Emergency Generator (CSM/G)

The CSM/G (Constant Speed Motor/Generator) is the emergency layer of the three-layer electrical defence described in the Electrical System Overview. When AC BUS 1 and AC BUS 2 are both lost, it turns Green hydraulic pressure back into 8 kVA of electrical power and dedicates that power to the essential (ESS) bus family. It is the hardware protagonist of the ELEC EMER CONFIG — this article takes the hardware and the mechanism to the bottom and leaves the cockpit procedure to Emergency Electrical Configuration.

The AMM treats the whole assembly as one system, the HEGS — Hydraulic Emergency Generation System = the CSM (hydraulic motor) + the generator + a dedicated GCU. Everything in this article is a part of, or a controller for, that system.

[!note]- Mental model — an emergency genset that burns water pressure

Think of the CSM/G as a building's emergency diesel generator set, except it burns hydraulic pressure instead of diesel. When both mains rails fail (AC BUS 1/2), a UPS (the batteries through the static inverter) holds the ICU's critical loads for a few seconds; the genset runs a 3-second self-check to confirm this is not a momentary flicker, then opens its fuel valve (the solenoid valve admits oil), and within 10 seconds it is paralleled onto the emergency bus. While the water tower is full (engine pumps) it runs to the end; if all that is left is a fire truck's pump (the RAT), it runs at reduced power and must even give water back to the hydrant (flight controls) — so when the big tap opens (slats extend) the genset shuts down and the ICU drops back onto the UPS until dawn.

1. The emergency layer — where the CSM/G sits

In the overview's three-layer ladder (normal generators → emergency generator → batteries + static inverter), the CSM/G is the middle layer. Its job is narrow and absolute: after the total loss of normal AC generation, keep the ESS family alive.

This article covers: the CSM/G machine itself (hydraulic motor / three-stage generator / solenoid valve), its dedicated GCU, the three ways it is activated (automatic / manual / test), and the LAND RECOVERY sub-bus mechanism.

Left to neighbouring articles: the AC ESS feed and transfer circuitry (AC ESS Feed and Transfer); the static inverter itself (Static Inverter); the ELEC EMER CONFIG procedure and the QRH/FCTM detail (Emergency Electrical Configuration); the battery-only end-state (Battery-Only Flight); and the RAT body and its extension logic (ATA 29).

The five questions this article must let you answer:

1. What is the full automatic start-up sequence, why is there a 3-second delay, and who feeds the ESS buses during the gap?
2. What are the two uses of the inlet solenoid control valve, and where does its power come from?
3. When does the CSM/G stay "on line to the end" versus "step out mid-flight", and why does slat extension stop it?
4. Which protection does its GCU carry that the main-generator GCU does not, and why?
5. What does the LAND RECOVERY button recover, and what does it cost?

2. Architecture — the HEGS chain

The CSM/G is a Green-hydraulic consumer that happens to produce electricity. Read the chain top-to-bottom: pressure source → motor → co-axial three-stage generator → EMER GLC → the ESS family.

Green hydraulic system (pressure from one of two sources)
 ├─ Engine-Driven Pump (EDP, >=1 engine turning) ──► CSM/G at full 8 kVA
 └─ RAT pump (RAT auto-extended) ──► 3.5 kVA baseline (<260 kt);
        │                            rises above 260 kt (engine windmill)
        ▼  PRIORITY VALVE  (on low Green pressure, sheds the CSM to keep
        │                   pressure for flight controls + brakes)
        ▼  SOLENOID CONTROL VALVE  (28 V DC: BAT 2 at start ─► PMG once steady)
 ┌────── CSM/G  8XE  (zone 148) ─────────────────────────────────┐
 │ CSM : variable-displacement axial-piston hydraulic motor       │
 │       + servo-valve speed regulator (governed by the GCU)      │
 │       └─ constant 8 000 RPM ──► generator (co-axial, same speed)│
 │ Three-stage generator : PMG (18-pole) ─► exciter (10-pole)     │
 │       ─► main generator (6-pole + rotating diodes)             │
 │ Cooling : stator = shaft-driven fan air ; rotor = hydraulic oil│
 └──────────┬──────────────────────────────────────────────────────┘
            ▼  115/200 V AC ±4 V . 400 Hz ±4 Hz . 3-phase
   EMER GLC 2XE  (driven by the CSM/G GCU 1XE ; POR is here)
            ├──► AC ESS BUS 9XP  +  AC SHED ESS BUS 4XP   (fed directly by EMER GLC)
            │       └─ AC ESS SHED segment 401XP : shed by relay 16XH on RAT / battery
            └──► ESS TR ──► DC ESS BUS 4PP  +  DC SHED ESS BUS 8PP
                            └─ 8PP : shed by contactor 1PH on RAT / battery

