Hi,
jcjeant,
1) 737 PCU malfunction: Only after Colo. Spgs crash ("testability problem") they started to detect a hidden problem. i don't know if we can classify this as a "design problem". Far more subtle than the Pitot's problem.
2) 741 Flight 811: Indeed a design fault with MANY incidents before this case.
3) 741 TWA 800: IMO doesn't fit as a design problem.They improved the design reducing the risk of a similar case.
Mac
We need to be cautious dealing with "design problem" classification. Everything could fit in that category. E.g. "lack of redundancy is a Design Problem? Not necessarily.
The use of Redundant OBSOLETE sensors (triple redundancy) that "FAILS" near simultaneously IMO is a SERIOUS DESIGN PROBLEM. The use of just one PCU (737) compared to the (redundant) 727 IMO is not a Design Problem. And in the 737 the lack of redundancy magnified the (hidden) problem.
The first case (COS 35 final) was shocking. The plane just rolled diving vertically from ~ 1,000 ft AGL when in the final. I can't imagine the feeling of both crew. Gusty winds masked the analysis. And it was a subtle (not easily testable) problem. Few days before the crash there was "strange" behavior involving rudder.
Intermittent failure in a "closed loop" System.
Testability is a complex issue. For some problems only TIME can test.