One example of an engine mounted inlet plenum for a rotorcraft includes an inlet flange, an outlet flange and a plenum duct that connects the inlet flange and the outlet flange. The inlet flange, which defines an inlet to flow air into a barrier filtration system, is configured to be substantially aligned with a closed cowl door assembly of the rotorcraft. In some implementations, the plenum duct is configured to extend to an outer mold line (OML) of the rotorcraft such that the inlet flange is aligned with the OML. The outlet flange defines an outlet to flow air filtered by the barrier filtration system into an engine of the rotorcraft. The plenum duct allows air to flow from the inlet to the outlet.