray) / DIFFERENTIAL / INPUT SEAL
   └─ ROTOR(S) · STATOR(S) · DIODES  (exciter + main-gen cooling)
                                                │
   used oil → SUMP ─┬─ SCAVENGE PUMP ──┐
                    └─ INVERSION PUMP ──┤
                                        ▼
   OIL OUT TEMP BULB → SCAVENGE FILTER → ΔP (oil-filter ΔP indicator)
        → COOLERS BY-PASS VALVE (cold-oil bypass) → OIL OUT PORT
                                        ▼
   IDG COOLER MATRIX (ACOC): free-stream air (cowl) + fan-delivery air

Job 1: transmission fluid. Part of the charge-pump output is "motive flow" feeding the hydraulic units and the control system — the hydraulic horsepower passed between variable pump and fixed motor, and the pressure that drives the control piston off the servo valve, are all this oil. The oil is not merely a lubricant; it is the gearbox's working medium. AMM §7.A pins down what the low-pressure switch watches:

"An oil temperature sensor monitors the oil inlet temperature and the other one the outlet temperature. The second enables oil temperature ADVISORY and OIL OVERHEAT detections. A pressure switch operates in the event of a low charge oil pressure."

Per AMM 24-21-00 §7.A. So LO PR watches the charge-oil pressure — exactly the pressure source of the "transmission fluid" job — and the outlet-temperature sensor is the common source of both the advisory (151 °C) and overheat (185 °C) detections.

Job 2: cooling. Oil enters from the side opposite the drive end and is distributed to the differential gears, the input seal, the charge pump and the generator stator and rotor (per AMM §6.F: differential oil also sprays-cools the PMG stator and lubricates the disconnect spline; the input housing has rectangular oil grooves around the main-generator stator core to drop the surface temperature). The return path runs through the scavenge pump and the inversion pump (the return oil collected in the sump is drawn out by these two) → deaerator / outlet-temperature bulb → scavenge filter → out to the external cooling pipework.

The cooler (ACOC). The Air-Cooled Oil Cooler (FIN 5001XT) is a heat-exchange matrix on the left side of the LP-compressor casing at about the 4 o'clock position (26 bolts); LP-compressor cooling air passes through the matrix fins to carry off oil heat, and the spent air leaves through an opening in the left fan cowl door via an S-shaped duct (per AMM §6.F). Note again: the verbatim source for the ACOC cooling mechanism is this section only (24-21 §6.F), not ATA 70 — do not treat it as cross-chapter content hung off elsewhere. Between the matrix inlet and outlet ports is a pressure-relief (cold-oil bypass) valve:

"If the oil is cold and will not flow easily through the matrix the valve will open. This will let the oil flow directly from the inlet connection to the outlet connection and back to the source. When the temperature of the oil increases, the valve will close and the oil will flow through the matrix."

Per AMM 24-21-00 §6.F. On a cold winter start the oil is thick as honey; forcing it through a fine matrix would build excessive pressure, so the bypass lets it through first and routes it to the cooler only once it is warm and thin.

Two clog indicators (one local, one remote):

"A clogged filter indication is provided by a local visual pop out indicator. The indicator is installed on IDG opposite side of the drive end. In addition, a differential pressure indicator, located in the IDG scavenge oil system, sends a ground signal to the GCU when the filter is clogged which, in turn, send a status message to the EWD warning that the DPI has switched."

Per AMM 24-21-00 §7.D. A local visual pop-out indicator (on the side opposite the drive end, read by maintenance on the spot), and a differential-pressure indicator in the scavenge system that grounds a signal to the GCU on a clog, the GCU then raising an EWD status "DPI switched" (a maintenance follow-up at landing). The same anti-cold-thickening idea is applied here: the differential-pressure sensing is automatically inhibited in the cold-oil regime to prevent a high-viscosity false "clog" (per AMM §6.F).

Maintenance side, for awareness only. Filling is through a pressure-fill quick connector and the tank is full when excess oil overflows the overfill drain — neither filling nor level-checking needs the engine to be motored; level is read on a vertical sight glass against red/yellow/green bands (per AMM §6.F).

6. Monitoring thresholds and the ECAM IDG fields

"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."