Reading notes taken from the schematics (AMM Fig.17100 / Fig.18650 / ASM 24-24-02):

• The detection set, not "1XP/2XP relays". 1XP and 2XP are the two AC main buses being monitored (AC BUS 1 = 1XP, AC BUS 2 = 2XP). The entities that detect their loss are 29XE (1XP VOLT REF/TEST) + 30XE (2XP VOLT REF/TEST) + 25XE (AC EMER DET RLY) + 26XE (EMER CONFIG DET). When both buses are unpowered, the 701PP HOT BUS (via 17XE) sends the GCU RESET start signal.
• Scenario and inhibit inputs. SFCC1 slat configuration (27TC1 SLATS EXTENDED), LGCIU1 nose-gear configuration, and the N2 > 50 % discretes for engines 1 and 2 (39RST / 39RSE) feed the inhibit logic and the EDP-vs-RAT scenario decision (see §5).
• Panel 211VU (EMER ELEC PWR): MAN ON (16XE) / EMER GEN TEST (15XE) / EMER GEN red FAULT legend (14XE) / LAND RECOVERY (8XC).

[!warning]- RAT extension is not triggered by N2 < 50 %

Two logics that students collapse into one. The RAT auto-extends on dual-engine flame-out, or on a Green + Yellow / Green + Blue dual reservoir low-level (per FCOM DSC-29-10-20). The N2 < 50 % discretes (39RST / 39RSE) are something else entirely: they are the interface signal the CSM/G uses to decide which scenario it is in — whether to fall to 3.5 kVA and step out at slat extension. N2 < 50 % does not extend the RAT.

The hydraulic side of the chain, read from AMM Fig.13350, puts the CSM/G firmly downstream of the priority valve:

GREEN HYD TANK ──► ┌ RAT pump  ┐
                   ├ ENG1 pump ┤──► GREEN BRAKE MANIFOLD ──┬──► TO OTHER USERS
                   ├ ENG2 pump ┤                           │    (flight controls / brakes, ref ATA 29)
                   └ elec pump ┘                           └──► PRIORITY VALVE ──► 8XE CSM/G
                                                                                     │ return
                                                                          LP RETURN MANIFOLD ──► TANK

The CSM/G is a Green-system user behind the priority valve. When Green pressure is short, the priority valve protects "TO OTHER USERS" (flight controls / brakes) first and puts the heavy CSM load last — the physical executor of the "give the water to flight controls" behaviour in §5. The ground TEST uses the electric pump shown on the same diagram (§8).

Numbers (AMM 24-24-00 §6.A(2)):

3. The machine — a hydraulic CVT, but in reverse

The IDG's constant-speed unit (Integrated Drive Generator) is "mechanical speed in, constant speed out". The CSM is the same idea with a different input: hydraulic energy in, constant speed out. A variable-displacement axial-piston motor takes high-pressure Green oil; a servo-valve, driven by the GCU's speed regulator, varies the swash-plate displacement and smooths the flow into a constant 8 000 RPM (per AMM 24-24-00 §6.A(1)(a)). The speed loop is structurally identical to the IDG's: the GCU compares PMG phase-1 and phase-2 frequency against a crystal reference and drives the servo-valve from the error (per AMM §6.B(1)(a)). The three-stage generator (PMG 18-pole → exciter 10-pole → main generator 6-pole with rotating full-wave diodes) is a scaled-down version of the brushless philosophy in GCU and AC Generation Control.

