Avionics Equipment Ventilation — AEVC, Blowing & Extraction

Ventilation Principles introduced the avionics bay as one of the ventilation objects with its own computer; Mixing & Recirculation showed the recirculation valves partly closing on a dual-pack failure to protect the avionics. This deep-dive opens up avionics ventilation itself: the Avionics Equipment Ventilation Computer (AEVC), the blowing sub-system, the extraction sub-system, the cooling-effect detector (CED), and the four operating configurations — including the DITCHING logic that is the reverse of the outflow valves.

1. Components — nine FINs + breakers

Breakers: 1HQ (AVNCS EXTRACT FAN), 4HQ (AEVC), 6HQ (AVNCS VENT CED), 8HQ (AVNCS VENT CTL).

2. The full chain

   BLOWING — air bled from the air-conditioning system, by one source:
     ① 2 cabin recirculation fans (normal)        ② 2 packs (if the fans fail)
     ③ 1 emergency ram-air inlet (dual-pack fail)  ④ 2 LP ground connectors (ground)
        ▼ via the recirculation path upstream of the mixer
     recirculated air filtered < 400 µm (better than ARINC 600)
        ▼
   AVIONICS BAY equipment cooled: rack 800VU (CPC / pack / zone controllers),
     fwd & weather-radar racks, ADIRU, main AC/DC power centres, battery,
     displays / FCU / pedestal, overhead, cockpit temp sensor
        ▼ hot air after the equipment
   EXTRACTION:
     CED 5HQ monitors cooling capacity (flow + temperature)
       → FWC (warning) → AEVC (maintenance) → mechanic call horn + AVNCS VENT 9HQ light
     extract fan 7HQ (three-phase 115/200 V AC, one speed, 155–165 °C thermal cutout)
     pressure switch 11HQ (fan outlet)
        ▼ AEVC selects the path
     underfloor valve 12HQ → fwd cargo underfloor → outflow valves → overboard
        OR
     overboard valve 13HQ → directly overboard

The EXTRACT pushbutton (10HQ) has AUTO (AEVC controls both valves) and OVRD (the overboard valve is partially open, the underfloor valve closed).

3. Blowing sub-system — four sources + the 400 µm filter

The required blowing airflow is bled from the air conditioning system by one of the following supply sources: 2 cabin recirculation fans, 2 air conditioning packs, 1 emergency ram air inlet, 2 LP ground connectors. Under normal conditions the system enables the required airflow to be supplied by both cabin recirculation fans. In case of failure of cabin recirculation fans, fresh air from the air conditioning packs is bled by a connection on the mixer unit. The recirculated air is filtered with a higher efficiency than ARINC 600 requirements (400 microns). — AMM 21-26-00 §3.B (1)

[!note]- The avionics use recirculation air from upstream of the mixer — not fresh pack air

Avionics ventilation ≠ cabin ventilation: it uses the recirculation air upstream of the mixer (filtered, already-used cabin air), not fresh pack air — except when the recirculation fans fail. Why: the recirculation air is already near the cabin target temperature (cool enough), and using fresh pack air would waste cooling capacity (the avionics need cooling, not fresh air). The 400 µm filter is better than the ARINC 600 requirement (the standard equipment-rack interface spec, including a minimum supply-air filtration).

4. Recirculation-valve behaviour

5. Extraction sub-system — fan + two valves + pressure switch

This system has these components: an extract fan, an underfloor extract valve, an overboard extract valve, an extract pressure switch. ... The fan has: one-speed motor (three-phase, 115/200VAC), power switches, a thermal switch that stops the electrical power supply to the induction motor if the temperature increases between 155 deg.C (311 deg.F) and 165 deg.C (329 deg.F). Energization of the aircraft electrical circuits (115VAC) directly controls the fan. — AMM 21-26-00 §6.B (1)

[!note]- The extract fan has no ON/OFF — it runs as soon as the aircraft is powered

Unlike the cabin fans (which the CAB FANS pb can switch off), the extract fan has no pilot ON/OFF. It starts the instant 115 V AC is available; the only stops are a full aircraft power-down or the 155–165 °C thermal cutout. The philosophy: the avionics must always be ventilated — no inadvertent crew shutoff is allowed. The cabin fan is a comfort item (switchable); the extract fan is an avionics-survival item (hard-wired on).