Per FCOM DSC-24-20. AMM §7.A spells out the full warning set (FCOM only mentions the ECAM caution; the AMM lists every annunciation):

"In case of abnormally low oil pressure or IDG oil overheat (outlet temperature ≥ 185 °C), the warnings are provided to the cockpit (FAULT legend on the corresponding IDG pushbutton switch, MASTER CAUT, single chime and messages on the Engine/Warning Display (EWD) and System Display (SD))."

Per AMM 24-21-00 §7.A. (The AMM writes outlet ≥ 185 °C, FCOM writes above 185 — a one-boundary difference; at pilot level "above 185" is enough.)

Reading the IDG fields on the ECAM ELEC AC page (per FCOM DSC-24-20):

The 151 °C advisory is not "just watch it" — the QRH gives an actionable response:

"IDG OIL TEMP — T > 151 °C — Reduce IDG load, if possible (GALLEY or GEN OFF). If required, restore when temperature has dropped. Restrict use of the generator to a short time, if temperature rises again excessively."

Per QRH 01.01A. So the 151 °C advisory drives a deliberate load reduction (GALLEY or GEN OFF) — the first manual intervention, ahead of the 185 overheat.

7. The disconnect mechanism — a one-way ticket with a built-in interlock

28 VDC ─[CB 1XT1(2) "IDG 1(2) DISC", 742VU, A76/D69, fed from FADEC A]─┐
                                                                       ▼
2XT1(2) IDG pb pressed ──► 4XT1(2) disconnect control relay            │
                              ▲ GCU UNDERSPEED signal cuts this relay = hardware gate
                              ▼
                         3XT1(2) disconnect supply relay ──► IDG 4000XU internal disconnect solenoid
   GCU ◄─ disc solenoid (request) / disconnection status ─┘  (pulls the input spline, mechanism springs apart)
   Panel legends: P/BSW ELEC/IDG —— FAULT (5 VAC ESS) / OFF (28 VDC)

Two points read off the disconnect-control figure. First, the 28 VDC for the disconnect circuit comes through CB 1XT1/1XT2 ("IDG 1/2 DISC", on panel 742VU, clipped A76/D69, fed from FADEC A) — the upstream origin of the disconnect supply chain (per AMM §4.A). Second, pressing the IDG pushbutton does not energise the solenoid directly — a disconnect control relay (4XT1) sits in between, controlled by the GCU UNDERSPEED signal: with the engine stopped or below idle the GCU declares underspeed, the relay drops out, and pressing the button does nothing. That is the electrical reality behind FCOM's "inhibited when the engine is stopped or below idle".

The IDG pushbutton carries two legends, FAULT and OFF (the FAULT legend at 5 VAC ESS, the OFF legend at 28 VDC, the GCU also driving an "IDG OFF" signal).

The FCOM operating red lines, from the panel description:

"CAUTION 1. If the pushbutton is pressed for more than about 3 s, damage may occur to the disconnection mechanism. 2. IDG disconnection is inhibited when the engine is stopped or below idle."

"Pressing this switch disconnects the IDG from its drive shaft. Only maintenance personnel can reconnect it."

Per FCOM DSC-24-20. The AMM makes the one-way nature concrete (note the reset-handle wording "shut down and stationary"):

"The IDG disconnection is irreversible in flight. … A mechanical reset handle is fitted to the IDG. The handle enables the drive to be reconnected while the engine is shut down and stationary on the ground."

Per AMM 24-21-00 §6.F.

The 50-hour red line — two manuals, two viewpoints. The AMM states the safety mechanism:

"If the IDG operates for more than 50 hours in the disconnected mode without a reset, the operator must replace it and send the removed IDG for a shop inspection. This is because there is a risk of damage to the ball bearing assembly on the input shaft and to the aircraft engine."

Per AMM 24-21-00 §7.B. The operator MEL adds the economic view — that the IDG should not run in the disconnected mode beyond 50 flight hours. The AMM gives the safety mechanism (bearing / engine risk); the MEL gives the economic consequence (forced replacement and shop inspection beyond the limit) — one 50-hour rule, two faces. The mechanism: disconnection only releases the spline so the CSD no longer transmits torque, but one end of the input shaft is still dragged round by the gearbox, and the bearing keeps wearing without normal oil circulation.

One easily missed footnote: the IDG FAULT light is inhibited when the engine is stopped (per AMM §7.C) — "no FAULT after shutdown" does not mean "the problem has gone"; the evidence is in the ECAM history and the maintenance record.

