Based on decades of experience and training.
Now we know who may have given the CAAFU people such a bad name
I say "may" because I very much doubt Mr Hamster was ever a CAA FE.
I wonder if Mr Hamster is "DFC"? The writing style is similar.
You do get some right cases on here
So a defect in any piece of equipment whose proper functioning is not mentioned as a condition or limitation in the flight manual is not unairworthy, as a matter of law.
I think that is more or less what I wrote, too, though less clearly.
This topic also touches on maintenance. In the USA, FAR Part 91, the same principle applies. Stuff in the Airworthiness Limitations section of the MM is mandatory. Everything else isn't (notwithstanding it being good practice, etc) unless called up in some equivalent reg elsewhere e.g. an ICA in an STC.