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

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

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.