The purpose of this pattern is to provide ground personnel with a visual cue that the engine is actually turning. This simple warning is an important safety feature that can often prevent tragic accidents when personnel are working near rotating engines. This precaution dates back to at least the 1930s when spirals appeared on the spinners of many propeller-driven aircraft. Otherwise, a propeller rotating at high revolutions per minute can become nearly invisible and very difficult to see to an observer on the ground. For example, think of the spinning blades of an electric fan or ceiling fan. As the speed of rotation increases, the blades become increasingly difficult to see.