1. Safety Procedure: Before starting the engine, pilots followed this step to ensure that there were no obstacles in the propeller's path. By manually rotating the propeller, they verified that no ground crew members or loose objects were in the way, preventing potential accidents or injuries during engine startup.
2. Engine Priming: In older piston-engine aircraft, priming was often necessary before starting the engines. This involved introducing a small amount of fuel into the cylinders to aid in the combustion process. Manually turning the propeller helped distribute this fuel evenly within the cylinders, improving the engine's chances of starting smoothly.
3. Checking for Engine Issues: Rotating the propeller by hand allowed pilots to detect any unusual resistance or irregularities in its movement. This simple check could reveal mechanical problems, such as tight spots or binding in the engine's components. If any issues were identified, pilots could address them before attempting to start the engines, preventing more significant problems from developing in the air.
4. Starting Unpressurized Engines: In certain older aircraft without pressurized cabins, manually turning the propeller could be necessary to draw air into the engine. This was particularly relevant for carburettor-equipped aircraft, as fuel needed to be mixed with air before entering the cylinders. By turning the propeller, pilots ensured proper airflow into the engine, aiding in its smooth operation.
5. Building Compression: Manually turning the propeller helped build compression in the engine cylinders, making it easier to start. This was especially important when engines had low compression ratios or were cold, requiring additional assistance to initiate the combustion process.
While modern aircraft with advanced starting systems no longer require manual propeller turning, this practice served as a crucial pre-flight safety procedure in the early days of aviation.
