The dual-flapper, swing check valve is used to prevent reverse flow to the Space Shuttle Main Engines. Upon inspection of the failed component, the spring tangs were found to be missing and heavy wear was observed on the i.d. of the spring coils. The fracture surfaces revealed that the metal had been steadily worn away until a simple overload caused the final fracture. A series of flow tests using water and a water/gas mixture was conducted to identify the flow phenomenon that produced the cyclic wear. A Plexiglas® outlet housing was utilized to view the flapper behavior under different flow conditions and to aid in high-speed photography. The tests revealed that flow instabilities induced two oscillatory flapper responses: 1) a rocking mode and 2) a chattering mode. Initially, attempts were made to reduce the spring-flapper oscillations. However, the final solution to the problem was a springless configuration that satisfied the valve's design requirements and eliminated the oscillations. The springless design relied on the inherent ability of the reverse flow momentum to close the flappers.