"The aircraft that had been built immediately before the one on the missing flight, the "Sovereign of the Skies", Pan Am Flight 6, serial number 15959, had encountered mechanical problems and had ditched in the Pacific Ocean on October 16, 1956, after two of its engines had failed.In that incident, all 31 passengers had been rescued, but the tail broke off on impact and the plane sank only 22 minutes after the forced landing, preventing a detailed investigation into the cause of the engine failures.