# Performance Class A (CS-25): Drift-Down and Stabilizing Altitude Data

> Determining the net ceiling and drift down path for obstacle clearance.

- **Subject:** 032 — Performance
- **Section:** 032-05 — CS-25-Applicable Operational Requirements Performance Class A – Use of Aeroplane Performance Data
- **Topic code:** 032-05-02
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/performance/cs-25-applicable-operational-requirements-performance-class-a-use-of-aeroplane-performance-data/drift-down-and-stabilising-altitude/

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## Service Ceiling vs. Net Ceiling

*   **Gross Ceiling (OEI):** The altitude where the aircraft can maintain a climb gradient of **0%** (level flight) with one engine inoperative.
*   **Net Ceiling (OEI):** The Gross Ceiling minus a safety margin (e.g., 1.1% gradient).
    *   This is the altitude used for planning obstacle clearance.
    *   Since the penalty is subtracted from the climb capability, the Net Ceiling is **lower** than the Gross Ceiling.

## Drift Down Graph analysis

Drift down charts allow the pilot to determine:
1.  **Distance**: How far the aircraft can travel while descending from cruise altitude to the stabilizing altitude.
2.  **Time**: How long the descent will take.
3.  **Fuel**: Fuel consumed during the maneuver.

### Procedure
1.  Enter with **Gross Weight** at the point of engine failure.
2.  Follow the lines for **ISA Deviation** (Temperature).
3.  Read the **Initial Altitude** (Start of drift down) and **Final Altitude** (Net Ceiling).
4.  Read the **Distance** travelled.

**Obstacle Check**:
The pilot must verify that at every point along the drift down path, the **Net Altitude** is at least **1,000 ft** (or 2,000 ft in mountains) above the terrain.

### Effect of Variables
*   **Weight**: Heavier aircraft $\rightarrow$ Steering descent $\rightarrow$ Lower Net Ceiling $\rightarrow$ Shorter Drift Down range.
*   **Temperature**: Higher ISA deviation (Hotter) $\rightarrow$ Lower engine thrust $\rightarrow$ Lower Net Ceiling.
*   **Wind**: Tailwind increases ground distance covered during drift down; Headwind decreases it.

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