When does the system switch from LP air to HP air?

When does the system switch from LP air to HP air?

• A. When the system needs more flow or volume, typically higher altitude or reduced power settings
• B. When engine RPM exceeds a threshold
• C. Only on the ground
• D. Never; always LP

Answer: (A)

Air supply to the packs switches from low‑pressure bleed to high‑pressure bleed when the demand for conditioned air exceeds what the LP source can provide. In practice this happens as you climb or when you reduce thrust: LP bleed pressure falls with altitude and lower power, so the system turns to HP bleed air to maintain the required air flow. It’s driven by actual flow needs and available pressure, not by engine RPM alone, and the switch is automatic.

Air supply to the packs switches from low‑pressure bleed to high‑pressure bleed when the demand for conditioned air exceeds what the LP source can provide. In practice this happens as you climb or when you reduce thrust: LP bleed pressure falls with altitude and lower power, so the system turns to HP bleed air to maintain the required air flow. It’s driven by actual flow needs and available pressure, not by engine RPM alone, and the switch is automatic.