The inlet solenoid control valve is the CSM's master throttle, and it does two jobs from one valve (per AMM §6.A(1)(a)):

• Isolate — in normal operation it keeps the CSM disconnected from the hydraulic system, so the machine does not windmill and wear.
• Shut down — on a GCU-detected fault it closes, cutting the hydraulic source at the same instant the electrical protection trips.

Its supply chain is deliberate: BAT 2 provides 28 V DC during start-up, then it transfers to the CSM/G's own PMG once running — the same arrangement as the GCU itself (BAT 2 for the first 12 s, PMG thereafter, per AMM §4). Once it is in steady state, the HEGS is fully self-sufficient: hydraulic power from the engine pumps or RAT, electrical control from its own PMG, depending on nothing in the already-dead aircraft network.

4. The automatic start-up sequence

The trigger is the loss of both main AC buses; the timeline is built around a 3-second settle and a 10-second connect.

"The CSM/G is automatically controlled when the AC BUS 1 and the AC BUS 2 are lost. As soon as these main AC busbars are lost, the start-up sequence is initiated by the generation of a DC signal from the HOT BUS 701PP to the CSM/G GCU RESET input. NOTE: The EMER GCU generates a signal for the solenoid control valve, 3 seconds + or - 0.5 seconds after it receives the starting signal from the AC logic circuit." [AMM 24-24-00, D/O §7.A, rev 01 APR 26]

"During the start-up sequence, 10 seconds maximum, (the time delay is performed by a time delay relay), BAT 2 supplies the solenoid control valve. Then, the CSM/G is connected to the hydraulic system and the generator begins to run. After de-energization of the time delay relay, the CSM/G GCU is supplied by the PMG. When the nominal CSM/G frequency and voltage are reached, the CSM/G GCU energizes the EMER GLC contactor (2XE)." [AMM 24-24-00, D/O §7.A, rev 01 APR 26]

As a timeline:

t = 0      AC BUS 1 + 2 both lost  (ECAM: EMER CONFIG red + MASTER WARN + CRC)
           Detection: 29XE/30XE VOLT REF/TEST + 25XE AC EMER DET RLY + 26XE EMER CONFIG DET
           701PP HOT BUS via 17XE ──► GCU RESET start signal ; BAT 2 takes over GCU supply
           [the gap]  the two main batteries feed DC ESS, and via the static inverter
                      (2.5 kVA single-phase, part of AC ESS only) bridge AC ESS
t = 3±0.5s GCU only now opens the solenoid control valve (settle: filter out a momentary
           supply flicker, do not open the valve on a false alarm)
           ── GCU internal K1 relay marked "3 S" closes when the timer expires ──
           Green oil floods the CSM ──► spins up to 8 000 RPM ──► PMG comes alive
t <= 10s   Time-delay relay de-energises : GCU / valve supply transfers BAT 2 ──► PMG
           ── GCU internal block "SWITCH OFF AFTER 10 S" performs the BAT2-to-PMG cutover ──
           Frequency + voltage in window ──► EMER GLC 2XE closes
           AC ESS (direct) + DC ESS (via ESS TR) taken over by the CSM/G ; static inverter steps back

The static-inverter bridge is not only "the start-up moment" — the AMM gives three precise conditions for it:

"In flight, in emergency configuration (busbars 1XP and 2XP lost): - as long as the CSM/G is not connected to the essential network (CSM/G starting phase), - if the CSM/G is not available (at slat extension with RAT extended) and the aircraft speed above 50 knts, - at landing, with aircraft speed lower than 50 Knts and if the two BAT1 and BAT2 pushbutton switches are pushed." [AMM 24-28-00, D/O, rev 01 APR 26]

The second condition is exactly the downstream of the RAT-scenario "slats out, CSM/G stops, batteries + static inverter take over" of §5 (with a 50 kt hand-over threshold); the third is the landing-phase bridge. Keep one thing here: the static inverter is only 2.5 kVA single-phase and feeds only part of AC ESS — a heavy reduction from the CSM/G's 8 kVA three-phase. The numerical detail of the inverter belongs to Static Inverter.

