Performance Class A (CS-25): Descent Theory
Emergency Descent
In case of rapid cabin depressurization, the aircraft must descend rapidly to a safe altitude where passengers and crew can breathe without supplemental oxygen (usually FL100 or 10,000 ft).
Procedure
- Thrust: Idle.
- Speed: $V_{MO}/M_{MO}$ (Maximum Operating Speed).
- Drag: Speedbrakes extended (and potentially Gear Down if structural limit allows).
Oxygen
- Oxygen masks drop automatically (usually at cabin altitude 14,000 ft).
- Chemical generators provide oxygen for a limited time (e.g., 15-22 minutes).
- The descent profile must ensure the aircraft reaches safe altitude before the oxygen supply runs out.
Obstacle Clearance (Depressurized)
Similar to OEI drift down, the "Net" path for a depressurized descent is the Gross path minus a safety gradient.
- The aircraft must be able to clear all obstacles to reach a suitable aerodrome.
- This is critical on routes over high mountain ranges (e.g., Himalayas, Andes), necessitating specialized "Escape Routes" charts.