Cross-System Interfaces — ATA 21 as a Central Node

Stages A and B mentioned, piece by piece, where ATA 21 touches the rest of the aircraft. This article gathers them: ATA 21 is one of the most-connected systems on the aircraft, interfacing with 11+ external ATA chapters because it needs bleed air, electrical power, temperature and pressure data, ground/flight state, fire signals, envelope data, APU coordination, engine coordination, and cabin communication. Understanding these interfaces is the key to understanding fault-propagation paths.

1. The interface map

                       ┌──────────────────────────────┐
                       │   ATA 21 — air-con / press /  │
                       │   ventilation                 │
                       │   CPC · pack · zone · AEVC ·  │
                       │   VC + valves/fans/sensors    │
                       └──┬───┬───┬───┬───┬───┬───┬────┘
       ┌───────┬──────────┘   │   │   │   │   │   └──────────┬───────┐
       ▼       ▼              ▼   ▼   ▼   ▼   ▼              ▼       ▼
   ATA 36   ATA 31         ATA 26 ATA 24 ATA 35          ATA 22-80 ATA 32
   bleed    ECAM           fire   power  oxygen          FMGES     gear
   (5 pts)  (3 SD pages)   (SDCU) (301PP)(>9550 ft)      (ldg elev)(LGCIU)
       ▲                                                              ▲
   ATA 49 ─ APU bleed + ECB        ATA 73 ─ EIU/EIVMU + OIL LOW PRESS
   ATA 27 ─ ADIRS air data         ATA 23 ─ CIDS zones + FAP/AAP + PSCU

2. ATA 36 bleed — five interface points

1. Bleed → PFCV inlet — hot bleed through the ozone converters → PFCV → packs + trim-air valves; low bleed pressure auto-closes the PFCV (ata-21-06).
2. Zone-controller → BMC, 200 → 150 °C — if cooling demand cannot be met the zone controller asks the Bleed Monitoring Computer to reduce the bleed temperature; inhibited with wing anti-ice ON (ata-21-09).
3. Compressor-outlet overheat → pneumatic PFCV close — > 235 °C dumps the PFCV actuator air → the PFCV closes pneumatically, independent of the controller (ata-21-06, ata-21-08).
4. Trim-air manifold bleed tap — bleed taps off upstream of the PFCV into the hot-air manifold (the trim-air pressure-regulating valves keep cabin + 4 PSI) (ata-21-09).
5. Pack-bay turbofan — bleed-driven — on the ground the pack-bay turbofan is driven by bleed (ata-21-15).

[!note]- Bleed is the lifeblood of ATA 21 — five independent uses

ATA 21 uses bleed for: pack cooling (PFCV → ACM); trim-air heating (the hot-air manifold); ACM anti-icing (bleed to the turbine inlet, ata-21-07); wing anti-ice (shared with ATA 30); and the ground pack-bay turbofan. Five uses, one source — a bleed failure affects ATA 21 and other systems at once.

3. ATA 26 fire

The SDCU (or CIDS-SDF) signals the VC on cargo smoke → it closes that compartment's isolation valves + stops the fan (forward also the cold-air valve) (ata-21-16). On a freighter, main-deck smoke triggers the five-action cascade (shutoff valves 253–256HG + recirculation fans stop + one pack off + the other to 85 %) (ata-21-10).

4. ATA 31 ECAM — three SD pages

   pack / zone / VC / AEVC / CPC ──ARINC 429──▶ SDAC 1+2 ──▶ FWC 1+2 ──▶ E/WD + SD pages
   (emergency discrete: pack overheat / pack-controller fault → SDAC directly, bypassing the zone controller)

5. ATA 24 power

On emergency power (RAT / battery / emergency generator), relay 26XE signals the VC → it sheds all fan relays; the packs keep working (per the emergency-power priority); the cabin breathes on pack fresh air.