[!note]- The extract-fan cutout (155–165 °C) is lower than the cabin recirculation fan (180 °C)

The extract fan normally runs hotter (it draws the avionics' hot air), so a lower threshold lets the protection act before things go abnormal; and the consequence differs — a cabin-fan failure affects cabin humidity, an extract-fan failure affects avionics cooling (more sensitive). After the thermal cutout, the AEVC detects it → ECAM caution + the AVNCS VENT light (with the external horn on the ground).

6. The four operating configurations

In normal operation, fresh air is blown by the extract fan: On ground, engines not running: Through the OVBD extract valve (the underfloor extract valve is closed). In flight, or on ground, with engines running: Through the underfloor extract valve (the OVBD extract valve is closed). If OVRD is selected on the EXTRACT pushbutton, air is blown through the OVBD extract valve which is partially open (the underfloor extract valve is closed). When the DITCHING pushbutton is ON, the OVBD extract valve is closed and the underfloor extract valve is open, whatever the position of the EXTRACT pushbutton. — FCOM DSC-21-30-20

[!warning]- DITCHING closes the overboard valve and opens the underfloor valve — the reverse of the outflow valves

The outflow valves close on DITCHING ("close the below-the-line openings to keep water out"). The avionics extraction logic is reversed: the overboard valve (a fuselage-skin opening) closes (keep water out, consistent), but the underfloor valve (an internal duct toward the cargo underfloor) opens so the avionics' hot air can still flow after the overboard valve closes. Why: the avionics must keep ventilating — unlike the cabin, they cannot go briefly without air; stop the avionics cooling and they overheat within minutes, losing the aircraft's capability. So the DITCHING pb is not "close every below-the-line part" — it is "close the parts at water-ingress risk + keep the parts essential for internal flow". The underfloor valve is the latter.

7. Cooling-effect detector (CED) — thermistor + heater

The cooling effect detector is a duct-type detector. It provides an electrical signal when the cooling capacity of the blown air is abnormal. The cooling capacity is defined from both data: air temperature and air flow. The setting threshold of the flow/temperature complies with the ARINC 600 requirements (maximum continuous conditions). ... The cooling effect detector permanently monitors the cooling capacity of the blowing airflow by measuring the cooling of a heating unit. Measurement and heating are ensured by a thermistor and resistors. — AMM 21-26-00 §6.A

[!note]- The CED measures how much a heating unit is cooled — not airflow speed directly

Inside the CED a small heating unit is driven to a fixed power; a thermistor measures its temperature; the airflow carries heat away (cooling = flow × temperature difference); the heating-unit temperature changes → the thermistor output changes. High flow + low temperature = strong cooling → the unit stays cool; low flow or high temperature = weak cooling → the unit heats up. The electronics compare the unit temperature against the set threshold — exceeded = abnormal cooling capacity = alarm. The advantage: one part senses both flow and temperature together (no separate sensors), and it directly reflects "is the equipment being effectively cooled", not an indirect flow or temperature reading. The threshold follows ARINC 600 (maximum continuous conditions).

8. CED — three outputs (including the only external warning in ATA 21)

The cooling effect detector detects an abnormal cooling capacity and triggers an electrical signal for: the Flight Warning Computer (FWC) (warnings), the AEVC (maintenance), the mechanic call horn and the external warning AVNCS VENT caution light (9HQ). — AMM 21-26-00 §6.A (1)

[!warning]- The CED is the only ATA 21 part that triggers an external horn + external caution light

An avionics cooling failure overheats the equipment within minutes → loss of aircraft capability. In flight the crew see the ECAM — fine. But on the ground (maintenance working), the crew may not be in the cockpit → no one watches the ECAM. So the external horn + the AVNCS VENT 9HQ light (on panel 925VU outside the avionics bay, visible before a mechanic enters) tell the ground crew directly: "avionics ventilation abnormal — act now." This is the only "external warning hardware" in all of ATA 21 — reflecting the very high priority of avionics cooling.

