Yet again when these discussions on full CAT A arise, I firmly agree with Nick and I find it almost incomprehensible that the argument goes around so many times.
It appears straight forward.
Risk = Frequency X Consequence. As engine failure rates are not accurately recorded, statistics are very hard to argue without significant flaws being apparent, so I will avoid the statistical minefield in terms of frequency of engine failures.
Examining three types of aircraft (single engine, Twin engine with limited CAT A, Twin engine with OEI hover), we can assume that engine failures are relatively consistent between the three types of aircraft and thus, we can assume that frequency of engine failure is the same across the three types. Except of course, frequency is doubled in a twin because they have two engines!
Given frequency is a constant; we need to measure risk in terms of consequence. .
If you want a single engine aircraft you are vulnerable to engine failure 100% of the time. Consequence is generally severe for all but clear area ops and experienced pilots. Thus generally, singles have high risk (due high consequence).
If you go for the Twin engine with limited CAT A, then I concede that your frequency of engine failure could be considered double that of a single in this simplified analysis, and the highest consequence is also similar to the single (i.e. severe) but not as likely to be severe due the other engine being available to reduce severity of the forced landing. Accordingly, the risk is still high, but less than that of a single.
The third type is the AW 139 – “Full CAT A” OEI hover capable with no deadman’s curve. Frequency is double the single, same as limited CAT A, but consequence is low. Thus over all risk is low.
Now the tricky bit – exposure to risk. This is a function of how often the aircraft is exposed to the negative consequence, or in other words, exposure is the amount of time that the frequency (number of engine failures) of the risk can become involved and cause the consequence (crash landing). If you are exposed for longer, the chances of an in-frequent event occurring are higher than if the exposure time was very low. Thus when we assess risk, we should also consider exposure to that risk in order to assess it’s real impact.
There are two extremes. The single is exposed nearly all the time from hover to hover. Lets say for about 80% to be conservative. The twin with hover OEI is not exposed.
The middle ground is actually not very middle at all! The twin with limited CAT A is exposed for less than 0.17% (illustrative only - Nick – can you recall the exact number here? I will amend when I have the correct figure). The aircraft is only exposed when operating in very narrow time segments of take off and landing over adverse terrain. It is not exposed enroute, nor during those narrow time segments when operating over terrain that would sustain a landing, albeit with minor aircraft damage (as opposed to human damage).
Many corporate owners happily fly around in singles because they are cheap and they assess engine failures as remote, but they almost universally fail to consider consequence. Similarly, many corporate owners want full CAT A because they either do not understand exposure, or because they don’t want any exposure to the same risks that many corporate owners consider negligible.
Now cost: singles are cheaper, twins with OEI hover are very expensive (20,000 lbs operating costs to haul 15,000 lbs of goods). The limited CAT A fits nicely in the middle BUT it allows compromise. If you want to have full CAT A, fit the aircraft with less seats and voila! You have it. If you then accept the exposure rate of 0.17% to a possible (not likely) consequence, you can load it up and operate cost effectively. I agree that limited CAT A is a smart (and probably a mandatory) step to make away from singles for high net worth individuals and work place safety reasons for fare paying passengers because it significantly reduces exposure (80%ish to 0.17%) for a more reasonable cost increase, but I cannot justify spending considerably more on reducing a mere 0.17% to about zero.
The outcome of making all new twins hover OEI is that more and more people will buy singles because cost effective operations are no longer viable in twins given that there is no aircraft available for those who accept the occasional 0.17%.
But what is most surprising to me about all this pprune (pilot as opposed to owner) demand for hover OEI twins that will actually drive people to singles is that a crucial piece of the puzzle always seems to escape the argument: it is NOT ENGINE FAILURES THAT ARE RESPONSIBLE FOR THE LARGEST CHUNKS OF OUR ACCIDENT STATISTICS.
Revolutionary idea follows; lets push owner funding into anti CFIT devices like multi axis autopilots, synthetic and night vision devices, 3D nav equipment, wire detection and avoidance capabilities, advanced weather radar and ground to air situational information linking, terrain and traffic detection and avoidance capabilities, and lots more motion simulator training for the pilots. CFIT risk is proven to be much more than the 0.17% exposure to possible risk that we are trying to overcome by demanding OEI hover capable twins and driving people back to singles. Why do we stay so obsessed by such an insignificant risk when the meanest monster gets so little attention???????
