APU Generator and GAPCU

The APU generator is the "versatile substitute" of the power ladder: the same rating as a main generator (115 kVA), able to carry the whole aircraft on its own on the ground and to replace any failed IDG in flight. This article is built around its similarities and differences with the IDG generation channel — the same brushless philosophy, but with no CSD, with oil drawn from the APU, and with the GAPCU as its housekeeper instead of a GCU. The GCU article has already worked through the GCU channel, so much of this article is given by contrast — and the differences are themselves the test points.

This article owns the APU generator itself, the GAPCU's control and protection of the APU channel, and the APU GLC. The GAPCU's other two jobs — twin external-power control (External Power) and the NBPT frequency reference (No-Break Power Transfer) — and the APU body itself (start, ECB, bleed) belong elsewhere.

1. APU GEN versus the IDG generator — one comparison table

The generator's oil-inlet temperature normal band is 60–135 °C — distinct from the 185 °C in §3, which is the generator scavenge-oil outlet at which the ECB shuts down; two different measurement points.

2. The channel at a glance

APU (tail cone)
   │ "directly" driven, constant 24 000 ±300 rpm (no CSD!)
   ▼   ※ actually geared down through the APU accessory gearbox — see §3
APU GEN 8XS (brushless, oil-spray cooled, 28.3 kg)
   PMG (8-pole) → exciter → 3-diode single-wave bridge → main generator
   │ Oil: from the APU oil system (in/out through two ports in the mounting
   │      flange, scavenged by the APU)
   │ Outlet oil temp bulb → APU ECB: > 185 °C for 1 s → ECB auto-shuts the APU
   ▼ 115/200 VAC 3-phase · 400 Hz ±5 Hz · 115 kVA
APU GLC 3XS ──→ middle of the transfer circuit (BTC1—SIC—BTC2 corridor)
   ▲              routed by priority to AC BUS 1 / AC BUS 2
   │ Control: GAPCU 40XG + ECMU 1 (double signature)
   │ Panel: APU GEN pb (235VU)
   └ CT 21XS (6-hole CT, two cables per phase: T1/T4=A · T2/T5=B · T3/T6=C)

Four points the schematic confirms: the outlet oil-temp bulb wires straight to the ECB (not the GAPCU) — the figure behind the counter-intuitive point of §3; the 6-hole CT carries two cables per phase, the physical basis for the open-cable protection ("one cable < 18 A while the other > 38 A on the same phase"); the GAPCU internally is voltage regulator + generator-control relay + differential protection + a protection-module supply that rectifies the PMG output into DC for protection/control and excitation; and the GLC three-phase output feeds the BTC—SIC corridor entry.

3. Direct-drive constant frequency — the trade for omitting the CSD

"The APU directly drives the APU GEN at constant speed. This maintains the generator frequency constant."

Per AMM 24-00-00. An engine's speed must follow the thrust demand, so the IDG must carry a CSD; the APU's job is to run at constant speed (it provides no thrust; the ECB pins its speed), so the generator can be driven directly and be constant-frequency. The price of that trade is written in the frequency tolerance: the IDG has a hydraulic servo-valve loop that trims to ±0.3 Hz; the APU GEN's precision is the APU governor's precision, rated only ±5 Hz.

[!warning]- Does "direct drive, no CSD" mean the generator shaft is bolted to the APU main shaft?

No. "Directly" is relative to the IDG's hydraulic CSD — it means "no CSD, the APU is constant-speed by itself", not "the generator shaft is on the APU main shaft". Physically the APU GEN mounts on the APU accessory gearbox and is geared down: per AMM 49-26-00 the gearbox turns at 41 730 rpm, distributed to the generator shaft at 24 111 rpm — consistent with the 24 000 ±300 rpm here. What is omitted is the CSD (the hydraulic gearbox), not the gear; remember "no CSD constant-speed, geared down through the accessory gearbox", and leave the depth to ATA 49.

