Ignition System

"The ignition system ensures: ‐ Engine start on ground (automatic and manual engine starts) ‐ Engine relight in flight ‐ Flameout prevention. It consists of two identical independent circuits for each engine, normally controlled by the FADEC's channel A, with channel B on standby. Each FADEC channel can control both igniters."

The hardware is almost minimal — two identical "exciter–lead–plug" chains per engine. Everything sophisticated is in the command logic: when to fire one igniter, when to fire two, and when the system fires them without being asked. The body of this article is that decision table.

1. The signal architecture — and two back doors

 ENG MASTER 1     ENG START selector      ENG MAN START pb
      │          (CRANK/NORM/IGN START)         │
      ├──────────────────┬─────────────────────┤
      ▼                  ▼                     ▼
 ┌───────────────────────────────┐   ┌───────────────────────┐
 │        EIVMU 1 (primary path) │   │ ALTERNATE START &      │
 └───────────────┬───────────────┘   │ IGNITION LOGIC         │
                 ▼                   └──────────┬────────────┘
 ┄┄ MASTER hardwired backup signal ┄┄┐          │
                 ▼                   ▼          ▼
 ┌──────────────────────────────────────────────┐
 │            FADEC 1   (CH A ┊ CH B)            │
 └──────┬──────────────────┬────────────────────┘
        ▼                  ▼
   START VALVE        IGNITERS (systems A + B)

Three reading points from the architecture diagram. The three cockpit controls feed the EIVMU as the primary path — but alongside it sit an alternate start & ignition logic box and a hardwired backup signal from the MASTER lever running straight to the FADEC. That pair of back doors is the diagram's proof of an FCOM sentence worth quoting whole:

"In the event of an EIVMU/EIU signal loss, all the functions, except manual start and wet crank, will remain available by using both a backup signal from the ENG MASTER lever, and the alternate start/ignition signal."

Lose the EIVMU and you lose exactly two things — manual start and wet crank; automatic starting, dry crank and continuous ignition all survive through the back doors. The third reading point: both the start valve and the igniters are drawn with dual-channel arrows — either FADEC channel can reach both.

The hardware, system by system (A and B are mirror images)

Two details deserve a pause. The power supplies are deliberately split: the PCU feeds ignition system A from the emergency 115 V AC supply and system B from the normal supply, with EEC-commanded relays able to cross-connect — so in an emergency electrical configuration, at least system A still has its spark (article 33). The wiring schematic sharpens this to the busbar level: IGN SYS A's circuit breaker hangs on the static-inverter bus — the emergency 115 V produced from the batteries — while system B hangs on a normal AC bus. System A's "emergency credential" runs all the way down to the busbar: with every generator gone and only batteries left, the A plug can still fire. The schematic also shows that the EEC exchanges both a drive line and a status line with each ignition unit — ignition is commanded and its execution is reported back, the feed for minor-fault-level monitoring (article 19). And the semiconductor surface-discharge plug is chosen because energy sliding across a semiconductor surface produces a fat, fouling-tolerant spark — oil film and moisture do not silence it the way they would an air-gap plug (synthesis). Positions 10 and 16 put the two flame sources roughly opposite each other on the annular combustor, so flame propagates around the ring from both sides.

2. Start ignition: one is enough, two stand ready

"During a first automatic start attempt only one igniter is supplied. The FADEC automatically alternates the igniters used in successive start sequences."

Single-igniter starting is life management (synthesis): one 8–12-joule spark train lights atomised fuel with margin to spare, so the second plug is held in reserve and the two alternate start by start, sharing the wear. The escalation to both igniters follows a fixed pattern:

A second temperature gate governs hot restarts:

"When residual EGT is above 100 °C, the engine is cranked until the EGT goes below 100 °C."

Keep the two gates apart: 100 °C is the cool-down gate before ignition is introduced; 150 °C is the power-up gate for the second attempt's dual ignition. The common logic behind every dual-igniter case: where the margin for error is small — in flight, manually, or on a second try — the system does not gamble on one plug.

The AMM completes "alternating" into a double rotation — not just the igniters but the power supplies rotate too:

"The EEC alternates power supply and ignition to avoid hidden failures on the total system."

Four starts walk through every combination of two supplies × two igniters (synthesis): if a supply path or an igniter has quietly died, it must reveal itself within four starts at most — routine operation standing in for a dedicated test. The physical allocation behind the supplies: of the aircraft's two 115 V AC inputs, Normal feeds PCU channel B and Emergency feeds channel A (article 04).

3. Continuous ignition: manual fires one, automatic fires both

"In flight, continuous ignition is selected, when the ENG START selector is on IGN/START, provided the assigned engine is running. Only one igniter is selected. If failed, both igniters are automatically selected. On ground, after starting, since ignition is automatically cut off, to select continuous ignition, turn the ENG START selector to NORM, then back to IGN/START."

The ground "detour" — out to NORM and back to IGN/START — is the same gesture language as the thrust-lever rating change of article 08: the system recognises a departure and return as a fresh instruction. Automatic continuous ignition, by contrast, always uses both plugs:

"Continuous ignition (igniters A + B) is automatically selected: ‐ If engine anti-ice is selected on and relevant EIU/EIVMU is inoperative ‐ For 10 s, in the event of engine flame out condition detected by FADEC on ground or in flight (auto relight function) ‐ For 10 s in the event of inadvertent cycling of the Master lever with the engine running provided N3 is above 50 %."

