# Performance Class A (CS-25): En-route One-Engine-Inoperative (OEI) Theory

> Regulations for handling engine failure during the cruise phase.

- **Subject:** 032 — Performance
- **Section:** 032-04 — CS-25-Applicable Operational Requirements Performance Class A – Theory
- **Topic code:** 032-04-04
- **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/en-route-one-engine-inoperative/

---

## Engine Failure Strategy

Unlike Class B, Class A aircraft **must** be able to continue flight to a suitable aerodrome after engine failure. If the failure occurs above the OEI Net Ceiling, the aircraft must **Drift Down**.

### Obstacle Clearance Requirements

The Net Flight Path (Gross path - 1.1% gradient penalty for 2 engines) must clear all obstacles by:
*   **Vertical Margin**: At least **1,000 ft** (2,000 ft in mountainous terrain).
*   **Horizontal Corridor**: Within 5 NM (or 10 NM depending on nav accuracy) either side of track.

### Drift Down Procedure
1.  **Set MCT**: Maximum Continuous Thrust on remaining engine(s).
2.  **Decelerate**: Maintain altitude until speed bleeds off to Green Dot (Optimal Drift Down Speed).
3.  **Descend**: Descend at that speed. This provides the shallowest descent angle.
4.  **Level Off**: At the Gross height (Net height + 1,000 ft clearance).

## Aerodromes

*   **Take-off Alternate**: Required if departure weather is below landing minima or if return is impossible (OEI performance). Must be within 1 hour (2 engines) or 2 hours (3+ engines) at OEI cruise speed.
*   **En-route Alternate**: Suitable airports along the route for diversion.
*   **ETOPS (Extended Range Twin Operations)**: For twin-engine jets flying more than 60 minutes from an alternate. Requires special certification for 120, 180, or 240+ minutes diversion time.

---

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