One embodiment is an inlet plenum assembly for a rotary aircraft that includes an inlet plenum defined on a first side by an inlet plenum wall and on a second side by a forward firewall assembly, the forward firewall assembly having an inlet aperture configured to receive a drive shaft to rotatably coupled to an engine; and a seal connected to the forward firewall assembly via a seal retainer disposed around a periphery of the inlet aperture. The forward firewall assembly may further include a forward firewall upper portion and a forward firewall lower portion, the forward firewall upper portion configured to removably seat to the forward firewall lower portion. The seal retainer may be attached to the forward firewall assembly via a plurality of bolts disposed around the periphery of the seal retainer. In some embodiments, the seal is connected to an inlet duct of the engine.