Overview
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Replace with overview of APU operating envelope, bleed/electric load capability, and start sequence.
The 747-400 APU is a Honeywell GTCP331-200 located in the tailcone. It provides electrical power (one 90 kVA generator, same rating as the engine IDGs) and bleed air for pack operation and engine start. Operating envelope is on the ground and in flight to maximum altitude with electrical-only; bleed extraction is limited to a lower altitude.
Architecture
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<APUSchematic /> — air intake, compressor, combustor, turbine, generator, bleed load valve.Components
| APU Model | Honeywell GTCP331-200 |
| Generator | 1 × 90 kVA, 115V AC, 400 Hz |
| Bleed Output | Sufficient for 2-pack operation or 1 engine start |
| Fuel Source | Main Tank 2 (primary) via APU fuel pump |
| Start Source | Battery (cold start) or external power |
| Fire Detection | Single loop (with auto-discharge on ground) |
Operating Envelope
- Electrical only: Up to maximum operating altitude
- Bleed extraction: Typically limited to ≤ 17,000–20,000 ft (verify per FCOM)
- Start in flight: Up to ≈ 25,000 ft
Start Sequence
- APU switch — START (momentary)
- RPM accelerates; fuel ignites at ~7% RPM
- Generator available at 95% RPM
- Bleed available shortly after rated speed
Failures & Abnormals
MEMORY ITEM — APU Fire
- APU fire handle — PULL
- Rotate handle — DISCHARGE
Limits
- EGT during start: monitor per FCOM red line
- Generator load: 90 kVA max
Notes / Personal
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User notes.