8. When and how to disconnect

8.1 What leads to the disconnect step

The trigger chain is an ELEC IDG 1(2) OIL LO PR or ELEC IDG 1(2) OIL OVHT ECAM caution (the handling procedure is in article 22) → the procedure leads to disconnecting the IDG. The AMM writes the decision as a standard action:

"If an IDG is faulty (overheat or abnormal oil low pressure), the FAULT legend comes on. The pilot must then open the safety guard and push the IDG pushbutton switch. This action results in the mechanical disconnection of the faulty IDG."

Per AMM 24-21-00 §7.B. The essence of disconnection is cutting your losses to save the fleet: the oil system can no longer support this "gearbox-plus-generator", and forcing it to keep turning would convert heat and metal particles into mechanical damage; releasing the spline lets it spin down quietly, and the electrical job is handed to the APU GEN or the opposite GEN (the network reconfigures automatically — see article 21).

[!warning]- A single IDG disconnect/failure does NOT enter ELEC EMER CONFIG

Losing or disconnecting one IDG does not drop the aircraft into the electrical emergency configuration — the opposite GEN plus the APU GEN cover it. Only loss of both main generators that the APU also cannot make good reaches the EMER GEN. Per FCTM PR-AEP-ELEC: "The electrical emergency configuration is due essentially to the loss of all main AC BUS …". Do not confuse "one IDG disconnected" with the "emergency configuration".

Note that the AMM passage above is a mechanism description — the actual handling always follows the ECAM/QRH procedure (for some fault forms the procedure does not disconnect immediately; article 22 checks the procedure roll step by step).

8.2 The flight deck after a disconnect

• IDG pb: the FAULT light goes out (per FCOM DSC-24-20, "It goes off when the IDG is disconnected" — once disconnection takes effect FAULT extinguishes); the OFF legend comes on (the GCU drives the "IDG OFF" signal).
• ECAM ELEC AC page: that side's IDG label and number turn amber, and an amber DISC appears (per AMM §7.B / FCOM DSC-24-20).
• EWD: an ELEC IDG 1(2) DISCONNECTED memo/caution (annunciated on the ground; the MEL has both true- and false-warning dispatch branches).

9. Dispatch view (MEL)

One main "AC generation system (IDG, GCU, line contactor)" inoperative may be dispatched under MEL 24-22-01 (Category C, 2 installed / 1 required). The core conditions, per the operator MEL (MI-24-22):

1. No ETOPS beyond 180 min.
2. The APU and AC auxiliary generation system operating and used throughout the flight.
3. All busbars energised.
4. The ELEC AC SD page verifies that the remaining AC main + auxiliary generation indications are operative.
5. The operating-side IDG has no OIL SYS FAULT warning.
6. No FUEL APU AFT PUMP FAULT warning.
7. When AC main generation 2 is inoperative, check the AC BUS 1→2 automatic transfer before first flight and daily thereafter.
8. Check the landing-recovery-configuration APU aft fuel-pump shedding function before first flight and weekly thereafter.
9. Check the APU oil quantity is sufficient for the flight before each flight.
10. Must be operative for departure from a main base.
11. Must not be inoperative at a high-altitude-airport take-off station.

Why must the APU run throughout? After dispatch the aircraft has a single main generator left — losing one more would drop straight into EMER CONFIG; keeping the APU GEN on line for the whole flight pre-positions the "second generator" and restores the N-1 redundancy. The dispatch fuel planning must account for the APU burn by altitude and configuration (per the operator MEL):

• Ground: 215 kg/h (two packs + APU GEN) / 140 kg/h (APU GEN only).
• In flight: 130 kg/h (FL200, packs + GEN) / 65 kg/h (FL300, GEN only) / 55 kg/h (FL410, GEN only).

The post-start cockpit check on a dispatch day: APU MASTER ON / APU GEN ON → start → operating-side GEN ON / APU GEN OFF (note: the galley/IFE may auto-shed momentarily at APU GEN OFF and restore afterwards), then confirm on the ELEC AC SD page that the operating-side IDG is white / oil outlet temp green / GEN white / load·voltage·frequency green.

The true/false-warning branches (each fault has two dispatch paths):

• IDG DISCONNECTED: spurious warning (troubleshooting confirms it false) → ignore the EWD at cockpit preparation, no downgrade; true warning → IDG genuinely disconnected → handle under 24-22-01 (the 11 conditions).
• IDG OIL SYS FAULT: spurious → check the oil circuit works and the sight-glass oil level is normal daily (each flight for ETOPS); true → (1) the IDG must be disconnected, (2) if a leak is confirmed, the IDG is fully drained, (3) handled under 24-22-01 as a main-generation failure.

