Which pair of APU-related limits is correct?

• A. APU Start up to Flight Level 370; APU can start a Main Engine up to Flight Level 210
• B. APU Start up to Flight Level 210; APU can start a Main Engine up to Flight Level 370
• C. APU Start up to Flight Level 390; APU can start a Main Engine up to Flight Level 250
• D. APU Start up to Flight Level 320; APU can start a Main Engine up to Flight Level 290

Answer: (A)

APU functionality during flight has two different altitude limits: one for starting the APU itself and another for starting the main engines using APU bleed air. The APU can be started and used for bleed air/power up to a high altitude (Flight Level 370), but starting a main engine with APU bleed air is more demanding and is limited to a lower altitude (Flight Level 210). This reflects the fact that providing enough bleed air to reliably start an engine becomes challenging as air density and pressure drop with altitude. So the correct pairing is that the APU can start up to FL370, while it can start a Main Engine up to FL210.