[!warning]- A red EMER GEN FAULT legend within the start window is normal

The EMER CONFIG flight deck is not dark at t = 0 — the batteries and static inverter bridge the ESS core first, and a few seconds later the CSM/G comes on line and widens the supply. During the start window the EMER GEN red FAULT legend (14XE) is lit, "until the emergency generator is available" (AMM §7.E). Do not judge the CSM/G failed inside 10 seconds — a legend that stays on after the window is the real fault (FCOM defines the red legend as: the emergency generator is not supplying and normal in-flight AC supply is lost — per FCOM DSC-24-20).

5. In line to landing, or stepping out — read the hydraulics

Whether the CSM/G runs to the end depends entirely on what is feeding the Green system.

"If the green hydraulic system is supplied with the hydraulic engine pump, the CSM/G remains in line until the end of the flight. If the green hydraulic system is supplied with the RAT pump when both engines are below 50 % N2, the CSM/G stops when the slats are extended. Then, the two main batteries supply then the DC ESS BUS and, via the static inverter, the AC ESS BUS until the end of the flight." [AMM 24-24-00, D/O §7.A, rev 01 APR 26]

The dividing line is N2 50 %, and the interface signal comes physically from the two engines:

"The emergency generation system receives the following information from the other systems: ‐ slat configuration from the SFCC1, ‐ nose landing gear configuration from the LGCIU1, ‐ N2 higher than 50% information for engines 1 and 2." [AMM 24-24-00, D/O §5.A, rev 01 APR 26]

The two scenarios also differ in cause (FCTM): the EDP scenario is usually a pure electrical disaster (a network short circuit) — all engine generators trip, the CSM/G is driven by the EDP, and the probability of recovering with the APU generator is low. The RAT scenario is usually a combination of electrical and engine failures — and the crew may actually be able to recover with the APU generator (depth in Emergency Electrical Configuration, per FCTM PR-AEP-ELEC).

• EDP scenario (engines still turning, a pure electrical disaster). Green pressure is abundant, the CSM/G runs at the full 8 kVA, supports even the AC/DC ESS SHED buses, and stays on line to landing.
• RAT scenario (both engines failed). The RAT's small flow is split between the flight controls and the CSM/G — the CSM/G falls to a 3.5 kVA baseline (half power; above 260 kt the output recovers somewhat because it takes credit from the engine windmill effect), and the SHED buses are dropped (relay 16XH sheds 401XP, contactor 1PH sheds 8PP, per AMM §3). At the moment the slats extend on approach, the CSM/G deliberately steps out, handing the RAT's entire flow to the flight controls and returning the ESS core to the batteries + static inverter.

"If the speed is higher than 260 kt IAS, the emergency generator electrical power is increased as it takes credit from the engine windmill effect." [FCTM PR-AEP-ELEC, rev 15 FEB 23]

The "give the water to flight controls" is executed by the priority valve — the CSM/G is on the valve's heavy-load list:

"In the event of low hydraulic pressure, a priority valve cuts off hydraulic power to heavy load users (emergency generator, nosewheel steering, landing gear) in order to keep the pressure for normal braking and flight controls." [FCOM DSC-29-10-30, p.2082, rev 14 JAN 26 (section eff. 22 MAR 16)]

So "slats out, CSM/G stops" is not the CSM tactfully bowing out — the hydraulic priority hardware (the valve sits between the Green brake manifold and the CSM inlet, AMM Fig.13350) actively cuts it. After slat retraction it can be brought back:

"... it can be reactivated after slats retraction via the MAN ON pushbutton." [FCOM DSC-24-10-30-30, rev 14 JAN 26]

FCOM also closes with two "exit rules". Automatic deactivation happens in only two cases — in flight at slat extension when powered by the RAT only, and on the ground after both engines stop — plus the well-known "once coupled, no hand-back":

"When the emergency generator is automatically or manually coupled, the emergency generator continues to supply DC ESS and AC ESS buses, even if one or more main generators are recovered." [FCOM DSC-24-10-20-10, p.1679, rev 14 JAN 26]

[!warning]- A recovered main generator does not automatically take the ESS family back

Once the CSM/G is coupled, the ESS family stays on it even after AC BUS 1/2 return. (Integrative reasoning, not verbatim: a just-recovered main generator is of unknown reliability, and the ESS family is the survival floor — better to keep it on the proven, stable 8 kVA and leave the hand-back to a deliberate crew action in the procedure.) The hand-back is a step in the recovery procedure (ATA-24-29), not an automatic event — do not be surprised that "the ESS buses are still on the emergency source".