ECAM → MEL routing: IDG OIL OVHT → 24-22-01; IDG DISC true caution → 24-22-01 / false caution → 24-09-01; IDG OIL SYS FAULT → 24-09-03; GEN 1(2) OFF → not within the MEL (pilot-selected, not a failed part); GEN 1(2) OVERLOAD → 24-26-01 galley supply system (the dispatch means being confirmation that the galley auto-shed works, echoing the overload-sheds-galley script — not "outside the MEL"). The full dispatch treatment is in MEL Dispatch View.

10. Flight-deck scenarios

1. Cruise ECAM: ELEC IDG 2 OIL OVHT. On the ELEC AC page: IDG 2 oil temp amber > 185 °C, IDG 2 label and number also amber. The procedure reaches the disconnect step: open the guard, press briefly (< 3 s) the IDG 2 pb, confirm FAULT out + OFF lit + ECAM DISC. The electrical supply was never lost — GEN 2 handed over the moment the GCU protection acted (article 22).
2. Oil temp first reaches the 151 advisory. Do not wait for 185. The QRH calls for a deliberate load reduction (GALLEY / GEN OFF), restoring once it drops; a steady climb towards 185 is the real problem. On a cold-day first leg, a pulsing advisory should first bring the ACOC bypass-valve logic to mind (cold-oil cooling efficiency is inherently low).
3. Regret after disconnecting. "Oil temp dropped — press it again to reconnect?" — there is no such thing. Disconnection is a mechanical release; the reset handle is on the IDG body, operable only on the ground after shutdown by maintenance.
4. Wanting a precautionary disconnect on the ground with the engine stopped. The button will not act. The GCU underspeed signal gates the disconnect control relay (§7) — a poka-yoke, because the disconnect mechanism could jam half-way in a non-rotating state.
5. A MEL 24-22-01 dispatch day. Before take-off, confirm the APU GEN is on line and work the 11 conditions one by one; carry the APU burn by altitude (ground 215/140, air 130/65/55) into the remaining-fuel mental sum throughout; remind the next crew that the "50 flight-hour" window is ticking.
6. ENG 2 failure on top of IDG 1 already lost. Mind the QRH note — in this combination FLAPS are lost (IDG 1 is not an ordinary generator; the cross-system consequence reaches the flight controls — see articles 02 / 22).

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

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

1. "Once the oil temp has dropped after a disconnect, press again to reconnect the IDG." — False. Disconnection is a mechanical release; the reset handle is on the IDG body, and disconnection is irreversible in flight — reconnection is only possible on the ground after shutdown, by maintenance (AMM §6.F). In the air it is a one-way ticket.
2. "To disconnect the IDG precautionarily on the ground with the engine stopped, just press it." — False. With the engine stopped / below idle the GCU underspeed signal gates the disconnect control relay (§7) — pressing does nothing, by design.
3. "In case it did not disconnect cleanly, hold the IDG pb a little longer." — False. Disconnection is instantaneous; holding for more than about 3 s damages the disconnect mechanism (FCOM caution). A brief press is enough.
4. "Once the IDG is disconnected it is safe and can be left running indefinitely." — Half true. After the spline is released, the input shaft is still dragged round by the gearbox and the bearing keeps wearing; running disconnected beyond 50 hours without a reset means a forced replacement and shop inspection (AMM §7.B).
5. "No IDG FAULT light on the panel after engine shutdown means the problem has gone." — False. The FAULT light is inhibited at shutdown (AMM §7.C) — "not visible" ≠ "gone"; the evidence is in the ECAM history and the maintenance record.

Self-test

[!note]- Q1. How does the CSD turn a varying speed into a constant speed?

By differential "speed summing" plus two planets in series. The engine feeds varying speed in through the carrier shaft (4500–9120 rpm); a hydraulic trim unit (a variable-displacement pump driven by the carrier shaft, hydraulically coupled to a fixed-displacement motor mechanically tied to the trim ring gear) supplies a trim speed; the servo valve adjusts the variable unit's swash-plate angle, setting the magnitude and direction of the trim speed; the two inputs are added or subtracted in the differential, the second (output) planet meshing with the first planet and the output ring gear (ratio 1:2), and the output ring turns at a constant 24 000 rpm to drive the generator — corresponding to 400 Hz.

