The investigation of the first incident (United Airlines 585) didn't result in enough evidence to be able to state a probable cause. The board decided (voted?) to publish the final report with "probable cause undetermined", only the 4th NTSB report in it's history that was so published. It wasn't until the 2nd incident (USAir 427) and the 3rd (Eastwind 517, which occurred during the USAir 427 investigation) that enough evidence was able to be recovered and tested that the conclusions could be drawn. The hydraulic servo unit in United 585 was too damaged to test. But USAir 427's unit was functional, and Eastwind 517 didn't crash, so it could be tested as well. Also, the pilots of Eastwind 517 were interviewed, which wasn't possible on the other two incidents since they were both total fatalities. After the evidence was gathered, USAir 417 and Eastwind 517's final reports reflected the hydraulic servo failure as the probable cause, and United 585's report was amended to state that the hydraulic servo failure was the probable cause. The reports resulted in action by the FAA to compel Boeing to replace the hydraulic servo unit with a redesigned unit on all Boeing 737's.