A filter system for a tiltrotor engine comprising: a tiltrotor engine nacelle (208a, 208b) having a forward end, an aft end, a bottom, a top, a side, and an engine in the nacelle; a ram air inlet comprising one or more bypass doors positioned to control air flow into the ram air inlet and capable of at least partially or fully blocking a ram air flow through the ram air inlet; a ram air inlet duct; a barrier filter inlet positioned in the side, the bottom, and/or the top of the tiltrotor engine nacelle (208a, 208b); a selector duct comprising one or more openings; and one or more selector duct doors or covers for each of the one or more openings; wherein the selector duct isolates the barrier filter inlet from the ram air inlet when the one or more selector duct doors or covers are closed; a filter plenum between the barrier filter inlet and the one or more openings of the selector duct; wherein the barrier filter inlet, the filter plenum, the one or more openings in the selector duct, the selector duct, and the engine are in fluid communication; wherein the ram air inlet, the ram air inlet duct, the selector duct, and the engine are in fluid communication; wherein, during hover operations, the one or more bypass doors in the ram air inlet are closed and the one or more selector duct doors or covers are opened to provide only filtered air flow to the engine through the barrier filter inlet, and wherein, during cruise operations, the one or more selector duct doors or covers are closed and the one or more bypass doors are opened to isolate the filter plenum from the ram air flow through the selector duct to prevent a ram air flow out of the barrier filter inlet. a tiltrotor engine nacelle (208a, 208b) having a forward end, an aft end, a bottom, a top, a side, and an engine in the nacelle; a ram air inlet comprising one or more bypass doors positioned to control air flow into the ram air inlet and capable of at least partially or fully blocking a ram air flow through the ram air inlet; a ram air inlet duct; a barrier filter inlet positioned in the side, the bottom, and/or the top of the tiltrotor engine nacelle (208a, 208b); a selector duct comprising one or more openings; and one or more selector duct doors or covers for each of the one or more openings; wherein the selector duct isolates the barrier filter inlet from the ram air inlet when the one or more selector duct doors or covers are closed; a filter plenum between the barrier filter inlet and the one or more openings of the selector duct; wherein the barrier filter inlet, the filter plenum, the one or more openings in the selector duct, the selector duct, and the engine are in fluid communication; wherein the ram air inlet, the ram air inlet duct, the selector duct, and the engine are in fluid communication; wherein, during hover operations, the one or more bypass doors in the ram air inlet are closed and the one or more selector duct doors or covers are opened to provide only filtered air flow to the engine through the barrier filter inlet, and wherein, during cruise operations, the one or more selector duct doors or covers are closed and the one or more bypass doors are opened to isolate the filter plenum from the ram air flow through the selector duct to prevent a ram air flow out of the barrier filter inlet.