[!note]- Q2. What triggers and inhibits the IDG FAULT light, and what is the ECAM label/number colour logic?

Two triggers — oil-outlet overheat (> 185 °C) or low (charge) oil pressure; inhibited at stop / below idle. The warning set is FAULT light + MASTER CAUT + single chime + EWD/SD messages. On the ECAM: the IDG label goes amber for overheat / low pressure / disconnected (any of the three); the IDG number is white with the engine running and amber when stopped; oil temp is green / green-pulsing advisory at 151 / amber overheat at 185; LO PR (inhibited N2 < 14 %) / DISC. The 151 advisory has an actionable QRH load-reduction step.

[!note]- Q3. Why does "holding more than 3 s damage" the mechanism and why is disconnection "irreversible in flight"?

Pressing energises the disconnect solenoid, pulling the input spline so the mechanism springs apart — done instantaneously; continued energisation burns the solenoid / mechanism (the 3 s red line). Reset is purely mechanical (a reset handle on the IDG body, AMM §6.F), possible only on the ground with the engine shut down and stationary — in the air it is a one-way ticket. After disconnection the input shaft still spins, and beyond 50 hours without a reset a replacement and shop inspection are mandatory (bearing / engine risk).

[!note]- Q4. What are the oil's "two jobs" and which pumps does the circuit pass through?

(1) Transmission fluid: the charge-pump-pressurised oil is the working fluid that transmits power from the variable pump to the fixed motor and the control-pressure source for the servo-valve control piston (this charge-oil pressure is what LO PR watches). (2) Coolant: stator-core oil grooves, internal rotor passages, PMG spray, and disconnect-spline lubrication. Pumps: charge pump (boost) + scavenge pump + inversion pump (return); plus a deaerator, the air-cooled ACOC (LP-bleed cooled), and a cold-oil bypass valve to handle thick cold oil.

[!note]- Q5. What are the MEL 24-22-01 core conditions, and the APU fuel burn?

The 11 core conditions: no 180-min ETOPS / APU + auxiliary generation operating throughout / all busbars energised / SD verifies the remaining generation / no OIL SYS FAULT on the operating side / no APU AFT PUMP FAULT / when AC 2 is failed, daily AC BUS 1→2 automatic-transfer check / weekly APU aft-pump-shed check / APU oil quantity check before each flight / must be operative for departure from a main base / must not be inoperative at a high-altitude-airport take-off station. APU fuel burn: ground 215/140; in flight FL200 130 / FL300 65 / FL410 55 kg/h.

Key takeaways

References

Per FCOM DSC-24-10-20-10 (overview-level IDG drive and 115 kVA) and DSC-24-20 (IDG pushbutton: 3 s caution, inhibit, FAULT conditions, maintenance-only reset; ECAM IDG fields — label three triggers, number colour, 151/185, LO PR/DISC); AMM 24-21-00 D/O (IDG two parts, QAD installation, accessory-gearbox drive, GCU three functions, differential speed summing with the two planets in series and 1:2, servo-valve frequency loop, 24 000 rpm, the oil circuit's dual role with the charge/scavenge/inversion pumps + deaerator + full distribution, ACOC + cold-oil bypass, clog ΔP inhibit in cold oil, fill without motoring, the GCU–IDG interface table and three connectors, 151/185 thresholds and the warning set with single chime, the two oil-temperature sensors and charge-oil low pressure, irreversible disconnection + 50 h replacement, the §6.F reset handle, FAULT inhibit at shutdown, the dual clog indicators) and AMM 24-00-00 D/O (the top-level drive/disconnect/ground-reset sentence) and AMM §4.A (the disconnect CB 1XT1/1XT2, 742VU, A76/D69, fed from FADEC A); QRH 01.01A (the 151 °C advisory load-reduction response); FCTM PR-AEP-ELEC (a single IDG failure is not the emergency configuration); the operator MEL 24-22-01 / 24-09 (single-generation dispatch conditions, APU fuel burn, post-start SD check, the DISCONNECTED and OIL SYS FAULT true/false branches, ECAM → MEL routing). The "cutting losses to save the fleet" reading, the "APU throughout = restoring N-1" motive, and the disconnected-shaft bearing-wear mechanism are integrative syntheses of the above and contain no facts from outside the library.

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