Dervish,
Hope I can help a bit here. 'Fitness for purpose' or 'fit for purpose' appears all over the MAA regulations set, in a number of contexts. Personally, I think it's a bit of a vague phrase, and used too liberally, probably because the politicians started appropriating it. I have a sinking feeling that the MAA probably thought it would look jazzy in the regs.
But to address a very good point you made - when looking at whether an aircraft's design (or a modification) meets operational 'fitness for purpose', my feeling is that the MAA regs have not much to offer. There is one clear link to the phrase in a recent update to 00-970 that includes the following:
Def Stan 00-970 is not intended to provide requirements to ensure the fitness for purpose of Type Designs or Major Changes.
Here's my take. It's probably wrong, but here goes anyway. When assessing a project, I put requirements into three (probably overlapping) bins:
1. Pure 'air safety' - the 00-970 stuff
2. Fitness for purpose
3. Operational effectiveness
To illustrate the idea, here's those three 'bins' applied to requirements for a new radio:
1. Pure air safety - does it catch fire? Does it fall out of the aircraft and kill someone? Does it irradiate users? EMC, Etc.
2. Fitness for purpose - can it be used by the aircrew without spending excessive time heads in? Does it clearly tell the user what it's condition and state is? Can it be maintained? Is it reliable? Does its basic performance meet the requirement (range, clarity, etc) Lots of HMI stuff here.
3. Operational effectiveness - does the HQ/crypto function work? Does it meet operational requirements for coalition/joint ops? Will the OEU accept it for service?
Like I say, these three may overlap, especially the last two, but I found that they help project teams work out the boundaries between the ATEC ITE folk and the OEU. They also help build a good requirements set, which is the foundation of any project.
So who's responsible? My call is that the project team are responsible for delivering a product to the user that is 'fit for purpose' as defined in the requirements PLUS (important point) any other aspects that they could reasonably be expected to apply to the job. (After all, they are supposed to be the procurement professionals, right?). And they're accountable to the user for doing this.
The Duty Holder is then responsible for accepting the system. Or rejecting it. Once accepted, the user maintains that 'fitness for purpose' via their CAMO and aircrew and personnel training systems.
In both cases, they are required to apply the relevant MAA regs while they discharge these responsibilities.
Now, does all this actually happen all the time in the real world? That's the big question, to which I don't have the answer.
Hope this helps a little, best regards as ever to all those reading these big books,
Engines