[!warning]- The RPCU is permanently on essential bus 301PP — independent of the main electrical state

The RPCU (ata-21-13) is one of the few parts directly on the essential bus, bypassing the rack — so even with both CPCs failed + main AC lost it can still drive the outflow-valve manual motors to release residual pressure (provided the four conditions are met).

6. ATA 35 oxygen — cabin altitude > 9550 ft

   cabin altitude rising → CPC monitors → > 9550 ft → CPC discrete
        ▼                                      ▼
   ATA 35: passenger masks drop automatically + signs ON
        ▼ also
   ECAM EXCESS CAB ALT → crew: don masks / communicate / emergency descent / PA+ATC+MAYDAY / monitor

[!warning]- The > 9550 ft mask drop is an ATA 21 + ATA 35 interaction, not one system alone

The CPC judges the threshold (using its internal vibrating-cylinder sensor) and outputs the "excess cabin altitude" discrete; ATA 35 receives it and drops the masks + signs automatically. No crew action is needed for the drop — a hardware interaction.

7. ATA 22-80 FMGES

FMGEC → CPC: landing-field elevation (the automatic descent profile in LDG ELEV AUTO), cruise FL, approach phase, flight-phase data. On an FMGEC failure the pilot pulls the LDG ELEV selector past the mechanical stop and sets the elevation by hand (−2000…14,000 ft) → semi-automatic mode (ata-21-11).

8. ATA 32 gear — LGCIU

LGCIU gives the ground/flight state to the pack controller (ram-air-inlet anti-dust), the CPC (the five-mode switching + an RPCU condition), the VC (cargo/lav/cabin-fan modes), and the AEVC (the extraction configuration).

[!note]- LGCIU is the single ground/flight source shared by many ATA 21 parts

All of ATA 21 (CPC / pack / VC / AEVC) takes the ground/flight state from the LGCIU — avoiding inconsistent independent judgements. An LGCIU failure affects several ATA 21 mode switches at once, but the LGCIU is itself dual (1 + 2) with ground-speed-relay backup.

9. ATA 27 flight controls — ADIRS

ADIRS → CPC: aircraft altitude (cabin-altitude schedule), V/S (the profile + descent mode), airspeed (an RPCU condition, < 100 kt).

[!note]- The CPC does not measure aircraft altitude directly

Aircraft altitude comes from the ADIRS (external); cabin pressure from the CPC's internal vibrating cylinder; ΔP = aircraft-external − cabin = computed by the CPC. The ADIRS and the vibrating cylinder are complementary.

10. ATA 49 APU

APU bleed → PFCV: single-pack operation + APU bleed ON forces HI flow (ata-21-06). APU bleed → pack-bay turbofan on the ground. The zone controller raises the APU flow demand (via the ECB) when cooling is short (ata-21-09). APU OFF → the ground pack-bay turbofan loses its bleed source → a pack-bay overheat risk.

11. ATA 73 engine

EIU/EIVMU IGN mode → PFCV closes for 30 s (start); EIVMU master ON + N3 < 50 % → PFCV closed; EIVMU takeoff mode → CPC takeoff mode + prepressurisation; EIVMU → zone controller (raise idle for bleed pressure); engine FIRE pb → PFCV auto-close; engine OIL LOW PRESS → AEVC (an input).

12. ATA 23 CIDS

CIDS → zone controller: the three-zone boundary programming (which trim-air valves belong to FWD/MID/AFT) + the FAP/AAP zone trims (±3 °C). CIDS-SDF → VC (cargo smoke on some aircraft, replacing the SDCU). PSCU → CPC + VC (door state → CPC ground-mode limit + VC cargo-heating stop with a door open).

13. External-ATA failures and the reverse

The reverse also holds: a dual-pack failure → emergency descent + ATA 35; a pack-bay overheat → wing anti-ice (shared bleed); a zone-controller failure → lost ECAM data (ATA 31); a VC failure → lost cargo/recirculation ventilation (degrading the ATA 26 smoke response).

