[taseal]

Originally Posted by Berkut

In addition to what's been stated above, you don't leave your Jeep running when it's on its back. Once the engine has stopped, the oil pools on the undersides of the pistons and then starts to seep past the rings into the cylinder.

Even if your aircraft was positioned in such a way that oil would flow towards the inverted cylinders, you won't get enough seeping past the rings to hydraulic lock a running engine. Take a radial engine, for example. It's not uncommon for the lower cylinders on a radial engine to hydraulic lock after sitting for a while, but you don't see it happening in flight.

another good point

thanks guys!