Complex sentence: He tried to ascertain if it was a turbine or mechanical engine, having had some experience in the reserve marines assisting with Helicopters, however the only sound it made was the blades swishing in the air and downdraft of wind.
Split into simpler sentences: He tried to ascertain if it was a turbine or mechanical engine, having had some experience in the reserve marines assisting with helicopters. However, the only sound it made was the blades swishing in the air and downdraft of wind.