In one embodiment, there is described an inlet plenum assembly for a rotary aircraft, including: an inlet plenum defined on a first side by an inlet plenum wall and on a second side by a forward firewall assembly; the inlet plenum wall having a mechanical interface for receiving a reduction gearbox (RGB) to provide reduction gearing to the drive shaft; and the forward firewall assembly having an inlet aperture configured to receive a drive shaft to rotatably couple to an engine; wherein the forward firewall assembly includes a forward firewall upper and a forward firewall lower, the forward firewall upper configured to removably seat to the forward firewall lower.