ENGINE EXHAUST REVERSE FLOW PREVENTION

A cover (40) system selectively allows a gas flow through a heat exchanger duct upstream of an exhaust duct (30) in an aircraft engine (10). A fluid is at a first pressure during an engine (10) operating condition and a second pressure lower than the first during shutdown. A plate (42) extends across the heat exchanger duct with aperture(s) extending therethrough. A closure is movable between an open position with gas flowing through the apertures (44) into the exhaust duct (30), and a closed position with the closure closing the apertures (44). A biasing member operatively connected to the closure closes the closure with a biasing force. During the engine operating condition, the first pressure creates a first force acting against and greater than the biasing force to urge the closure open. At shutdown, the second pressure creates a second force acting against and lower than the biasing force, the biasing force biasing the closure closed.