The ECAM amber thresholds follow the same logic as the engine-generator symbol:

"(D) The GEN load is normally green. It becomes amber, if the load is greater than 108 %, for more than 10 s. (E) The GEN voltage is normally green. It becomes amber below 110 V, or above 120 V. (F) The GEN frequency is normally green. It becomes amber below 390 Hz, or above 410 Hz."

Per FCOM DSC-24-20. Note the two event layers: the ECAM frequency amber line is 390/410 Hz (the display layer), while the APU GEN rated tolerance is ±5 Hz, i.e. 395–405 Hz (the generator layer) — ±5 Hz is still inside the ECAM green band, so the looser tolerance does not trigger an ECAM amber, and a pilot cannot tell the APU GEN is "rougher" than an IDG. Likewise the 108 % load amber is a display layer, distinct from the 135/180 kVA overload capability (the protection layer); do not merge them.

This is also the root of NBPT's "15 s":

"Synchronization may take up to 15 s for the APU GEN with GPU, and some milliseconds in all other cases. If synchronization is not achieved within the allowed time, transfer is performed anyway (without simultaneous connection of two sources). This function has a backup in the GCU."

Per FCOM DSC-24-10-20-30. Two sources that both drift (the APU GEN at ±5 Hz and the GPU) are much harder to bring to the same frequency and phase than "one steady, one drifting" — which is why 15 s is an order of magnitude longer than "a few milliseconds"; past the limit it transfers anyway (mechanism in No-Break Power Transfer).

3.1 The oil — living in someone else's house

"The oil is supplied from the APU. It flows in and out of the APU generator through two ports in its mounting flange. … A high oil temperature greater than 185 °C during 1 second leads to an automatic shut down of the APU (by the ECB)."

"The oil enters the drive end of the APU generator and is then diverted to the PMG, exciter, diode package, the main generator. Then the oil is drained at the bottom of the unit and is scavenged by the APU."

Per AMM 24-23-00. The generator has not a drop of its own oil — that is what "living in someone else's house" means literally.

[!warning]- The APU generator oil overheats — how does the system handle it?

By the IDG script you would answer "light the FAULT and let the pilot disconnect the generator" — wrong. The APU GEN has no disconnect mechanism, and its oil is the APU's oil: 185 °C at the generator end means the APU oil system itself is in trouble, and the ECB's response is to shut the whole APU down — generator and bleed together. One 185 °C threshold, two outcomes: the IDG loses one generator, the APU GEN loses the whole APU. That is why this sensor wires to the ECB (the APU's brain), not the GAPCU.

Two precision points (depth to ATA 49): the 185 °C measures the generator scavenge oil, not the inlet (per AMM 49-94-00, the ECB monitors the generator scavenge oil and shuts down when it exceeds 185 °C for 1 second); and this shutdown is not always active — it is a "subsequent shutdown" type, inhibited between first-engine start and last-engine shutdown while APU speed > 95 % (per AMM 49-61-00), so in cruise (engines running) the generator-overheat shutdown is masked, avoiding a needless APU loss at a flight-critical phase. (Overspeed / ECB 1A / emergency shutdown are never inhibited.)

3.2 GAPCU — one housekeeper, three jobs, and the self-excitation loop

The GAPCU (40XG) does for the APU channel exactly what a GCU does for an IDG channel — voltage regulation at the POR, PWM excitation, the three relays (GCR/PRR/SVR), protection, NBPT, and BITE. Its peculiarity is the moonlighting: it is the APU GEN housekeeper and the twin external-power housekeeper and the system BITE letter-box, collecting the GCU fault codes over an ARINC 1553 bus and forwarding them to the CMC.

The self-excitation loop is the channel's self-sufficient power root:

"The GAPCU receives this AC power and rectifies it into DC power. This DC power is used by the GAPCU for the system control and protection circuitry, as well as for generator excitation."

The PMG three-phase comes in, is rectified, and feeds the protection/control circuitry on one path and the excitation field (PWM) on the other. As with the IDG/GCU: as long as the APU turns, the PMG generates, and neither the GAPCU nor the generator field draws from the public network — the power precondition for the APU GEN to "stand alone" during NBPT and in emergency.

