Ignition — High-Energy Dual System, Start 7–50 %, Flameout-Relight 95–50 %
The fuel system delivers the fuel; this article gives the spark. The APU ignition is a high-energy dual system that lights the start and, crucially, re-lights automatically if the APU droops in normal operation — a built-in flameout guard.
The ignition system gives the constant high-energy ignition which ignites the fuel/air mixture... It operates during the start sequence until the APU speed is between 7 % and 50 %. It also operates during the APU operation if the APU speed decreases to between 95 % and 50 %. The ECB 59KD controls the ignition system. — AMM 49-41-00
1. When ignition fires — two windows
Per AMM 49-41-00, the ECB drives ignition in two distinct windows:
- Start: power to the exciter at 7 % APU speed, removed at 50 % (once self-sustaining).
- In operation: if APU speed drops to between 95 % and 50 % (and no shutdown is commanded), the ECB re-energises ignition — an automatic relight.
[!warning]- Ignition re-lights itself if the running APU droops to 95–50 % — a flameout guard The headline behaviour: during normal running, if the APU speed sags into the 95–50 % band, the ECB automatically turns ignition back on (AMM 49-41-00) to prevent/recover a flameout — no crew action. Conceptually like the main engine's auto continuous ignition on a detected flameout, but keyed here to a speed droop. Below 50 % the logic treats it as a failed run (shutdown territory, 13).
2. The components — exciter, two leads, two plugs
...a minimum ignition spark energy of 1 joule. It is a dual system with two igniter plugs. The main components are: ‐ the ignition exciter, ‐ the ignition leads, ‐ the igniter plugs. The ignition exciter is installed on the lower RH side of the APU in a hermetically sealed container.
...A transformer supplies 4 joule nominal stored energy at 18 kV high-voltage to the ignition leads. The primary circuit regulation makes sure of a minimum spark rate of 2 sparks per second.
Per AMM 49-41-00, it is a dual system, two igniter plugs, with at least 1 joule spark energy. The exciter (59KA10) — in a hermetically sealed container (moisture/altitude protection) — transforms 28 V DC into high voltage: 4 joule nominal stored energy at 18 kV, with a guaranteed minimum 2 sparks/second across all temperatures and input voltages. Two teflon-insulated leads (A, B) carry it to the plugs.
[!note]- Dual plugs + high energy = reliable light in a tailcone start (integrative synthesis) Two igniter plugs (redundancy) + 4 joule / 18 kV / ≥ 2 sparks per second (AMM 49-41-00) give a robust light despite altitude, cold and moisture — important because the APU often starts at altitude or after cold-soak, when ignition is hardest. The sealed exciter container guards the high-voltage electronics from exactly those conditions.
3. Power and interfaces
The ignition system gets the electrical power from the DC system of the aircraft. It supplies 28 V DC from the APU BAT BUS 309PP through the circuit breaker 1KD (ECB SUPPLY) to the ECB.
Per AMM 49-41-00, power is 28 V DC from the APU BAT BUS (via the ECB supply) — so ignition is available on battery alone, the basis for a battery-only start (06). It interfaces with the EIS/SD APU page, the CMS, the ECB and the emergency shutdown.
4. Counterintuitive points
[!warning]- The running APU re-lights itself on a speed droop (95–50 %) Not just a start function — ignition comes back on automatically if a running APU droops to 95–50 % (AMM 49-41-00), guarding against flameout with no crew action. Below 50 % it is treated as a failed run.
[!warning]- High energy + dual plugs for the hardest lights 4 joule / 18 kV / ≥ 2 sparks per second, two plugs (AMM 49-41-00) — sized for altitude / cold-soak starts where ignition is hardest, and battery-powered (28 V DC).
Self-test
[!note]- Q1. The two ignition windows? Start: on at 7 % APU speed, off at 50 %. In operation: back on if speed droops to 95–50 % (flameout guard).
[!note]- Q2. The system's energy and redundancy? Dual system, two plugs; ≥ 1 joule spark; exciter 4 joule / 18 kV nominal, ≥ 2 sparks/second.
[!note]- Q3. Why a hermetically sealed exciter, and what powers it? Sealed against moisture/altitude; powered by 28 V DC from the APU BAT BUS (battery-only start capable).
[!note]- Q4. What happens below 50 % in operation? Treated as a failed run → shutdown territory (13), not re-ignition.
Key takeaways
| Point | Detail |
|---|---|
| Windows | start 7 % on / 50 % off; running 95–50 % auto re-light (flameout guard) |
| System | dual, two igniter plugs, ≥ 1 joule |
| Exciter (59KA10) | hermetically sealed; 4 joule / 18 kV, ≥ 2 sparks/s; two teflon leads (A/B) |
| Power | 28 V DC APU BAT BUS (battery-only start capable) |
References
- AMM 49-41-00 (Ignition — Description and Operation) — constant high-energy dual system (two plugs, ≥ 1 joule); start 7 %→50 %, in-operation re-light 95–50 %; exciter 59KA10 (sealed, 4 joule / 18 kV, ≥ 2 sparks/s); two teflon leads; 28 V DC APU BAT BUS power.
- FCOM DSC-49-10-20 — ECB controls the ignition (overview).
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.