Checklists / SOPs are not foolproof, and aircraft are not operated by fools.
Similarly humans in design are not immune to error, but they are part of a process which has more time, skill, and, facilities, than the average operator for checking their proposals. But even this does not ensure an error free design, just something which is acceptable to the regulator, who, with the manufacture is open to operator feedback / incident reporting to trigger design improvements. A ‘design flaw’ as such might only represent the limits of knowledge at the time of design, in service knowledge provides opportunity for review and change if necessary – crew checks are an easy change, but not necessarily ‘foolproof’.
One of the cornerstones of modern safety thinking is that error, in all forms, cannot be totally eliminated; thus the object is to minimize the occurrence (design / checklist input), detect and correct the error (operator / checklist input), or minimize the consequences (design / operator input). This is the basis of an in-depth defense, which further reduces the occurrence of the combination of factors that contribute to an accident.
Even the better designed TOWS have weaknesses; e.g. what duration of operator input (button press) is required for a system to achieve a successful test - 20 ms or 2 sec - do the crew know this, do they test the system for the required time?
[Accident report somewhere (BAe146?) indicated a 20 sec TOWS reactivation time after reset, this was the 20 sec during takeoff before V1 – why did the crew reset the system, no checks, no SOPs, no knowledge, no thinking?]
One attribute of this forum is that the reality of everyday operation surfaces.
TOWS saves “More often than you want to imagine …”, and no doubt the engineers will recount similar human failings / saves during maintenance or with system reliability.
If these events are ‘relatively’ frequent, then what in our high safety industry stops them coming together? Identifying this aspect will improve safety, and might well indicate changes to checks / SOPs, which is part of an essential process of gaining experience – industry experience which goes into design/certification, corporate experience for every operator, and individual experience, which has to be shared – not left residing with each chief pilot (many of whom in my experience are not well equipped to judge modern system designs).