scope:

Description of Driving Event:

During post ascent shutdown of an orbiter OV-102 Auxiliary Power Unit (APU) on shuttle mission STS-2, the unit increased in temperature. Flight data indicated stoppage of coolant (water) flow to the APU. Post flight tests indicated proper operation of the coolant control valves and no flow restrictions. During disassembly, a coolant inlet filter interface fitting was found to be cross threaded. It was concluded that this fitting leaked during ascent and while on orbit. The leak caused water to flash freeze in the line, blocking the flow. The ice thawed on entry and removed evidence of the blockage. Initial installation and maintenance procedures were revised to add leak checks and visual inspection for proper alignment of fittings.