Various embodiments herein pertain to apparatus and methods that utilize the water and existing chemicals to generate a foam. The foam can be introduced at that gas-path entrance of the equipment, where it contacts the stages and internal surfaces, to contact, scrub, carry, and remove fouling away from equipment to restore performance. Some embodiments include mixing gas with liquid to create a supply of foam, streaming the foam into a gas turbine engine installed on an airplane, rotating the spools of the engine while streaming, and re-rotating the spools of the engine after a spool has stopped. In yet other embodiments there is a method that includes streaming the foam into an engine installed on an airplane, quantifying an improvement to a family of engines achievable by foam washing, operating a specific engine, determining that the specific engine should be washed, and scheduling a foam washing of the specific engine.