Complex sentence: An emergency hand crank, could be attached to reposition the turret from inside the airplane fuselage in the event of a power failure.this permitted crewman to crank the turret into the vertical position to allow the gunner to exit.
Split into simpler sentences: An emergency hand crank could be attached to reposition the turret from inside the aircraft fuselage. In the event of a power failure another crewman would use this to crank the turret into the vertical position to allow the gunner to exit.