6. The dedicated GCU — five protections, one extra

The CSM/G GCU carries the same protections as a main-generator GCU, plus one the main generator does not have (per AMM §6.B(1)(c)):

A trip does three things at once: open the EMER GLC (2XE) + de-excite + close the solenoid control valve (cutting the hydraulic source too). The fault is latched in the GCU until a 28 V DC reset signal arrives (per AMM §6.B(1)(c)).

The extra OS (overspeed) item is the engineering tell of what the CSM is. (Integrative reasoning, not verbatim:) the CSM is a hydraulic motor — if the servo-valve jams or the speed loop fails, full Green pressure can drive it to a runaway, so the fast 200 ms trip + valve closure is a purpose-built guard against the CSM-specific "hydraulic drive runaway" failure mode. A main generator is physically speed-limited by its engine or the APU, so the 17-item protection table in GCU and AC Generation Control has no generator OS item (it has an underspeed item instead). Threshold comparison: the emergency unit's voltage/frequency thresholds (128/100/433/363) are comparable to or slightly tighter than the main generator's (130/101.5/435/361), but the tolerances are wider (±2.5 V / ±3 Hz versus the main generator's ±1.5 V / ±1 Hz) — emergency equipment, judged fast, not precisely.

[!warning]- Gravity gear extension also protects the CSM/G from a spurious trip

The fast protections (UV 4.5 s / UF 4 s / OS 200 ms) explain why the approach insists on gravity gear extension — it is not only to save hydraulic fluid:

"When the emergency generator is powered by EDP, this avoids strong fluctuation of the green hydraulic pressure which may cause a spurious disconnection of the emergency generator." [FCTM PR-AEP-ELEC, GENERAL GUIDELINES, p.393, rev 15 FEB 23]

A normal gear extension draws a large gulp of Green oil → Green pressure dips and fluctuates → the CSM's speed/voltage wobble → they may cross the UV/UF/OS thresholds → the CSM/G protection trips spuriously. Gravity gear extension keeps the CSM/G from being killed by its own protection.

The GCU front face also carries a red LED: lit for roughly 10 seconds while connected to BAT 2 (start-up or an MCDU test) is normal; steady-on = a detected fault for maintenance (per AMM §6.B(1)(d)).

7. LAND RECOVERY — the buy-back button

In the emergency cruise phase the DC LAND RECOVERY sub-buses (407PP, 805PP) are not supplied — that power is saved for more critical loads. Before the approach, pressing LAND RECOVERY (8XC) buys back the equipment needed for landing. FCOM gives the precise list and price.

Recovered (with the EMER GEN running): LGCIU 1, SFCC 1 (the flap channel is not recovered when on RAT supply), BSCU channel 1 (not on RAT), left windshield anti-ice (not on RAT), left landing light (not on RAT). The cost: the remaining fuel pump (if any) is lost (on RAT supply that pump is shed at 260 kt anyway — "260 kt or LAND RECOVERY, whichever comes first"); and pressing the button loses ADR 3, which loses AP 1 (per FCOM DSC-24-20). The escape hatch (QRH footnote): setting the AIR DATA selector to "CAPT ON 3" preserves ADR 3 (but AP is still lost) (per QRH 16.02C footnote (h)).

Note the "discount buy-back" on RAT supply: four of the items track the RAT reduction (flap channel / BSCU / windshield anti-ice / landing light do not come back) — 3.5 kVA is all the budget there is. The downstream consequence of BSCU channel 1 not being recovered (FCTM): the RAT-scenario landing is on pedal braking with no anti-skid; and the AP / pitch / rudder trim are unavailable, with the aircraft out of trim in roll due to right outboard aileron upfloat — depth in Emergency Electrical Configuration.

