Ecology valve for draining a manifold of a gas turbine engine, fuel system and method for draining fuel

A fuel system (10) including a fuel pump metering unit (FPMU) for delivering fuel to an engine manifold with an ecology valve (20) for draining and storing fuel from the engine manifold. The ecology valve (20) includes a housing (21) having a piston (24) dividing the housing (21) into a first side in fluid communication with an output of the FPMU (23d) and a second side in fluid communication with the engine manifold (23b). An assembly connected between the FPMU and engine manifold selectively creates a pressure differential across the first and second side of the housing when the FPMU delivers fuel to the engine manifold. In a run position, the piston (24) moves to decrease a volume within the housing interior (22) as a result of the pressure differential. In a drain position, the piston (24) moves to increase the housing volume (22) within the interior and thereby pull and store fuel from the engine manifold.