What should be done before starting a radial engine that has been shut down for over 30 minutes?

Before starting a radial engine that has been shut down for more than 30 minutes, it is essential to turn the propeller by hand to check for liquid lock. This practice is crucial for ensuring that there is no fuel or oil accumulated in the cylinders, which can cause significant damage if the engine is started while in this condition.

Liquid lock occurs when liquid fuel or oil enters the combustion chamber and prevents the piston from moving, potentially bending connecting rods or damaging the crankshaft. By manually rotating the propeller, the mechanic can feel for any resistance which indicates a liquid lock situation. If resistance is encountered, it is imperative to address the issue before attempting to start the engine.

Turning the propeller not only helps to identify potential hazards but also ensures the engine components are lubricated adequately and can move freely, thereby promoting a safe and effective start of the engine. This step is a critical preventative measure and aligns with best practices in engine operation and maintenance.