Patent ID: 8820045

Claim:
An auxiliary power unit comprising: a gas turbine engine comprising: a combustor section including a combustor case a fire enclosure joined to the auxiliary power unit to circumscribe the combustor case; and a drain assembly configured to permit fluid within the combustor case to drain out of the fire enclosure, the drain assembly including: a piston seal configured to prevent combustor air from leaking into the fire enclosure; a scupper extending outward from the combustor case to form a trough; a drain fitting extending through an opening in the fire enclosure to align with the trough; and an orifice extended from the trough by a cylindrical body penetrating into the drain fitting to form an expansion joint; wherein the piston seal comprises a piston ring positioned around the cylindrical body within the drain fitting; wherein the combustor case includes perforations within a perimeter of the scupper; wherein the drain fitting includes a cup in which the piston ring is positioned; and wherein the cup comprises: a first collar extending away from the fire enclosure to receive the cylindrical body; a platform extending from the first collar and upon which the piston ring rests; a sidewall extending from the platform alongside the piston ring; and a retaining ring engaged with the sidewall to prevent the piston ring from being displaced from the cup.