FCOM limits the GAPCU's frequency/voltage regulation to "when NBPT is required":

"… A Ground and Auxiliary Power Control Unit (GAPCU): ‐ Regulates, via the APU Electronic Control Box, the frequency and voltage of the APU generator when the No Break Power Transfer (NBPT) is required."

Per FCOM DSC-24-10-20-10. So in the normal state, voltage regulation is done by the GAPCU's own VR through PWM excitation at the POR; only at the NBPT synchronisation moment does the GAPCU trim APU speed via the ECB to match frequency and phase. This explains why "fine" frequency control detours through the ECB — the APU GEN has no servo valve to turn the frequency directly.

3.3 The protection table — two key differences from the GCU

The threshold values almost replicate the GCU (OV 130 / UV 101.5·68 / OF 435·452 / UF 361·343 / OC-OL 75 A·+15 A / DP 50 A / OPC 18–38 A / GLCF / SRD), with two key differences:

1. No underspeed protection — replaced by APU READY. An IDG uses engine speed to decide "should I be generating"; the APU channel listens to the ECB — absence of the ECB ground signal (APU ready) for 80 ms trips the PRR, and the UV/UF protections are also inhibited by "absence of APU ready" (low voltage/frequency before rated speed is not a fault). The FCOM panel line "APU GEN FAULT light is inhibited when APU speed is too low" is the deck face of this logic.
2. IPT is a single tier (220 ms) and trips PRR + GCR — the IDG IPT is three tiers, and trips the BTC; the APU GEN sits mid-corridor and is not directly on any one BTC, so an inadvertent-paralleling trip disconnects the unit itself (PRR + GCR). The reset remains the "two attempts" family.

Three depth notes from the protection table: GLCF trips only the GCR, not the PRR (it judges "GLC closed/PR true but current > 25 ±5 A" — de-exciting via the GCR cuts the current); SRD (rotating-diode failure) is detected from AC ripple on the excitation-field current versus load current (a failed diode puts ripple that should not be there on the field current); and UV/UF are two-tier (UV1 101.5 V/4.5 s network fault, UV2 68 V/160 ms VR-loop fault; UF1 361 Hz/4 s, UF2 343 Hz/160 ms), all blocked by "absence of APU ready".

3.4 Coming on line and dropping off — two thresholds with hysteresis

"When the pushbutton switch is pressed-in, the APU generator is excited if: the APU generator frequency is greater than 335 Hz and, no protection function is triggered. After the APU ready becomes true, the Power Ready Relay (PRR) closes."

"When the APU generator speed decreases: the APU GLC opens when the generator speed is below 22,800 RPM (this corresponds to frequency of 380 Hz), the APU generator is de-energized when its speed is below 19,200 RPM (this corresponds to a frequency of 320 Hz)."

Per AMM 24-23-00. The come-on-line chain mirrors the GCU channel: excite (335 Hz gate) → APU ready true → PRR closes → 28 VDC to the APU GLC + a ground signal to ECMU 1 → ECMU 1 validates the control logic → GLC closes → the generator joins mid-corridor. Note the AMM wording — "in flight the ECMU1 validates the control logic of the APU GLC" is the in-flight branch; on the ground the GLC closes via the NBPT synchronisation branch (up to 15 s), a different path. Drop-off is two-stage: 22 800 rpm (380 Hz) off the network first, 19 200 rpm (320 Hz) then de-excited — in a normal APU shutdown the generator "leaves first, then dies".

3.5 Where it joins the network decides its role

The APU GLC feeds the middle of the BTC1—SIC—BTC2 corridor, not any one AC BUS directly. So who it supplies is decided entirely by the ECMU priority order:

"1XP = IDG 1/APU GEN/EXT PWR B/EXT PWR A/IDG 2, 2XP = IDG 2/EXT PWR A/APU GEN/EXT PWR B/IDG 1."

Per AMM 24-23-00. Two identities: for AC BUS 1 the APU GEN is second priority (after the side's own IDG 1); for AC BUS 2 it is behind EXT A. That is the root of the FCOM tongue-twister ("APU priority left, EXT A priority right").

