Such is the fickleness of National Airworthiness Authorities that in the days when UK CAA had anything to do with aircraft certification, the Super Puma family was forbidden to have MGB fire warning systems fitted. But now with the switch to EASA, now we are forbidden to remove the system
But there is a bit of history - when the fire detectors "detect" they open their contacts, creating a resistance in the circuit. Originally the fire detectors (which are just bimetallic switches such as you might find operating the thermostat of your electric fan heater) were open to the environment, and prone to contamination by oil, fumes, soot etc such that spurious warnings were routine. There were probably at least 1000 false warnings for every real one. The CAA decided that, since there was no fire protection (extinguishers) in the transmission area, and the only course of action is to land/ditch, one day there would be a ditching in bad weather and people would drown due to a spurious warning. So they made N Sea operators remove them.
After many years, the manufacturer decided to sheath the detectors in a thin metal envelope, and this resulted in a massive reduction in false warnings - perhaps down to 100 false warnings for every real one (no, probably not that good) and its these same detectors that are still fitted today.
Of course "only the Brits" have a problem with these detectors - well actually others do as well but if you are operating onshore you just land in a field, kick the detector and carry on, no big deal. Even though Norway followed suit by removing them, EASA were hoodwinked into accepting the detectors as part justification for meeting some elements of JAR29 and before we knew it, the crappy old system was mandatory on the 225. Yes there are 2 circuits and yes its pretty unlikely both systems will activate spuriously at the same time, but nevertheless it takes a brave (and foolish) pilot not to rtb following an activation of one of the circuits. I suppose if we can build up a big enough library of sprurious activations we can go to EASA with a case, but that will take many years.
Regarding the unfortunate MHS accident, the problem as I understand it was a dodgy hydraulic system, not the absence of fire detectors. I am sure there are scenarios where the detectors might have helped (though no use if you are over unlandable terrain (primary jungle etc) or IMC over high ground). There are other scenarios where the unreliability of the detectors could cause loss of life for no good reason. I have operated the 332L without the detectors for many years, now I fly with them and I don't feel strongly either way (apart from hoping I never see both systems activated at once!)
HC