A slow turning system is fitted in an engine air starting system to prevent potential damage caused by the accumulation of oil or water in the cylinders. During extended periods between engine operations, oil or water can leak into the cylinder and accumulate. If the engine is started with a full blast of starting air, the sudden pressure increase can cause hydraulic lock or mechanical damage to the engine.

The slow turning system ensures the engine rotates slowly before starting to identify and clear any such accumulation, protecting the engine from damage.

If the engine has not been operated for an extended period, the timer triggers the slow turning system. This action blocks the main automatic starting valve and allows air to pass through the slow-turning valve. The engine is rotated slowly to complete one revolution, ensuring that any accumulated oil or water is cleared from the cylinders before the main starting air valve opens for normal engine operation.