Flatiron
Well said about the technical red herrings BUT the analogy of driving too fast in fog is strange - was has emerged from the inquiries and from inputs to this thread is that they were flying in clear air (excellent visibilty for miles at their altitude) with a clear view of the sea towards a landmass whose ground detail was obscurred by a localised (indeed ground hugging) mist that they were to turn away from (close in - on the shoreline, in fact) and go up the coast at low level.
It was that the turn had not been initiated in time that caused the crash.
Now, I agree that if (at the high speed that they were travelling at) they were relying upon visual judgement alone of their distance off to start this turn then they were indeed negligent - without clear familiar objects or topography to give them spatial awareness they would have to have slowed down considerably to come in so close, and as you said:
<<All the hypotheses about engines failing, computers going on the blink etc are seen as a red herring because if the crew had been flying prudently, they would have had time to cope. >>
The real nub is what made such an experienced crew continue in so close at high speed: given the RAFs obfuscation on the actual flying conditions and the intended route, I doubt very much that at this late stage they will make anything clear and thus it remains for others to try and get to the truth. My view is that, without an on board radar and distrusting the accuracy of the TANS, the logical explanation could be that they were using their TACAN to measure their distance to go to their planned turning point. This is something that someone reading this thread should be able to ascertain for certain one way or the other but, alas, no one has done so to date - do you have the contacts?
You see, if there was a concession allowing use of TACAN (on this regular route in such common conditions) for this one difficult-to-judge turn on an otherwise perfectly reasonable VFR flightplan, then the performance of the TACAN ground equipment should have been considered. I understand the simplistic arguement that under VFR the pilots should not have relied upon navaids for terrain avoidance but common sense suggests that this scenario is a special case if you look at the actual weather conditions and their intended path - one can surely understand that if you have a nice digital readout telling you how far away you are and your visual reference is a grey amorphous blob then the digital readout may have a strong influence on your visual judgement WHETHER YOU ARE SUPPOSED TO BE RELYING ON IT OR NOT. If the ground equipment was set incorrectly (and this can so easily be done with this equipment) and was reading, say, 1 mile more than was the case then it may have biased their visual judgement. If this sounds incorrect then please consider how often pilots turn to their instruments when momentarily disorientated (there's a whole thread on this subject in another services website that I am sure many pilots reading this thread would be familiar with) - in this case, only disorientated in one dimension but the principal is the same, ie it's hard to judge the distance off that grey mass with the mist moving up the slope, ever changing the texture, just the shoreline clear but nothing of familiar size (lighthouse buildings not visible and seagulls not big enough, etc), you suddenly think you are getting too close so your eyes flit to the TACAN, you calm down and continue looking at that grey mass, the TACAN has told you that you've still got time to make your judgement, perhaps that odd little turn to the right was the left hand pilot nudging the nose over so he can see the shoreline better himself (bear in mind the angle of approach to the Mull).
ZD576s TACAN CU was set on the right channel for Mac - this should have been discussed at least - whether or not there was no line of site to the aerodrome TACAN - whether or not there was a portable (DME only) equipment near the lighthouse for such transits - whatever - this navigation aid should have been discussed fully at the inquiries given the flightplan and the lack of radar, innaccuracy of TANS, and that the CU was set accordingly.
As you would be aware if you look back through the postings, TACAN is a taboo subject in this case.
If the use of a TACAN at the crucial turning point was the usual practice by those flights, even if there was no official relaxation of the rules such that the pilots were still responsible for avoiding terrain visually, then surely the case could not be closed without consideration of the state of the ground equipment. Had this scenario been explored then the concern may have been much more than the reputation of the pilots - for if such ground equipment had been tampered with, then by who, etc..
At the very least, the pilots should not have been blamed after such inadequate inquiries which failed to clearly establish at the very beginning the basics which may have given the many experts and learned gentlemen a flying start at getting to what happened; ie:
the intended flightplan (VFR low level turning close in to the shoreline and going up the coast, never intending to enter the ground hugging mist, never intending to fly over the hill);
the actual weather conditions over the sea where the helo was flying (excellent vis for miles laterally and the sea clearly in view);
that the Mull ahead was clearly visible but its ground detail was obscurred by ground hugging, localised mist and that such conditions are common there.
Add to these basics that the only precision navaid available to ZD576, its TACAN, hardly got a mention - never mind whether it should have been relied upon or not, nor whether the MAC TACAN could have been used or not; it should have been discussed at the inquiries where the right questions could have cleared this matter up.