N.B. Try Article 9 point 7. I'd say that anything listed on the TCDS or in the POH as "mandatory" is by definition "necessary for the airworthiness of the aircraft".
I don't think that there's any doubt that if the AFM (POH) requires something, the aircraft is not airworthy without it. I don't think the TCDS has the same authority as far as operations are concerned. Art 9(7) doesn't actually cover U/S kit, only kit that has been repaired/modified or replaced.
I think Art 8(1) is sufficient authority for the AFM requirements and limitations. My scepticism, and understand that this is
mild scepticism because you are, after all, Genghis, purveyor-of-pretty-damned-reliable-information, comes from not having seen an MEL in an AFM.
If an MEL is a routine part of an AFM, why does the FAA go through all the fuss of 91.213?
Could it not be argued that having an aircraft take off without having a means of knowing in which direction it is flying, is acting negligently, and in a manner that may endanger the aircraft?
The point is that the technical argument does not just affect that scenario. I can take off in my aircraft with an EHSI/gyrocompass, standby DI and two GPSs. I'm not going to get lost! But it does worry me that someone, particularly someone from an insurance company, could question the legality of such a flight because the wet compass is leaking a little.
For that reason, it seems important that we establish the basis for the "approved build state" which Genghis eloquently describes. I'm enjoying the debate too.