# Aircraft Propellers: Constant Speed, Feathering, and Critical Engine

> Understand aircraft propeller systems, including constant speed control, feathering procedures, and critical engine effects in multi-engine aircraft.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-10 — Piston Engines
- **Topic code:** 021-10-09
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/piston-engines/aeroplane-propellers/

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The propeller converts engine brake horsepower into thrust.

## Propeller Types
*   **Fixed Pitch**: Blade angle is set by manufacture. Compromise between climb and cruise performance.
*   **Variable Pitch (Constant Speed)**: The blade angle can be changed in flight to maintain a selected RPM. This allows the engine to operate efficiently across different airspeeds.

## Constant Speed Unit (CSU)
A **Constant Speed Propeller** uses a governor (CSU) to automatically adjust blade pitch to maintain the RPM selected by the pilot.
*   **Cockpit Control**: Blue lever.
    *   **Forward**: Fine Pitch / High RPM (Take-off, Landing).
    *   **Aft**: Coarse Pitch / Low RPM (Cruise).
*   **Operation**:
    *   If airspeed increases or throttle is added -> RPM tends to rise -> CSU increases pitch (coarser) to increase drag and maintain RPM.
    *   If airspeed decreases or throttle is reduced -> RPM tends to fall -> CSU decreases pitch (finer) to maintain RPM.

## Operational Procedures
*   **Take-off / Landing**: Propeller lever **Full Forward** (Fine pitch). This allows the engine to reach maximum RPM and power.
*   **Cruise**: Propeller lever set to a lower RPM (Coarser pitch) for fuel efficiency and engine longevity.
*   **Pre-flight Check**: Cycle the propeller lever.
    *   **Action**: Move lever from High to Low RPM and back.
    *   **Indication**: **RPM should drop** and recover. Manifold pressure (MAP) may rise slightly. Oil pressure should remain stable.
    *   **Purpose**: Check governor function and circulate warm oil into the hub.

## Feathering
Featured on multi-engine aircraft.
*   **Feathered Position**: Blades turned approx. 90° to airflow (edge-on).
*   **Purpose**: **Minimize drag** when an engine fails and stops. Prevents "windmilling" which causes immense drag and can damage the engine.
*   **Control**: Pulling the prop lever fully aft (past a detent) feathers the prop.
*   **Unfeathering**: Moving lever forward returns blades to fine pitch to allow windmilling for restart.

## Reduction Gearing
Many engines use a reduction gearbox between the crankshaft and the propeller.
*   **Purpose**: To keep propeller tip speeds below the speed of sound (efficient operation and noise reduction) while allowing the engine to run at higher, more efficient RPM.

## Critical Engine (Multi-Engine)
The **Critical Engine** is the one whose failure results in the most adverse effects on handling (yaw).
*   **P-Factor**: The descending blade produces more thrust than the ascending blade.
*   **Clockwise Rotating Propellers**:
    *   **Right Engine**: Thrust line is further from the CG (Longer arm).
    *   **Left Engine**: Thrust line is closer to the CG (Shorter arm).
*   **Critical Scenario**: If the **Left Engine fails**, the Right engine (with the longer arm) produces a strong yawing moment. Therefore, the **Left Engine is the Critical Engine**.

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