its badly-designed and poorly-maintained bow visor lock finally (cumulatively) failed......
And we again emphasize that this is not to accuse Meyer Werft of incompetence, dereliction, or negligence. Some properties of designs that bear on safety and effectiveness are inherent. Some are parasitic. Others are combinatorial. Clamshell designs for ferry doors are fail-safe, whereas visor designs are not. Parasitic properties would include the notion that no kind of bow door can exist without affecting the structural integrity of the hull. Combinatorial design factors are usually secondary factors that we manipulate as a system to improve overall desirability, but which may present their own failure modes.
For years, airliner overwing exits were "plug" doors, meaning that the bevel of the door frame prevented inadvertent opening in flight. If the inside and outside air pressure were too different, the greater indoor pressure would simply force the door harder into its frame, seating it more firmly. It would have been physically impossible for some deranged passenger to open the door in flight and depressurize the airplane. It simply would have taken superhuman strength to pull against all that pressure. Similarly clamshell doors are forced tighter together by wave action and so would retain watertight integrity even if elements of the mechanical locks failed. Not so a visor design, which requires the locking mechanisms to absorb the wave loads. If the locks fail, the visor loses its watertight integrity.
The problem with airliner plug doors is that they must open inward by definition. In most cases they came free entirely and required the operator to throw the 20 kg door out through the opening, or (later) to lay the door along the armrests. This proved to be a serious impediment for evacuation—the primary purpose for the overwing exit. If your common or primary use case is marred by some design factor in favor of a secondary need (to prevent in-flight operation), then you want to rethink the solution.
Nowadays the doors open outward and (generally) upward. The plug-door concept has been abandoned. Instead there are robust interlocks and pressure sensors that prevent inadvertent opening of the door in flight. An inherently "unsafe" design can be
made safe by the complication of combinatoric factors. That is, making it more complicated in some ways that might seem flaky may make the whole design better suited for its primary purpose. Similarly, Meyer Werft undoubtedly believed that the inherently fail-prone visor design could be
made safe by the employment of redundancies, requirements for inspections and upgrades, and restrictions on operation. But absent any agreed-upon standards for how to do this, they were left to their own devices.
