Performance Class A (CS-25): Descent Theory

032-04-05Pablo Asensio Martínez2026-04-011 min

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

  1. Thrust: Idle.
  2. Speed: $V_{MO}/M_{MO}$ (Maximum Operating Speed).
  3. 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.