The LAND RECOVERY sub-bus supply across the three configurations (anchored to the FCOM distribution table, which is cleaner than the OCR-risky internal AMM table):

"(1) Supplied when LAND RECOVERY pushbutton is at ON." [FCOM DSC-24-10-30-40, p.1709-1710, rev 14 JAN 26]

Three nailed-down conclusions: (i) in the battery-only configuration the LAND REC bus (which carries 407PP) is supplied without pressing the button — landing is imminent and the demand is direct (no (1) footnote on this row, the key difference from the EMER GEN configurations); (ii) the SHED LAND REC segment exists only in the EDP scenario with the button ON (always empty on RAT); (iii) in the battery-only configuration the DC BAT BUS is lost after 7 seconds (footnote (2) Lost after 7 s).

8. EMER GEN TEST — the ground exercise

The TEST pb (15XE, guarded, press and hold) validates the whole HEGS on the ground: the CSM/G spins up and supplies DC ESS + AC ESS. The precondition is Green pressure — from the Green electric pump or a ground hydraulic source (the engines do not need to be running) (per AMM §7.C). The test is inhibited when the slats are extended (FCOM). The line in the overview AMM power-distribution table, "EMER GENERATION OPERATIONAL TEST ON GND ONLY WITH HYD ELEC PUMP", is this test's electrical footprint.

9. Activation, scenarios and common misconceptions

9.1 Three ways to activate

9.2 Six flight-deck scenarios

1. EMER CONFIG in cruise. MASTER WARN + CRC, screens shrink — count the beats: the battery bridge is already working; at +3 s the valve opens, within 10 s the EMER GEN red legend should go out and the ELEC AC page should show EMER GEN voltage/frequency. A legend that does not go out → a real fault → try MAN ON once (procedure in ATA-24-29).
2. Dual-engine failure (RAT scenario). A reduced 3.5 kVA CSM/G with the SHED buses already dropped. Before a relight or before extending slats on approach, expect the CSM/G to step out — as the slats come out, the ESS family returns to the batteries + static inverter and the screens shrink one more notch. This is by design, not a new fault.
3. RAT scenario, slats retracted again (before a go-around / successful relight): MAN ON can bring the CSM/G back (FCOM: "it can be reactivated after slats retraction via the MAN ON pushbutton").
4. Pressing LAND RECOVERY before the approach. Before the left hand goes down, the right brain runs the cost list — AP 1 will drop (ADR 3 lost), the remaining fuel pump will be lost; on RAT supply do not expect the flap channel / BSCU / windshield anti-ice / landing light back.
5. A main generator recovers mid-flight (e.g. a successful GEN reset). AC BUS 1/2 are back, but the ESS family is still on the EMER GEN (coupled, no hand-back) — perform the ECAM/QRH recovery hand-back (ATA-24-29); do not be puzzled that the ESS buses are still on the emergency source.
6. A routine ground EMER GEN TEST. Pressurise with the Green electric pump → press and hold TEST → watch the ELEC AC page for EMER GEN parameters and the ESS buses changing source → release to restore. Cannot be done with the slats extended (inhibited).

9.3 Common misconceptions — predict, then check

1. "The CSM/G is the emergency generator that only appears on a dual-engine failure." — Conditional. The trigger is the loss of both AC BUS 1 and 2, which is a different matter from whether the engines have stopped (AMM: "automatically controlled when AC BUS 1 and AC BUS 2 are lost"). A pure electrical disaster with engines still turning brings it on too (the EDP scenario, §5).
2. "The red EMER GEN legend at the start of EMER CONFIG means the emergency generator is broken." — Wrong. The start sequence has a 3-second settle plus a 10-second connect window; a red legend inside that window is normal (§4). Only a legend that stays on afterwards is a real fault.
3. "In the RAT scenario the CSM/G suddenly stops at slat extension — that is a new failure stacking on top." — Wrong. By design: at slat extension the CSM/G stops, handing the RAT's whole flow to the flight controls, and the ESS family returns to the batteries + static inverter (§5). It can be revived with MAN ON after slat retraction.
4. "As soon as a main generator recovers (AC BUS 1/2 back), the ESS family switches back to the main network automatically." — Wrong. Coupled = no hand-back: FCOM, "even if one or more main generators are recovered" the EMER GEN keeps supplying DC ESS and AC ESS (§5). The hand-back is a manual recovery step (ATA-24-29).
5. "Doing an EMER GEN TEST on the ground needs the engines running to make hydraulic pressure." — Wrong. The only precondition is Green pressure — the Green electric pump or a ground hydraulic source is enough, the engines need not run (§8); the test is inhibited with slats extended.
6. "LAND RECOVERY is free money — it buys all the landing equipment back; without it the 407PP landing essentials have no power." — Half right. It has a price: pressing it loses ADR 3 and then AP 1, and the remaining fuel pump (§7); and in the battery-only configuration the LAND REC bus is supplied without pressing the button — the landing essentials are fed directly, without waiting for you (§7, FCOM distribution table).