9. AEVC — a digital computer

The AEVC is a digital computer which ensures warning and monitoring functions: (a) For the air conditioning compartment ventilation ... (b) For the avionics equipment ground cooling ... (c) For the avionics equipment ventilation. — AMM 21-26-00 §6.C (1)

[!warning]- The AEVC is a single-channel digital computer — not dual-channel like the pack / zone / cabin-pressure controllers

The pack controller, zone controller and CPC are all dual-channel "hot-backup". The AEVC is not. Why (integrative): an AEVC failure is less severe than a CPC failure (a CPC failure = cabin pressure out of control = life; an AEVC failure = the extract fan still runs on direct power, so the avionics still get basic cooling). On an AEVC failure the extract fan keeps running, and the valve positions hold at the last command or the manual OVRD. The cost is the loss of precise control (no automatic configuration switching) — but basic ventilation continues. The AEVC is the "precise control + monitoring + BITE" layer; lost, it can still work coarsely.

[!note]- The AEVC reports class-2 faults — shown on the ECAM after flight

Fault classes (general avionics BITE): class 1 affects the flight in real time (immediate ECAM in flight); class 2 does not affect the current flight but needs maintenance (shown on the ECAM after landing); class 3 is recorded in the CMS only. Most AEVC faults are class 2 (no in-flight distraction, post-flight troubleshooting). But the CED-triggered abnormal cooling is a real problem — it generates a class-1 warning via the separate FWC path (real-time ECAM).

10. AEVC BITE — four modes

The BITE can operate: during aircraft energization, permanently, when controlled by the CMC, during line maintenance. These tests are performed on the ground after a power failure longer than 5 seconds, in the sequence specified below: Central Processing Unit (CPU) test, Random Access Memory (RAM) test, Erasable Programmable Read Only Memory (EPROM) test, Discrete input test, Outputs test (discrete and power outputs), Cooling effect detector test, Pressure switch test. — AMM 21-26-00 §7 + §8.A

11. Two cabin-fan configurations

TWO CABIN FANS. Two electric fans operate continuously, as long as the aircraft's electrical system is supplied. (one configuration) TWO CABIN FANS. Two electric fans operate, one is active and the other is in standby ... (another configuration) — FCOM DSC-21-30-20

[!note]- The cabin-fan operation differs by configuration — affecting the recirculation flow

On some aircraft both fans run together (dual-fan flow = 2× a single fan); on others one runs and one is on standby (dual-fan flow = 1× a single fan, the standby not contributing). Both configurations cut off via the CABIN FAN pb. Avionics-cooling implication: with the active/standby configuration a single-fan failure switches to the standby (little flow change); with the both-running configuration a single-fan failure halves the flow (slightly weaker avionics cooling). The configuration is read directly from the FCOM for the specific aircraft.

12. ECAM, fault classes, dual-fan failure & dispatch

ECAM AVNCS VENT FAULT: the CED detects abnormal cooling → FWC → E/WD warning; the typical procedure checks the CABIN FAN pb (not inadvertently off), the EXTRACT pb (not inadvertently OVRD), and the packs (not a dual-pack failure). On a dual cabin-fan failure, the AEVC switches the blowing source to fresh pack air via the mixer connection, the extract fan keeps running, and the avionics still get basic cooling.

MMEL 21-26-02A / B / C AVIONICS OVERBOARD EXTRACT VALVE — AMM 21-26-00-040-801-A

Dispatch with the overboard extract valve (13HQ) inoperative is addressed by three MMEL branches (e.g. stuck fully open / stuck not fully open / inoperative with operational limitations) — the specific conditions are in the operator MMEL.

