Ignition System — Dual Independent Circuits, Alternating Igniters

Three-Spool Architecture noted two igniter chutes in the combustor's outer liner. This article opens that ignition system: the three jobs it does, the two redundant circuits, how the FADEC alternates between the two igniters, and when continuous ignition cuts in. It is the basis for start faults, relight and flameout prevention.

1. Three functions, two circuits

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.

Per DSC-70-80-20: three jobs — ground start, in-flight relight, flameout prevention — done by two identical independent circuits per engine, normally on FADEC channel A (B on standby), and either channel can drive both igniters.

2. Dual circuits — supply and cross-control

Per the AMM igniter schematic:

 AIRCRAFT 115VAC EMERGENCY BUS ─► PCU-A ─► IGNITER BOX A ─► IGNITER PLUG A (2.7-3 KV)
                                    ▲
                   EEC-A ─DEMAND─┐  │  (relay cross-control)
                   EEC-B ─DEMAND─┴──┤
                                    ▼
 AIRCRAFT 115VAC NORMAL BUS ─────► PCU-B ─► IGNITER BOX B ─► IGNITER PLUG B (2.7-3 KV)

• Two independent circuits: A (Emergency bus → PCU-A → Igniter Box A → Plug A), B (Normal bus → PCU-B → Igniter Box B → Plug B).
• EEC-A and EEC-B (the FADEC's two channels) cross-control both igniter boxes via DEMAND signals — confirming "either channel can drive both igniters."
• Igniter-plug discharge 2.7–3 KV.

Confirmed by AMM:

The ignition power supply includes two ignition units. The top ignition unit is system B and the bottom ignition unit is system A. Each unit has a case assembly and an ignition exciter.

Two inputs of 115VAC 400Hz are supplied by the aircraft (Normal to channel B of PCU and Emergency to channel A of PCU).

[!note]- Why circuit A is on the Emergency bus and B on the Normal bus (integrative synthesis) System A is fed from the Emergency bus, system B from the Normal bus — putting the two circuits on different busbars means a single busbar loss still leaves one ignition circuit available. This reads the verbatim busbar allocation (AMM-74-10); the availability rationale is engineering reasoning.

3. Start-ignition logic (ground)

ON GROUND During a first automatic start attempt only one igniter is supplied. The FADEC automatically alternates the igniters used in successive start sequences. When residual EGT is above 100 °C, the engine is cranked until the EGT goes below 100 °C.

Note: 1. In the case of a second automatic start attempt after the failure of one igniter to light up, both igniters are supplied when the EGT goes below 150 °C.

Per DSC-70-80-20: a first automatic start supplies one igniter only, the FADEC alternating igniters across successive starts; with residual EGT above 100 °C the engine is cranked until EGT < 100 °C; on a second attempt after a light-up failure, both igniters are supplied once EGT < 150 °C.

[!note]- The logic behind three start numbers (integrative synthesis across verbatim) ① First attempt, one igniter (enough, and it spares igniter life) → ② second attempt, two (raise the light-up probability) → ③ residual EGT > 100 °C, crank first to < 100 °C (clear residual fuel/heat, prevent a hot start — see Start Faults); the second-attempt threshold relaxes to 150 °C. The three numbers (100/100/150 °C) are DSC-70-80-20 verbatim; the chaining is the reasoning.

4. Continuous ignition

Continuous ignition is either selected manually or automatically to protect engine combustion. MANUAL SELECTION 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. AUTOMATIC SELECTION Continuous ignition (igniters A + B) is automatically selected: ‐ If engine anti-ice is selected on and relevant EIU/EIVMU is inoperative…

Per DSC-70-80-20, continuous ignition guards combustion in high-risk phases (icing, turbulence, take-off/landing). Manual: in flight, ENG START to IGN/START with the engine running — one igniter (both if it fails); on ground after start, NORM then back to IGN/START. Automatic (A + B): when engine anti-ice is on and the relevant EIU/EIVMU is inoperative, among others.

5. FADEC control and EIVMU failure

The FADEC controls and monitors the ignition and starting system... In normal operation, the FADEC receives its inputs from the EIVMU/EIU. 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.

Per DSC-70-80-10, on an EIVMU/EIU signal loss everything remains available except manual start and wet crank, the FADEC running on the ENG MASTER backup signal. (Compare 02, where DSC-70-80-40 names the loss of manual start; here DSC-70-80-10 adds wet crank.)

6. Inadvertent-shutdown auto-relight

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.

Per DSC-70-80-30, an inadvertent ENG MASTER OFF-then-ON triggers an automatic FADEC relight, regardless of selector position.

7. Counterintuitive points

[!warning]- A start fires one igniter, alternated each time Not both at once — a first automatic start supplies one igniter, the FADEC alternating between them (DSC-70-80-20). Two are supplied only on a second attempt (EGT < 150 °C). Purpose: enough, and even igniter wear.

[!warning]- Residual EGT > 100 °C cranks first, doesn't fire Under hot-start risk, residual EGT > 100 °C cranks to < 100 °C before ignition (DSC-70-80-20) — the hot-start defence, directly tied to Start Faults.

Self-test

[!note]- Q1. The ignition system's three functions? Ground start (automatic + manual), in-flight relight, flameout prevention.

[!note]- Q2. How many ignition circuits per engine, and who controls them? Two identical independent circuits, normally FADEC channel A (B standby), either channel driving both igniters.

[!note]- Q3. How many igniters on a first ground start, and why alternate? One only, the FADEC alternating across successive starts (even wear). Two on a second attempt once EGT < 150 °C.

[!note]- Q4. What if residual EGT is high? The threshold? Residual EGT > 100 °C → crank to < 100 °C before ignition (hot-start defence).

[!note]- Q5. Igniter-plug voltage, and which busbar feeds each circuit? 2.7–3 KV; circuit A Emergency bus → PCU-A → Box A → Plug A, circuit B Normal bus → PCU-B → Box B → Plug B.

Key takeaways

References

• FCOM DSC-70-80-10 — FADEC controls ignition/starting; EIVMU loss removes manual start and wet crank.
• FCOM DSC-70-80-20 — three functions; two circuits (channel A control, B standby); start EGT 100/150 °C; continuous ignition (manual + automatic).
• FCOM DSC-70-80-30 — inadvertent OFF→ON triggers automatic relight.
• AMM 74-10-00 §1 — two 115VAC 400Hz inputs (Normal → channel B, Emergency → channel A).
• AMM 74-11-00 §1.A — two ignition units (top B, bottom A) with ignition exciters.
• AMM igniter schematic — dual-circuit supply, EEC-A/B cross-control, plugs 2.7–3 KV.

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.