An intake duct assembly for directing intake air between an enclosure of an auxiliary power unit and an exterior wall of an aircraft, the interior duct assembly comprising: a first inlet adapter configured to be mounted on a first opening through the enclosure, the first inlet adapter including a duct wall defining at least one hole through the duct wall; a second inlet adapter configured to be mounted on a second opening through the exterior wall; and a fire containment apparatus extending between the first inlet adapter and the second inlet adapter to form a first passage to direct intake air in a downstream direction through the second opening in the exterior wall of the aircraft, through the second inlet adapter, through the fire containment apparatus, through the first inlet adapter, and into the enclosure through the first opening, the fire containment apparatus comprising: an inner duct mounted on the first inlet adapter at a first position upstream of the at least one hole and mounted on the second inlet adapter; an outer sheath mounted on the first inlet adapter at a second position downstream of the at least one hole and mounted on the second inlet adapter, the outer sheath circumscribing the inner duct to form at least a portion of a barrier annulus between the inner duct and outer sheath in fluid communication with the first passage via the at least one hole in the first inlet adapter, wherein the barrier annulus has a first closed end at the first inlet adapter and a second closed end at the second inlet adapter; and an isolation shield mounting the outer sheath on the first inlet adapter, the isolation shield including a first bracket secured to the first inlet adapter and a collar member at least partially supported by the first bracket, the collar member defining a mounting surface for the outer sheath. a first inlet adapter configured to be mounted on a first opening through the enclosure, the first inlet adapter including a duct wall defining at least one hole through the duct wall; a second inlet adapter configured to be mounted on a second opening through the exterior wall; and a fire containment apparatus extending between the first inlet adapter and the second inlet adapter to form a first passage to direct intake air in a downstream direction through the second opening in the exterior wall of the aircraft, through the second inlet adapter, through the fire containment apparatus, through the first inlet adapter, and into the enclosure through the first opening, the fire containment apparatus comprising: an inner duct mounted on the first inlet adapter at a first position upstream of the at least one hole and mounted on the second inlet adapter; an outer sheath mounted on the first inlet adapter at a second position downstream of the at least one hole and mounted on the second inlet adapter, the outer sheath circumscribing the inner duct to form at least a portion of a barrier annulus between the inner duct and outer sheath in fluid communication with the first passage via the at least one hole in the first inlet adapter, wherein the barrier annulus has a first closed end at the first inlet adapter and a second closed end at the second inlet adapter; and an isolation shield mounting the outer sheath on the first inlet adapter, the isolation shield including a first bracket secured to the first inlet adapter and a collar member at least partially supported by the first bracket, the collar member defining a mounting surface for the outer sheath. an inner duct mounted on the first inlet adapter at a first position upstream of the at least one hole and mounted on the second inlet adapter; an outer sheath mounted on the first inlet adapter at a second position downstream of the at least one hole and mounted on the second inlet adapter, the outer sheath circumscribing the inner duct to form at least a portion of a barrier annulus between the inner duct and outer sheath in fluid communication with the first passage via the at least one hole in the first inlet adapter, wherein the barrier annulus has a first closed end at the first inlet adapter and a second closed end at the second inlet adapter; and an isolation shield mounting the outer sheath on the first inlet adapter, the isolation shield including a first bracket secured to the first inlet adapter and a collar member at least partially supported by the first bracket, the collar member defining a mounting surface for the outer sheath.