I've read all the reports that originated from the RAE about the Comet disaster and the observation that shocked me was that there were some cracks in the window (passenger and ADF) that had been stopped drilled but continued to propagate. The problem was one of material and production methods (not helped by the shape of the opening). How many metal fuselage designs had de havilland built before the dh106? They were learning the art whilst pushing the envelop.