10. Dispatch and operating limits

Dispatch (the operator MEL) — a clean "system no-go, indicator-light go" split:

The teaching point: the indicator light (the 14XE red legend) is dispatchable, but the system it indicates (the CSM/G) is not — §4 repeatedly uses the 14XE legend to judge a fault, and the MEL adds "a broken light ≠ a broken system, but a broken system is no-go". There is also no standalone "EMER GEN INOP" ECAM warning — a genuine loss of the CSM/G presents as ELEC EMER CONFIG (no dispatch). The CSM/G has no crew reset (it is not in the QRH reset table); the only manual handle is MAN ON (activation, not reset); the GCU fault reset is an internal 28 V DC signal.

QRH cruise/approach numbers (the depth of the SUMMARY belongs to ATA-24-29; the values a pilot should hold are here):

[QRH 16.01A ELEC EMER CONFIG SUMMARY, rev 21 AUG 24] — the most counter-intuitive number to hold is MINIMUM RAT SPEED 140 KT (the RAT must not go too slow before slat extension, or it stalls), with the 200 KT ceiling on gravity gear extension.

Self-test

[!note]- Q1. Walk through the automatic start-up sequence. Why the 3-second delay, and who feeds the ESS buses during the gap?

AC BUS 1 + 2 both lost → the 701PP HOT BUS sends the GCU RESET start signal and BAT 2 takes over the GCU and solenoid-valve supply (during the gap the batteries feed DC ESS and, via the static inverter, bridge AC ESS) → at +3 ±0.5 s the GCU opens the solenoid control valve (the delay filters out a momentary supply flicker so the valve is not opened on a false alarm) → Green pressure drives the CSM to 8 000 RPM → the sequence completes in ≤ 10 s (a time-delay relay), the supply transferring to the PMG → frequency and voltage in window → EMER GLC 2XE closes → AC ESS fed directly + DC ESS via the ESS TR, and the static inverter steps back.

[!note]- Q2. What are the solenoid control valve's two functions, and how is it powered?

(i) Isolate — keep the CSM/G disconnected from the hydraulic system when not in use, to prevent idle windmilling wear; (ii) shut down — on a GCU protection trip, close the valve and cut the hydraulic source (alongside opening the GLC and de-exciting — the three-part trip). Power: 28 V DC from BAT 2 during start-up, then the CSM/G's own PMG once steady — in steady state the HEGS electrical control is fully self-sufficient.

[!note]- Q3. When does the CSM/G stay on line to landing, when does it step out mid-flight, what single signal decides, and does a recovered main generator take the ESS family back?

The split is whether both engines are above N2 50 %. EDP scenario (≥1 engine turning): full 8 kVA including the SHED buses, on line to the end of the flight. RAT scenario (both engines below N2 50 %): 3.5 kVA (< 260 kt; rising above 260 kt from windmill credit), SHED buses dropped (16XH / 1PH), and the CSM/G stops at slat extension (the priority valve hands the flow to the flight controls), after which the batteries + static inverter take over; MAN ON can revive it after slat retraction. Automatic deactivation is only two cases: in flight at slat extension (RAT only) and on the ground after both engines stop. And no — once coupled it does not hand back automatically even if a main generator recovers.

[!note]- Q4. Which protection is unique to the CSM/G GCU, why, and what does a trip do? And why does EMER CONFIG insist on gravity gear extension?

