Posted on another thread but probably best here.
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.
(A probability does not mean that a failure will occur after the reliability number has been reached, it can occur at any time but it should only occur once in the period.)
A helicopter certificated in Category A can depart or arrive utilising Category A procedures where failure of an engine should not result in damage.
A single-engine helicopter can depart or arrive using a Category B procedure where failure of the engine should not result in damage. It does this by accelerating below the HV avoid curve until it achieves a climb speed clear of the 'knee' of the HV curve.
Any departure other than that published in the Flight Manual - for either a Category A or Category B helicopter - could, following an engine-failure, result in a 'Hazardous' or 'Catastrophic' outcome (with a probability for a single of 1 x 10**-5, or for a twin or 2 x 10**-5).
An engine-failure in the cruise for a twin will be a non-event. An engine-failure in the cruise for a single might result in an outcome of 'Major' (as above) unless it is being flown over a hostile environment in which case the outcome might be 'Hazardous' or 'Catastrophic'.
In the recent accident, the profile flown was the AW169 Category A variable TDP procedure. Whatever caused the accident was not the result of having flown this procedure. It was within the defined and certified operational envelope.
JimL