Boeing 747-8 System Practice Exam 2026 - The Comprehensive All-in-One Guide to Master Your Certification!

Bleed air sources come from different compressor stages in the engine, and the environmental control system needs a reliable supply at varying engine powers. During descent and other low power settings, the high‑pressure bleed air remains available at sufficient pressure to feed the air conditioning packs and cabin pressurization. Low‑pressure bleed air would drop in pressure as the engine RPM falls, which could lead to inadequate conditioning and pressurization. External bleed air is used only when engine bleeds aren’t providing enough, which isn’t the normal case in descent. So the high‑pressure bleed air is the source used in descent and low power operations.