BOAC
The best way to understand this situation is to walk all over the site on the land, view it by boat, and view it from the air in both clear conditions and those that day (easy to schedule – almost any summer evening with a prevailing southerly blowing – you can count on it).
Do the chartwork, too.
What you would then see is an ample, firm, level area about ½ a footy ground in size being oblong with its longer axis following the strata that sticks out to sea – a line that corresponds to 035 mag (at the time) so users could line up nicely along that mini shoreline and with the big, white painted post (that has been uprooted for some reason).
Although almost 300 ft up, its edge is precipitous and drops down to the sea – so it is very much a perch to hop onto as apposed to an area to slowly approach – a heavy helo like a Chinook can get into vortex ring state quite easily hovering about well clear of ground plus there would be nasty up or down drafts near a cliff edge. But Chinooks used to land there before this accident so how did they do it? The obvious approach is a routine fast approach – a mini glide path, if you like, only flaring out when right over the area, coming to a stop in the comfort zone of the ground effect – like a fixed wing a/c stays above a stall speed until a runway threshold is reached - a manoeuvre requiring accurate judgment or measurement of range.
I do not believe that they would have approached there without an accurate measurement of range that they believed was intrinsically accurate – perhaps you could inquire quietly with your colleagues what sort of gear was used by SF/SH guys to extract personnel from 1995 on.
The ground there is not easy to judge distance from even in clear conditions – there are few suitable features for scaling, hence the white post perhaps. Visual judgement in the best of conditions can be dodgy for a fast approach – I posted a description with a link to a video of the crash of the Oz Blackhawk doing a fast approach to a ship from the side – nice sunny day - the captain was the handling pilot on that occasion and ignored his copilot's advice that, according to the TACAN, they were too fast too close, preferring his own visual judgement. So you should use the navaid for accurate range – so much so that it could conflict with visual cues and still be followed – that's the rub. A PRC112 has a UHF DME function like a TACAN but is mobile and has to be where the pilot thinks it is.
So you could be using a UHF DME for accurate range for your high speed approach but you would still need expect to see some ground as you got closer and had slowed down more – but what if it had been ½ mile further up the hill? You would have thought you were further out – you had been able to see the shoreline from a long way off and from a distance you can see that, as is common thereabouts, the mist only starts onshore but the rugged appearance of the shoreline makes range judgement difficult even in clear weather – so you are surprised when you run over that shoreline as they did with only 9 secs to impact – 4 secs were the evasive manoevre so they had only 5 secs of confusion.
Now to your specific points:
<<...not be discovered? The political fall-out would be enormous if it were.>> I have always hoped so – I would hate it if people didn't care – I would hope that the public would realise that anything goes when pushing hidden agendas and they need to understand who is really running their country (what's left of it, anyway). The chances of being caught out lying to make the case for war didn't deter Bliar – how many Brits and Iraqi people died? And the $$$ cost? - wasn't much fallout at all in that case was there? - not that mattered. Funny thing to muse over – I'll bet that if this Chinook had been exposed as a dirty trick soon after the crash, Brits may not have got into the Iraq war.
<< Start, therefore, looking at 'probilities' of success for it. I would venture to suggest that anything less than 99% would not be considered?>> I think you mean 99% for not being exposed/brought to account. It would have been a “free shot” - if they had avoided crashing then the pilot could have been fobbed off with an explanation of confusion on the ground … worth the effort even with a 25% chance of success? To get more than 50% chance of conditions being right at that time of year you only had to make sure that the flight started in the late afternoon/evening.
Points A, B, C, & D are, I believe, covered in the above overall scenario.
<< e) You need to ensure that the ensuing impact is of sufficient violence to kill the crew and pax and render a technical investigation difficult.>> ensure? Why? I would have thought it would have been extemely likely anyway but a few survivors aren't going to keep the team effort going, are they? Would the pilots have gone public if they had survived? All they could have said anyway was that an exercise had gone wrong. I'd bet that the team on board would have been suspicious if they had survived a near miss but what could they have proved? Regarding “technical investigation”, I do not think that you could count on the CPLS equipment being unrecogniseable and its presence would surely have given rise to inquiry as to its possible use – I have no idea whether a piece of wreckage could be “disappeared” if it was said to be sensitive – I dunno – it would have been easy to remove as it was stand alone and designed for easy removal/installation. Anyway, even if it was proved/admitted that such an exercise had been attempted, how could it be proved to have been wilfully interfered with? You have not got people of the calibre of that team to investigate it, you have no one like Airey Neaves to get action at the highest level. The “government” that wanted to wash its hands of NI would hardly want to undo the peace process it so loves by any nasty revelations. It was a free shot. That is why “accidents” are so convenient and commonly used to remove people problems.
<< f) You need to ensure that no 'survivors' know of the plan, as it appears to have been un-authorised and 'unknown'. Who tasked it? >> I would really like to know – then we could ask him who was it in MI6 who did the liason and thought of this jolly little demonstration. There was a post here some time back wherein someone who seemed acceptable to you “regulars” had said that it was the intention to use a Chinook several weeks before. Then Flt Lt Tapper did depart from routine by not breakfasting with his crew to go alone to a “met” brief – at Aldergrove? - perhaps he got his outbrief then. There were enough anomalies in the planning and preparation for this flight to suggest that something extraordinary was going on.
Your final remark <<At best, I arrive at around a 5-6% chance of success. Not good enough? Why would anyone take that risk? Credible? >> not a punter are you? I reckon at least a 25% chance of success for just about no risk at all.