Overspeed (OS) — PMG-referred 450 ±5 Hz, 200 ms fast trip — against the hydraulic-motor-specific runaway risk (a jammed servo-valve or failed speed loop letting full Green pressure drive it away). A trip does three things: open EMER GLC 2XE + de-excite + close the solenoid valve (cutting electrics and hydraulics). The other four are OV 128 / UV 100 / OF 433 / UF 363 (4–4.5 s delays). Gravity gear extension matters for two reasons: (i) it gives the Green hydraulic to the flight controls; (ii) a normal gear extension draws a large gulp of oil → Green pressure fluctuates → the CSM's speed/voltage wobble → they may cross the UV/UF/OS thresholds → a spurious disconnection — gravity extension avoids that self-inflicted trip (FCTM).

[!note]- Q5. What does LAND RECOVERY recover, at what cost, why is the battery-only case different, can the CSM/G be dispatched inoperative, and what are the headline limits?

Recovers: the LAND REC sub-buses — LGCIU 1, SFCC 1, BSCU channel 1, left windshield anti-ice, left landing light (the last four do not come back on RAT, and SFCC recovers only the slat channel). Cost: the remaining fuel pump is lost; ADR 3 is lost → AP 1 is lost (escape: AIR DATA to CAPT ON 3 preserves ADR 3 but AP is still lost). The battery-only configuration is different: the LAND REC bus is supplied without pressing the button (no (1) footnote on that row), and on RAT the BSCU is not recovered → landing with no anti-skid. Dispatch: the CSM/G system must work — no dispatch (MI-24-24, a pure no-go, no reset), although the 14XE red FAULT legend light alone is Category C (a broken light ≠ a broken system); the ELEC EMER CONFIG warning is also no-go. Headline limits: cruise MAX 330/.82, speedbrake DO NOT USE, AP 1+2 lost; approach CAT 2 INOP, FLAP 3, MINIMUM RAT SPEED 140 KT, LAND RECOVERY ON; gravity gear extension MAX 200 KT; go-around gear not retracted, MAX 200 KT.

Key takeaways

References

Per AMM 24-24-00 D/O (HEGS make-up, the two Green pressure sources, the automatic sequence with the 3 s / 10 s timing and 701PP RESET, the EDP-vs-RAT split, MAN ON / TEST preconditions, the CSM variable-displacement motor + servo valve, the three-stage 18-10-6-pole generator, the value table, the five-item protection table including OS 450, the three-part trip and fault latch, the red-LED semantics, the LAND RECOVERY sub-buses, and the EMER CONFIG red warning + 14XE legend window); AMM 24-28-00 (the static-inverter bridge's three conditions, 2.5 kVA single-phase feeding part of AC ESS); FCOM DSC-24-10-20-10 (8 / 3.5 kVA, the 260 kt windmill, coupled-no-hand-back), DSC-24-10-30-30 (automatic / manual activation, slat inhibit and reactivation, the two deactivation conditions, EDP-vs-RAT supply), DSC-24-10-30-40 (the EMER CONFIG distribution table — battery-only LAND REC direct, EMER GEN configuration (1) only with the button ON, DC BAT BUS lost after 7 s), DSC-24-20 (the ELEC EMER panel, the red-legend definition, the LAND RECOVERY list and its ADR 3 / AP 1 cost); FCOM DSC-29-10-30 (the priority valve cutting the heavy-load users); FCTM PR-AEP-ELEC (the two classes of cause, the windmill mechanism, EDP supply avoiding Green-pressure-fluctuation spurious disconnection, the RAT-scenario no-anti-skid landing and roll out-of-trim); QRH 16.01A / 16.02C / 24.01A (the cruise/approach limits, the AIR DATA → CAPT ON 3 footnote, the SMOKE manual EMER CONFIG); the operator MEL MI-24-24 / ME-24 / MI-24-01-02 (CSM/G system no-go, the FAULT light Category C, the ELEC EMER CONFIG warning no-go); read from the schematics ASM 24-24-02, AMM Fig.13350 / Fig.17100 / Fig.18650 (the detection set 25XE/29XE/30XE/26XE, the priority valve between the Green brake manifold and the CSM inlet, the K1 "3 S" and "SWITCH OFF AFTER 10 S" timing entities, the three-stage generator and servo/solenoid valves, the 39RST/39RSE N2 50 % discretes). The integrative-reasoning passages — the design motive for "coupled, no hand-back", the rationale for the OS item, and the "3 s = transient filter" note — introduce no facts beyond the manuals above.

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