Self-test

[!note]- Q1. Is the AEVC a dual-channel controller like the pack/zone/cabin-pressure controllers?

No — a single-channel digital computer. Why: an AEVC failure is less severe (the extract fan still runs on direct power, so the avionics keep basic cooling; the crew can also force OVRD on the EXTRACT pb). Dual-channel would add complexity/cost; single-channel + the four BITE modes is sufficient. The cost is the loss of precise control (no automatic configuration switching) — but basic ventilation continues.

[!note]- Q2. When does each of the four blowing sources work?

Normal: the 2 cabin recirculation fans (recirculation air from upstream of the mixer, near cabin temperature, 400 µm filtration better than ARINC 600). Dual cabin-fan failure: 2 packs via the mixer connection. Dual-pack failure: the emergency ram-air inlet (with the recirculation valves partly closing to favour the avionics). Ground: the 2 LP ground connectors. The four sources cascade by availability automatically — no crew action.

[!note]- Q3. Why does DITCHING close the overboard valve and open the underfloor valve — the reverse of the outflow valves?

The outflow-valve DITCHING philosophy is "close the below-the-line openings to keep water out". The avionics logic is reversed: the overboard valve (a skin opening) closes (keep water out, consistent), but the underfloor valve (an internal duct) opens so the avionics' hot air can still flow. Why: the avionics must keep ventilating — stop their cooling and they overheat within minutes. So DITCHING is "close the water-ingress-risk parts + keep the parts essential for internal flow"; the underfloor valve is the latter.

[!note]- Q4. How do the thermistor + heater measure cooling capacity?

A small heating unit runs at a fixed power; a thermistor measures its temperature; the airflow carries heat away (cooling = flow × ΔT). High flow + low temperature → the unit stays cool; low flow or high temperature → it heats up; the electronics compare against the threshold → exceeded = abnormal cooling. Advantage: one part senses both flow and temperature, directly reflecting "is the equipment effectively cooled". The threshold follows ARINC 600 (maximum continuous conditions).

[!note]- Q5. Why does the CED trigger an external horn + caution light — the only such part in ATA 21?

Because an avionics cooling failure overheats the equipment within minutes → loss of aircraft capability. The ECAM works for the crew in the cockpit, but on the ground the crew may be absent → no one watches it. The external horn + the AVNCS VENT 9HQ light (outside the avionics bay) tell the ground crew directly. This is the only external warning hardware in all of ATA 21 — reflecting the high priority of avionics cooling.

Key takeaways

Common misconceptions

Scope — what this deep-dive covers and defers

References

A330 specifics per FCOM DSC-21-30-20 (the system, the two cabin fans in their two configurations, the recirculation/extract components, the four operating configurations including the DITCHING logic, and the dual-cabin-fan-failure pack backup) and AMM 21-26-00 §1/§2/§3/§6/§7/§8 + §040-801-A (the cooled-equipment list, the nine FINs and four breakers, the blowing sub-system with its four sources and 400 µm filter better than ARINC 600, the extraction sub-system with the three-phase 115/200 V AC one-speed fan and its 155–165 °C thermal cutout / the underfloor + overboard valves / the pressure switch, the duct-type cooling-effect detector with its thermistor + heater and its three outputs to the FWC / AEVC / mechanic-call-horn + AVNCS VENT 9HQ light, the AEVC as a single-channel digital computer with discrete output to the SDACs for class-2 faults and ARINC 429 to the CMC, the four BITE modes and the seven-test energization sequence after a > 5 s power loss, and MMEL 21-26-02 for the overboard extract valve). The single-channel-vs-dual-channel rationale, the DITCHING reverse-logic reasoning, the thermistor/heater physics, the no-ON/OFF "avionics-survival" categorisation, and the fault-class definitions 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.