# Performance Class A (CS-25): Descent Theory

> Emergency descent and cabin depressurization scenarios.

- **Subject:** 032 — Performance
- **Section:** 032-04 — CS-25-Applicable Operational Requirements Performance Class A – Theory
- **Topic code:** 032-04-05
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/performance/cs-25-applicable-operational-requirements-performance-class-a-theory/descent/

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## 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.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.