What was found
FLIGHT RETURNED TO DEN, UNABLE TO MAINTAIN CABIN ALTITUDE AT CRUISE. *S/D* THE BOOT, REF: (P/N S212W301-9), IPC 36-12-04-005, ITEM 155) AT THE PENETRATION OF THE APU DUCT INTO THE AFT DOME OF FUSELAGE, WAS FOUND TORN IN THE SEAM. FURTHER EXAMINATION OF THE BOOT REVEALED THAT THE STEEL RING (INSIDE THE BOOT) SNAPPED AT THE WELD. THE BOOT WAS REPLACED ACCORDING AND THE A/C WENT TO REVENUE SERVICE.