Engine exhaust reverse flow prevention

A cover system for selectively allowing a flow of cooling air through a heat exchanger duct upstream of an exhaust duct in an aircraft engine, the heat exchanger duct including an air-cooled heat exchanger disposed therein, comprising:

• a source of pressurized fluid fluidly connected to the exhaust duct, the pressurized fluid flowing into the exhaust duct at a first pressure during an operating condition of the aircraft engine and at a second pressure during a shutdown condition of the aircraft engine, the second pressure being lower than the first pressure;
• a closure movable between an open position in which the closure fluidly connects the heat exchanger duct to the exhaust duct, and a closed position in which the closure fluidly disconnects the heat exchanger duct to the exhaust duct;
• and
• a biasing member disposed in the exhaust duct and operatively connected to the closure, the biasing member biasing the closure towards the closed position with a biasing force;
• wherein, during the operating condition of the aircraft engine, the first pressure of the pressurized fluid creates a first force acting against the biasing force of the biasing member, the first force being greater than the biasing force to urge the closure into the open position;
• and
• during the shutdown condition of the aircraft engine, the second pressure of the pressurized fluid creates a second force acting against the biasing force, the biasing force being greater than the second force to bias the closure into the closed position.

A method of operating an aircraft engine, comprising:

• the aircraft engine comprising:
• an exhaust duct receiving a gas flow from the aircraft engine, the gas flow being at a first pressure during an operating condition of the aircraft engine and at a second pressure during a shutdown condition of the aircraft engine, the second pressure being lower than the first pressure;
• a heat exchanger duct having an air inlet for receiving a flow of cooling air and an air outlet fluidly connected to the exhaust duct;
• an air-cooled heat exchanger disposed in the heat exchanger duct between the air inlet and the air outlet;
• a cover disposed in the heat exchanger duct downstream of the air-cooled heat exchanger, the cover including a closure movable between an open position to fluidly connect the heat exchanger duct to the exhaust duct, and a closed position to fluidly disconnect the heat exchanger duct from the exhaust duct;
• and
• a biasing member disposed in the exhaust duct, the biasing member exposed to the gas flow and operatively connected to the closure, the biasing member biasing the closure toward the closed position with a biasing force, a first force generated on the biasing member by the gas flow at the first pressure being greater than a second force generated on the biasing member by the gas flow at the second pressure, the first and second forces opposing the biasing force, the biasing force being greater than the second force and less than the first force;
• the method comprising:
• operating the aircraft engine in the operating condition that includes passing the flow of cooling air through the air-cooled heat exchanger towards the exhaust duct of the aircraft engine;
• and
• transitioning the aircraft engine from the operating condition into the shutdown condition, the transitioning including preventing exhaust gas from flowing from the exhaust duct towards the air-cooled heat exchanger, said preventing including the biasing member biasing the closure disposed fluidly between the air-cooled heat exchanger and the exhaust duct to the closed position.