Measurements of the quasi-steady acceleration environment on Columbia during STS-50 (the First United States Microgravity Laboratory) by the Orbital Acceleration Research Experiment and the Passive Accelerometer System differ in magnitude from those predicted from a numerical model. Known contributions to the orbiter quasi-steady acceleration environment from venting forces were investigated. The Flash Evaporator System (FES) vents steam through two opposing nozzles on either side of the aft orbiter. FES cycles were evident in recorded accelerometer data. Additional accelerations arose due to venting of waste and potable water through a vent on the orbiter's port side.