Starting — Altitude-Dependent Target Speed, Clutch Auto-Disengage, the Start Envelope
The ignition lights it; the starter spins it up. This article covers the start: the altitude-dependent target speed, the ECB-sequenced motor/clutch, and the start envelope the crew works to.
The starting system turns and accelerates the main shaft of the APU ‐ up to 25000 ft. to 50 % APU speed, ‐ above 25000 ft. to 54.5 % APU speed ‐ in cruise (above 31000 ft.) to 65 % APU speed... The ECB 59KD controls the start sequence. Three consecutive start attempts are permitted without cooldown. After the third start attempt, the starter motor must cool down for at least 60 minutes. — AMM 49-42-00
1. The target speed rises with altitude
Per AMM 49-42-00, the starter accelerates the shaft to a target speed that depends on altitude:
| Altitude | Starter target speed |
|---|---|
| up to 25000 ft | 50 % |
| above 25000 ft | 54.5 % |
| in cruise (above 31000 ft) | 65 % |
[!warning]- Higher altitude needs a higher starter cut-out speed The starter spins the APU to a higher speed before releasing as altitude increases (50 → 54.5 → 65 %, AMM 49-42-00). Thinner air means the APU needs more rotational speed to become self-sustaining — so the starter has to carry it further up the speed range before combustion can take over. This is why a high-altitude start is the demanding case (matched by the high-energy ignition and the cold-soak-tolerant load-compressor bearing).
2. The start sequence
When you operate the starting system the ECB 59KD energizes the main start contactor and the backup start contactor. Thus the 28 V DC is supplied to the starter motor. The starter motor turns the APU main shaft (through the starter clutch module and the gearbox)... The timed acceleration logic of the ECB 59KD controls the APU acceleration during the APU start. The timed acceleration logic refers to... ‐ the air inlet temperature, ‐ the altitude, ‐ the speed (rpm), ‐ the Exhaust Gas Temperature (EGT).
Per AMM 49-42-00, the ECB energises main + backup start contactors → 28 V DC to the starter motor → turns the main shaft via the clutch + gearbox. The ECB's timed acceleration logic schedules the run-up against air inlet temperature, altitude, rpm, and EGT — so the start adapts to conditions (a hot/high start is paced differently from a cold/low one).
3. Starter motor and clutch
The starter motor is a series-wound DC motor... totally enclosed and explosion-proof with an overspeed protection... a yellow brush-wear indicator-pin... For the manual drive of the APU main shaft, a manual drive shaft is installed on the front of the starter motor.
...there is a starter clutch which makes the mechanical connection... The starter clutch module disengages mechanically when the self sustained APU speed is more than the starter motor speed.
Per AMM 49-42-00: the starter motor (8KA) is a series-wound DC motor, totally enclosed + explosion-proof (it sits by the fire zone) + overspeed-protected, with a brush-wear indicator (yellow pin visible = brushes serviceable) and a manual drive shaft. The starter clutch module disengages mechanically once the APU's self-sustained speed exceeds the starter-motor speed — automatic, no command.
[!note]- The clutch frees itself the instant the APU outruns the starter (integrative synthesis) Like the main-engine starter's centrifugal clutch, the APU starter clutch disengages the moment the APU accelerates past the starter (AMM 49-42-00) — protecting the motor from being back-driven. Combined with the 3-attempt / 60-min cooldown, this manages the starter's thermal and mechanical limits.
4. The start envelope and limits
APU start is permitted throughout the normal flight envelope, except when APU battery only is supplying (Refer to LIM-APU Operational Envelope). (DSC-49-10-10)
Per DSC-49-10-10, APU start is allowed throughout the normal flight envelope — except on APU battery only, where the envelope is restricted (LIM-APU). Power: 28 V DC from the APU BAT BUS (via fuse 6KA + contactors) to the motor, and the ESS BUS (via CB 1KA) to the ECB.
[!warning]- Battery-only restricts the start envelope — and three attempts then 60 min A normal start works across the flight envelope, but battery-only narrows it (DSC-49-10-10 / LIM-APU) — relevant in an all-electrics-down scenario where the APU is the recovery source (ATA-70-31). And after three consecutive attempts, the starter motor needs ≥ 60 min cooldown (AMM 49-42-00) — distinct from the 3-min fuel-line repressurise wait between attempts.
5. Counterintuitive points
[!warning]- The starter cut-out speed climbs with altitude (50 → 54.5 → 65 %) Thinner air → the APU needs more speed to self-sustain → the starter carries it higher before release (AMM 49-42-00).
[!warning]- Two different "waits": 3 min (fuel) vs 60 min (starter) Between failed attempts, wait 3 min for fuel-line pressure (04); after three attempts, wait 60 min for the starter motor to cool (AMM 49-42-00). Different causes, different timers.
Self-test
[!note]- Q1. The altitude-dependent starter target speeds? 50 % up to 25000 ft / 54.5 % above 25000 ft / 65 % in cruise (above 31000 ft).
[!note]- Q2. When does the starter clutch disengage? Mechanically, when the self-sustained APU speed exceeds the starter-motor speed — automatic.
[!note]- Q3. What does the ECB's timed acceleration logic use? Air inlet temperature, altitude, rpm, EGT.
[!note]- Q4. The two waits, and the battery-only caveat? 3 min (fuel re-pressurise) between attempts; 60 min after three attempts (starter cooldown). Start is allowed across the envelope except battery-only (restricted, LIM-APU).
Key takeaways
| Point | Detail |
|---|---|
| Target speed | 50 % ≤25000 ft / 54.5 % >25000 ft / 65 % cruise (rises with altitude) |
| Sequence | ECB → main+backup contactors → 28 V DC starter motor → clutch + gearbox; timed accel logic (inlet temp/alt/rpm/EGT) |
| Starter motor (8KA) | series-wound DC, explosion-proof, overspeed-protected, brush-wear indicator, manual drive shaft |
| Clutch | disengages mechanically when APU self-sustained speed > starter speed |
| Limits/envelope | 3 attempts → 60 min cooldown; start across envelope except battery-only (restricted) |
References
- AMM 49-42-00 (Starting — Description and Operation) — altitude-dependent target speed (50/54.5/65 %); ECB + main/backup contactors + 28 V DC starter motor → clutch + gearbox; timed acceleration logic (inlet temp/altitude/rpm/EGT); series-wound explosion-proof starter motor (brush-wear indicator, manual drive shaft); clutch auto-disengage when self-sustained speed > starter speed; 3 attempts → 60-min cooldown; APU BAT BUS / ESS BUS power.
- FCOM DSC-49-10-10 — APU start across the normal flight envelope except battery-only (LIM-APU).
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.