U/S Instruments - should an aircraft be flown with them?

[frontlefthamster]

Let's consider things from the fundamentals:

A manufacturer builds an aircraft and provides a manual (let's call it the AFM) with it. The AFM makes no mention of operating with defects (this is usual). The regulator knows nothing of the aircraft's specifics and therefore also makes no provision for the aircraft to fly in a defective state.

The aircraft develops a defect.

Without an aleviation of some kind, it might be said that operating the aircraft with any defect at all invalidates the CofA (which is itself dependent upon operations being in accordance with the AFM). This is the UK regulator's traditional position (Cabair had huge arguments with the CAA about this many years ago when they wanted to, and were eventually allowed to, operate SEP and MEP aircraft off an AOC but with a quasi-MEL - another famous Belgrano climb-down).

That (notwithstanding the MEL considerations, which should only affect aircraft operated on AOCs in the UK) is how things stand.

Now, to the praticalities: An owner has an aircraft which develops a defect. Strictly speaking, this renders it unairworthy as described above. However, the owner flies it and nothing bad happens. A happy ending. If something bad did happen he might try to cover it up and might be found out. Not such a happy ending. The burden of proof, though, may be very difficult to shoulder. It is, in my view, most unlikely that, for example, negative consequences would arise in course of an insurance claim (insurers seem to lack both appetite and competence to defend claims in questionable circumstances, though less so now than some years ago).

So, my own advice would be:

Without an MEL everything on the aircraft must be serviceable before strictly legitimate flight.

Any defect renders an aircraft without an MEL 'unairworthy' but the practical consequences are few unless you have an accident which is blatantly connected to the defect and the insurer and loss adjuster have the bit between their teeth.

Hence my questions about Section 2 etc.

Please treat the above with a degree of caution; it's some years since I operated a G-reg airframe off AOC, though to the best of my knowledge, the words above are true.

(That said, my mind returns to a vacuum pump on a PARO which crashed after departing Inverness; I can't remember the details. Booze also springs to mind but I seem to remember a vac system defect - grateful for a reminder..?).