The Application of NASA Risk Management to the SOFIA Program

NASA and DLR (German aerospace center) have been working together to create the stratospheric observatory for infrared astronomy (SOFIA). SOFIA is a Boeing 747SP (Special Performance) aircraft extensively modified to accommodate a 2.5 meter reflecting telescope and airborne mission control system. This paper shows how the SOFIA program handled one safety issue through appropriate use of NASA´s risk management process. The safety issue was identified while reviewing the probabilistic risk assessment (PRA) of a depressurization scenario in the telescope cavity. The failure scenario itself was previously known where a leak in the telescope cavity door seal sucks air out from the telescope cavity creating a negative pressure differential between the telescope cavity and the aft cavity. Two negative pressure relief valves were designed to handle this and other cavity negative pressure scenarios. However, the proposed new scenario had a leak area that was beyond the original design basis. This failure scenario was deemed credible but with a lower probability of occurrence. After identification of the safety issue, both the Risk Management and the Engineering processes required an analysis of this depressurization scenario. Multiple models of the depressurization scenarios were created and analyzed at peak dynamic pressures. The results revealed that under some failure scenarios the relief valves might not be redundant. Both valves need to function for adequate pressure equalization without exceeding structural design loads. These conditions created a program risk state that needed to be mitigated. As a result, the program started a risk mitigation plan where by a test will be performed to characterize the seal failure scenario by intentionally deflating the seal at lower dynamic pressures. The test is inherently safe at lower dynamic pressures. This paper describes the risk management process for this technical risk and ends by describing how the program continues to tra- ck this risk and program options for controlling the risk.