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What is windmilling aviation?

Published in Aviation Engine Operations 4 mins read

Windmilling in aviation is a critical aerodynamic phenomenon where the rotating components of an aircraft's engine, such as the turbine and compressor spools in a jet engine or the propeller in a piston engine, continue to spin solely due to the force of the relative airflow, without the engine producing thrust or power.

Understanding Windmilling in Aircraft Engines

Windmilling typically happens when the gas turbine engine flames out, and at this time, the engine spool is powered by the momentum of the incoming airflow, just like the effect on a windmill. This means that even after an engine ceases to produce power, the air rushing through it can keep its internal components rotating. This rotational energy can be vital for various operational and emergency procedures.

How Windmilling Occurs

When an aircraft's engine, particularly a gas turbine, experiences a flameout (a loss of combustion), the engine's internal rotating assembly, known as the spool, no longer receives power from the burning fuel-air mixture. However, as the aircraft continues to move through the air, the incoming airflow passes through the engine's inlet and interacts with the compressor and turbine blades. This aerodynamic force pushes the blades, causing the entire spool to rotate. The speed of this rotation is directly proportional to the aircraft's airspeed and the density of the air.

Key Aspects of Windmilling

Windmilling is not merely an idle consequence of engine failure; it serves several crucial purposes and is influenced by specific factors.

Purpose of Windmilling

  • In-Flight Engine Restart: For jet engines, windmilling is fundamental for an in-flight restart attempt. The rotating spool provides the necessary mechanical energy to drive the compressor, which draws in air, and to turn the generator (starter-generator) for ignition, potentially allowing the engine to re-light and resume normal operation. This is often the primary method for restarting engines at altitude.
  • Accessory Power: Even without combustion, a windmilling engine can continue to drive essential engine-driven accessories, such as hydraulic pumps or electrical generators, providing vital systems with power for a limited time.
  • Helicopter Auto-rotation: While differing in mechanism, the concept of aerodynamic forces driving rotor blades is central to auto-rotation in helicopters. In the event of an engine failure, the pilot adjusts the collective pitch to allow the main rotor to windmill, converting potential energy (altitude) into kinetic energy to control descent and execute a safe landing.
  • Preventing Engine Damage: Maintaining some rotation helps prevent the engine from seizing due to thermal shock or lack of lubrication, as long as oil pressure can be maintained by a windmilling accessory pump or an auxiliary system.

Factors Affecting Windmilling

The effectiveness and speed of windmilling are influenced by several variables:

Factor Impact on Windmilling
Airspeed Higher airspeed leads to greater airflow through the engine, resulting in faster spool rotation and improved chances of a successful restart.
Altitude At higher altitudes, air density is lower, which reduces the aerodynamic force on the blades. This typically requires higher true airspeeds to achieve adequate windmilling RPM.
Engine Type Different engine designs have varying windmilling characteristics. Engines with lower compression ratios or more aerodynamically efficient turbines might windmill more effectively.
Engine Condition Internal friction, damage, or remaining oil can affect how freely an engine windmills.
Aircraft Attitude The angle of the aircraft relative to the airflow (e.g., during a dive) can significantly impact the amount of air forced through the engine.

Windmilling vs. Feathering (Propeller Aircraft)

It's important to distinguish windmilling from propeller feathering, although both relate to unpowered engines.

  • Windmilling is the uncontrolled or semi-controlled rotation of an engine's internal components due to airflow, often with the intent to restart.
  • Feathering, specifically for propeller-driven aircraft, is a deliberate action where the propeller blades are rotated to an angle nearly parallel to the airflow. This minimizes drag and prevents the propeller from windmilling, which can cause significant drag on the aircraft and potentially overspeed or damage the engine during a failure. Feathering is typically chosen when an engine cannot be restarted.

In summary, windmilling aviation describes the aerodynamic rotation of engine components without power, a crucial phenomenon that enables vital emergency procedures like in-flight restarts and helps maintain control during critical situations.