JimL
Engines should meet a reliability figure of 1 x 10**-5; in ICAO parlance, this qualifies them as very reliable (the reason for the low standard is that a failure, at worst, should only result in an outcome of 'Major' - i.e. 'physical distress including injuries).
Tail-rotors should meet a reliability figure of 1 x 10**-9 because a failure could result in an outcome of 'Hazardous' or 'Catastrophic' - i.e. a fatality or multiple fatalities.
Reliability targets for tail-rotors are therefore 4 orders of magnitude better than engines - i.e. 10,000.
The logical question to ask is, are engines actually more reliable these days than this "low standard" when the CAT A profile was thought up many moons ago?
If so the CAT A PC1 procedure could be due an update...