Why Preflight Prep for B-17 Engines is all Hands Up

A preflight activity took place last week during a visit from the B-17, Sentimental Journey. Ground crew members slowly rotated the four propellers by hand - much to the amazement of the crowd (see above video from last week's visit.) Why are they doing that, asked small children.

A person in the know told us that the propellers are turned over by hand to clear the pistons of oil and/or gas that may have leaked into the cylinder heads, which are typical of radial engines like those used on B-17s. Any liquid in the cylinders would damage the engine if started. Turning the engine over slowly by hand allows any liquid to be purged from the cylinders. Slowly rotating the engine before starting allows the collected oil to be recirculated.

Pulling the engine through several rotations by hand was standard procedure for many radial engine warplanes in World War II. Ground crews used to line up and each one in session would grab a prop blade and walk it forward in the direction of the blade movement. Know that.