Self-test

[!note]- Q1. How many external ATA chapters does ATA 21 interface with? The most complex?

11+: ATA 36 bleed / 26 fire / 31 ECAM / 24 power / 35 oxygen / 22-80 FMGES / 32 gear / 27 ADIRS / 49 APU / 73 engine / 23 CIDS. The most complex: ATA 36 bleed (five independent uses — pack cooling, trim-air heating, ACM anti-icing, wing anti-ice, pack-bay turbofan) and ATA 31 ECAM (three SD pages with data from the pack/zone/cabin-pressure controllers).

[!note]- Q2. How does an ATA 26 cargo fire trigger ATA 21 ventilation? Passenger vs freighter?

Passenger: the SDCU detects lower-deck smoke → the VC closes that compartment's isolation valves + stops the fan (forward also the cold-air valve) + ECAM CARGO SMOKE. Freighter additionally: main-deck smoke → the five-action cascade (shutoff valves 253–256HG + recirculation fans stop + one pack off + the other to 85 % to prevent mixer overpressure). On some aircraft the CIDS-SDF replaces the SDCU. The crew sees CARGO SMOKE → the fire procedure; the ATA 21 ventilation links automatically.

[!note]- Q3. Is the > 9550 ft mask drop decided by ATA 21 or ATA 35?

Both: the CPC (ATA 21) judges the threshold (its internal vibrating cylinder) and outputs the "excess cabin altitude" discrete; ATA 35 receives it and drops the masks + signs automatically — no crew action for the drop. The ECAM EXCESS CAB ALT also triggers → the crew's five steps (don masks / communicate / emergency descent / PA+ATC+MAYDAY / monitor).

[!note]- Q4. How does the CPC get the landing elevation if the FMGEC fails?

Normal: the CPC uses the FMGEC's field elevation (automatic descent profile, LDG ELEV in AUTO). On an FMGEC failure: the pilot pulls the LDG ELEV selector past the mechanical stop and sets the field elevation by hand (−2000…14,000 ft) → semi-automatic mode. The selector is a redundant dual potentiometer and cannot be set above 14,000 ft. Briefing the destination elevation lets the crew set it at once if the FMGEC fails.

[!note]- Q5. After a dual-engine-out + RAT out, what in ATA 21 still works, and what stops?

Still works: the CPC (dual unit + manual backup, individual supplies + the CPC #1 backup-section supply) — pressure still controllable; the RPCU (on 301PP) — ground residual-pressure release; the packs (with RAT power + APU bleed); the vibrating-cylinder sensor (digital, AC-independent); the safety + negative-relief valves (pure mechanical). Stops: on emergency power the VC sheds all fan relays — recirculation + galley/lavatory + cargo fans all stop; AEVC-controlled items may fail (ground pack-bay turbofan loses bleed). The cabin still breathes on pack fresh air (humidity and flow drop).

Key takeaways

Common misconceptions

Scope — what this article covers and defers

References

This article integrates the cross-chapter interfaces first cited in the stage-A and stage-B deep-dives: AMM 21-51-00 (bleed → ozone converter → PFCV), FCOM DSC-21-10-30 (the zone controller's 200 → 150 °C BMC demand), AMM 21-28-00 §5.G/§5.H (the SDCU → VC smoke link and the emergency-power fan shedding), FCOM DSC-21-20-20 + AMM 21-31-00 §5.B (the CPC's six ARINC inputs and discrete connections), AMM 21-26-00 §6.C (the AEVC inputs incl. OIL LOW PRESS / LGCIU), and AMM 21-21-00 §5 (the cabin-distribution interfaces). Each specific threshold/value was first cited and sourced in its stage-A/B chapter. The five-bleed-uses grouping, the RPCU essential-bus significance across electrical-loss scenarios, the 11-chapter count, and the bidirectional propagation are integrative syntheses. All engineering detail is from the A330 knowledge base; no cross-type comparison is made, and no fleet tail numbers appear.

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.