Each trigger has its own story. The first is self-protection when the messenger is down: icing conditions call for continuous ignition, but a failed EIVMU cannot relay your selection — so the system grants it unbidden. The second is the cockpit face of the auto-relight function from article 05 — and the AMM supplies its detection criterion: the EEC monitors the rate of change of N3 at idle against a minimum datum computed from P30; below the datum, flameout is declared and both igniters fire continuously until the condition corrects, plus ten seconds — once again the fuel-follows-P30 family, P30 being combustion's most honest witness. The third is the lightweight cousin of quick relight: N3 still above 50 %, the flame not yet fully dead — ten seconds of spark simply rescues it in place.

4. Windmilling quick relight: automatic redemption for an inadvertent MASTER OFF

"In case of inadvertent engine shutdown by cycling the Engine Master lever to OFF then ON, the FADEC will attempt automatically a relight regardless of the rotary selector position."

Assembled with the control-law detail of article 05, the full picture: a window of 30 seconds with N3 above 10 %, the usual start checks skipped, the rotary selector ignored, fuel and ignition immediately — and no automatic cancellation if the fuel fails to light (stopping it is the crew's job). Note the directionality: an accidental OFF-then-ON is rescued automatically; a deliberate OFF that stays OFF is not — relay R2 in the EIVMU cuts ignition power on MASTER OFF (article 06), the gate on the deliberate-shutdown side.

5. The selector's three positions, and the MASTER's absolute veto

"CRANK position: no ignition system is supplied but it is possible to perform engine dry and wet motoring. NORM position: … with engines running, the EEC automatically selects continuous ignition under the following condition: in-flight flameout detected. IGN START position: … normal autostart sequence / normal manual start sequence / manual control of continuous ignition."

The NORM clause is the reassurance worth underlining: returning the selector to NORM does not stand down the protection — auto relight remains on watch in every selector position (it is a standing function armed by MASTER ON plus a successful start). CRANK's guaranteed no spark anywhere is precisely the safety precondition for wet motoring (article 12). And above everything sits the MASTER:

"With ENG/MASTER switch in OFF position: the HP fuel shut off valve is closed, the engine is shutdown. This position of the ENG/MASTER switch overrides any FADEC function."

6. Where ignition meets the failure chapters

Self-test

[!note]- Q1. Where are the igniter plugs, and what type are they? Beside fuel nozzles No. 10 and No. 16 — roughly opposite sides of the annular combustor. Surface-discharge type with semiconductor-filled gaps, fed by capacitor-discharge exciters delivering 2.7–3.0 kV, 8–12 joules, at ≥ 60 sparks/min — fat sparks that fire through oil film and moisture.

[!note]- Q2. Today's first start used plug A. Who fires next start — and when do both fire together? The FADEC alternates start to start — next start is B (and the AMM shows the power supplies rotate too, EMER/NORM, so four starts exercise every combination and expose any hidden failure). Both plugs fire on: a second attempt (once EGT < 150 °C), any manual start, any in-flight start.

[!note]- Q3. Restarting five minutes after shutdown with 180 °C residual EGT — what does the sequence do? It cranks first, holding off ignition until EGT falls below 100 °C — the cool-down gate. (Keep it distinct from 150 °C, which gates the second attempt's dual ignition.)

[!note]- Q4. Name the three cases where continuous ignition arrives without being selected. ① Engine anti-ice ON with the associated EIVMU inoperative; ② FADEC-detected flameout, for 10 s (auto relight — N3 decay rate vs a P30-computed datum); ③ inadvertent MASTER cycling with the engine running and N3 > 50 %, for 10 s. All three fire both igniters.

[!note]- Q5. EIVMU 1 has failed completely. Can engine 1 still be started? Yes — through the MASTER hardwired backup signal plus the alternate start/ignition logic. The only functions lost are manual start and wet crank.

Key takeaways

References

• FCOM DSC-70 (ignition/start architecture and controls) — system roles, two-circuit architecture, EIVMU-loss survivals, single/dual igniter logic, the 100/150 °C gates, manual and automatic continuous ignition, quick relight.
• AMM 74-11 / 74-21 (exciters, leads, plugs, D/O) — output figures, mounting, surface-discharge construction, plug positions.
• AMM 74-31 (ignition starting and continuous relight, D/O) — supply-and-igniter double rotation table, auto-relight N3/P30 criterion.
• AMM 74-10 (ignition power supply, D/O) — Normal→PCU-B / Emergency→PCU-A allocation.
• AMM 74-30 (controls, D/O) — selector position semantics, MASTER override.
• ASM (ignition wiring schematic, read directly) — busbar-level supply split (static-inverter vs normal bus), drive/status signal pairs.
• Integrative synthesis (marked in text): the life-management reading of single-igniter starts; the four-start full-combination observation; the fouling-tolerance rationale of surface-discharge plugs; the small-margin logic of dual ignition.

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.