APU layout click a component
Simplified — not for operational use. Click the turbine, ECB, inlet door, generator, or fire bottle.
Click any component above to see what it does →
Altitude limits — bleed vs. electrical
TWO SEPARATE LIMITS
The APU generator can typically be used at any altitude as an electrical backup, but APU bleed air use is restricted to lower altitudes — thinner air up high makes bleed output far less useful and puts more strain on the unit. Above that bleed-air cutoff, the APU can still run and supply electrics, it just can't feed the packs or assist an engine start.
Quick reference
STARTING
- Can start on battery power alone
- No external ground power or running engine required
- Inlet door opens automatically when start is selected
OUTPUTS
- Bleed air for packs and engine starts
- Electrical power via its own generator
- Can supply both simultaneously within limits
PROTECTION
- ECB monitors overspeed, over-temperature, low oil pressure
- Automatic shutdown if a protected limit is exceeded
- Dedicated fire bottle separate from the main engine system