English: Damage from an explosion in the Propulsion Systems Laboratory’s Equipment Building at the National Aeronautics and Space Administration (NASA) Lewis Research Center. In the early morning hours of April 7, 1971 a massive explosion ripped through the Equipment Building. The building contained the compressors, exhausters, refrigeration equipment, and other apparatus used to condition the air for tests in several facilities. Its foundation, flooring, and footings were reinforced concrete. That morning, a valve between the exhausters and the exhaust stack was mistakenly sealed. The sound of the over-pressurization of the exhausters sent the operators scrambling to reduce the load. A large cloud of dust appeared, and the staff immediately sought shelter on the floor as the explosion tore through the building. Two men, 150 feet away from the building, were wounded by falling debris. The rupture of a large inverted dished head in the basement destroyed the 6-inch-thick reinforced-concrete floor above it, damaged I-beam supports, and tore a 30-foot diameter hole in the roof 35 feet above the floor. There was extensive damage to the equipment, piping, and windows. An Accident Investigation Board was established the following morning. The board presented its findings one week later to Center Director Bruce Lundin. The investigators found that an operator had inadvertently closed a 72-inch diameter butterfly valve, which caused the exhausters to begin perform as compressors. This soon caused the pressure in the line to increase until the large valve gave out. As a result, the system was modified to incorporate several large check valves. The accident, in conjunction with another over-pressurization incident several months earlier, led to NASA’s implementation of a Recertification Program for test engineers.