The substitute-priority verbatim and a boundary trap:

"ECMU provides automatic reconfiguration. Complete network remains supplied. Note: If a generator is lost due to overcurrent detection, reconfiguration does not occur and the related AC BUS is lost. The system automatically replaces the failed generator with: ‐ The APU generator if available or, ‐ The other engine generator (automatically shedding part of the galley load and some commercial loads)."

Per FCOM DSC-24-10-30-30. The substitute priority is APU GEN before "the other IDG" — the official basis for "the versatile substitute on call". And the trap: this automatic substitution only happens on an ordinary protection trip — if the generator tripped on overcurrent (the "lock-BTC family"), the ECMU does not reconfigure, the AC BUS is simply lost, and the APU GEN does not take over automatically. So the versatile substitute has a hard boundary: it cannot cover an overcurrent-type failure.

[!warning]- When can the "versatile substitute" not cover a failed generator?

When the generator tripped on overcurrent detection (the lock-BTC family) — the ECMU does not reconfigure and the AC BUS is lost; the APU GEN does not auto-substitute. By the same logic, in a pure-network short-circuit electrical emergency the probability of restoring power with the APU GEN is low (per FCTM); only with a combined electrical-plus-engine failure, and below FL 250 where the APU can start, can the normal configuration partly recover. And the EMER GEN, once connected, stays connected even if the APU GEN later comes back.

The ground dual-source split is also here: "The APU generator and the ground power unit connected to the EXT PWR A receptacle can be used at the same time on the ground" — "at the same time" is not paralleled: APU carries the left half, EXT A the right, with the SIC dividing them.

4. Operations and dispatch

The panel skeleton: "On: The APU generator field is energized and the line contactor closes provided parameters are normal. Each bus tie contactor 1 and (or) 2 automatically closes if its associated generator is not operative." The second sentence puts the BTC logic into the APU view: whichever side the APU GEN covers, that side's BTC closes to route the power. And the BUS TIE pb AUTO logic explains how a single APU GEN carries the whole aircraft:

"AUTO: The three BUS TIE contactors open or close automatically according to the priority logic in order to maintain power supply to all AC buses. The three contactors close when: ‐ only one engine generator supplies the aircraft, or ‐ only the APU generator or single ground power unit supplies the aircraft."

Per FCOM DSC-24-20. Single APU GEN supplying → all three BUS TIE contactors close → the BTC1—SIC—BTC2 corridor is fully through → both half-networks draw from that one source.

The fault script and reset: a protection trip opens the GCR + PRR, the APU GLC drops out, and the deck shows APU GEN pb amber FAULT + MASTER CAUT + single chime + EWD amber APU GEN FAULT; reset is the APU GEN pb cycle (OFF/R then ON). The procedure layer adds the trigger and the failed-reset action:

"This alert triggers when: ‐ The protection trip is initiated by the associated GCU, or ‐ The line contactor is open with APU GEN pb-sw set to ON. … APU GEN … OFF THEN ON / IF UNSUCCESSFUL: APU GEN … OFF / In flight: restrict use of APU to emergencies."

Per FCOM PRO-ABN-ELEC. Note that this OFF/R→ON cycle is an ECAM in-line step, not in the QRH system-reset table; do not confuse it with the QRH "GPCU/GAPCU" reset, which is an external-power function reset done on the ground.

Dispatch view (MEL)

ELEC APU GEN FAULT routes to MEL item 24-23-01 (AC auxiliary generation system — APU generator / GAPCU / line contactor), while ELEC APU GEN OVERLOAD routes to 24-26-01 (galley supply) — because all three OVERLOADs (IDG GEN / APU GEN / EXT PWR) dispatch by confirming the galley automatic shedding works. MI-24-23-01 has three sub-items (1 installed / 0 required, all with a placard, all conditioned on no ETOPS beyond 180 min): 24-23-01A (APU considered inoperative, Cat C, refer to powerplant item 49-10-01); 24-23-01B (electrical failure, Cat C, unrelated to a mechanical failure, APU GEN pb to OFF); 24-23-01C (disconnected or removed for mechanical failure, Cat D).

The operative consequence a pilot must carry: APU GEN inoperative is not an isolated "one less backup" — it takes out NBPT (so engine start must use external power or the battery), and on the ground at a destination without external power, the engine on the side opposite the refuel point must run during refuelling to keep a continuous electrical source available for engine start. Dispatch is possible, but the whole way of operating changes.

Self-test

[!note]- Q1. Why does the APU GEN need no CSD, and what is the price?

The APU is governed to a constant speed by the ECB (24 000 ±300 rpm), so the generator is constant-frequency when driven directly. The price is that frequency precision rests on the APU governor alone: tolerance ±5 Hz, an order of magnitude looser than the IDG's ±0.3 Hz (the IDG has a hydraulic servo loop). "Direct" means "no CSD", not "no gear" — it is still geared down through the accessory gearbox (41 730 → 24 111 rpm).

[!note]- Q2. What happens when the APU GEN oil overheats, and why is it different from the IDG?

Scavenge oil > 185 °C for 1 s → the ECB shuts the whole APU down (generator + bleed). The APU GEN has no independent oil loop — its oil comes from the APU oil system, so overheat at the generator end is an APU oil-system problem, and the sensor wires to the ECB, not the GAPCU. The IDG, with its own oil loop, only lights a FAULT at 185 °C and lets the pilot decide to disconnect that one generator.

[!note]- Q3. What are the come-on-line and drop-off thresholds?

On line: generator frequency > 335 Hz and no protection → excite; APU ready (ECB ground signal) true → PRR closes → ECMU 1 validates and closes the APU GLC. Off line in two stages: below 22 800 rpm (380 Hz) the GLC opens (off network), below 19 200 rpm (320 Hz) the generator is de-excited.

[!note]- Q4. What does the GAPCU protection table lose and gain versus a GCU?

It loses underspeed protection (an IDG uses engine speed); it gains APU READY (absence of the ECB ground signal for 80 ms trips the PRR), with UV/UF blocked by "absence of APU ready". The IPT is also simplified to a single 220 ms tier. The threshold values (OV 130 / UV 101.5·68 / OF 435·452 / UF 361·343 / DP 50 A …) match the GCU channel.

[!note]- Q5. With APU GEN and EXT A both connected on the ground, who supplies whom?

Split, not paralleled, by the per-bus priority: 1XP (left) = IDG1 > APU GEN > EXT B > EXT A > IDG2, so the APU carries the left network second in line; 2XP (right) = IDG2 > EXT A > APU GEN > EXT B > IDG1, so EXT A carries the right network. The SIC/BTC keep the two sources in their own half.

Key takeaways

References

Per AMM 24-23-00 D/O (§6.A generator construction, oil, single-wave bridge, frequency tolerance; §6.B GAPCU functions; §6.A(1)(b) self-excitation; §3.B priority order and ground dual source; §7.A come-on/drop-off thresholds; §7.C fault and reset) and 24-00-00 D/O §3.A(1) (direct-drive sentence); FCOM DSC-24-20 (ECAM thresholds, APU GEN/BUS TIE panel), DSC-24-10-20-10 (GAPCU regulation when NBPT required), DSC-24-10-20-30 (NBPT 15 s), DSC-24-10-30-30 (substitute priority, overcurrent no-reconfigure); FCOM PRO-ABN-ELEC (APU GEN FAULT triggers and action); AMM 49-26-00 / 49-94-00 / 49-61-00 (gear-down speeds, scavenge-oil shutdown, subsequent-shutdown inhibition — depth in ATA 49); FCTM PR-AEP-ELEC / PR-AEP-ENG (emergency-restore probability, FL 250, EMER GEN does not yield); the operator MEL 24-23-01 / 24-26-01 (dispatch, OVERLOAD routing). The "rooming-house substitute" framing is integrative synthesis grounded in the oil-source and ECB-shutdown facts above.

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