VFR
Even as little as ˝ mile vis would have been OK – the idea being that helicopters can always slow down.
An eyewitness on the NI coast at the (actual) time of the accident said that the Mull could be seen although it was shrouded in mist – he thought he could see the base of the lighthouse wall. This suggests the all too common conditions there of clear visibility offshore at low level (below the cloud base) with the mist localized right on the Mull – this is important as had the aircraft turned at (the actual geographical position of) waypoint A it would not have entered the mist and would not have breached the VFR for helicopters in that situation.
They did not turn at waypoint A as according to the SuperTANS but continued on, indeed making a small (correctional?) turn to the right – at this time, they could not have been experiencing any control problems (or other emergency) as they would not have bothered to select their next waypoint for route navigation on the SuperTANS.
The actual geographical position of waypoint A was already very close in to the landmass and had they believed the SuperTANS they should have commenced their turn up the coast (remember, at that point of waypoint change it seems reasonable to assume that they had control).
It should not be surprising that this crew would not believe the position given by the SuperTANS to any degree of accuracy (they had said so, clearly, that they had doubts about this system’s accuracy previously). Thus, if they had some other reference to go on (visual or instrument) ignoring the SuperTANS waypoint was reasonable – a common practice for helicopters crossing from NI to this part of the coast was to directly approach a point near the lighthouse and commence a turn close in (but before the prevailing mist) so as to go along the shoreline – a neat maneuver if the turn was executed in a timely fashion.
One simple explanation for this accident is that the use of their other reference misled them; ZD576 did not have radar so all that remained to them in that position was a visual reference or a DME beacon. I have not read that book but I gather from the comments on this thread that they had a visual reference but confused it – this would rather suggest that conditions were not good enough for a visual reference. This HC2 was equipped to interrogate the (UHF) portable DME sets which could have given them very accurate range and approximate bearing (that corrective steer to the right at waypoint change?) to a portable set located, say, at the lighthouse helipad. DME is a favorite NavAid for helicopters being used for a variety of applications – it is regarded as accurate an reliable by pilots; from a navigation systems viewpoint, a DME set at the Mull lighthouse helipad would have been an obvious solution to what would have been a common situation for pilots on that leg. Yes, I know, you are not supposed to rely upon a radio NavAid for terrain avoidance in VFR but to have used it to aid a turn which should have kept them in the clear is little different to using a DME for, say, a turn on to an approach – in theory, not running into the mist (which, as they would have known, started just after the shoreline) relying for safety ultimately on their visual judgment of the range to that amorphous gray blob with no significant ground detail visible and no perspective going straight at it at low level.
The last moments of the flight suggest that they were surprised by entering the mist – at about the time that they entered the mist, the following would have happened:
Mist is suddenly flashing past them;
The shoreline is suddenly right beneath them;
The RADALT warning would have gone off.
And what happened about this time? The aircraft (thus apparently under control) started an entirely reasonable evasive turn.
If they were using such a beacon it could have been giving a false reading in which case the pilots could hardly be blamed – although ultimately they should have used visual only for terrain clearance, the effect of having a clear readout from a trusted system would have given them a strong bias to what they thought they were seeing distance wise..
Maybe this scenario is not what happened but I believe that it is sufficiently reasonable to have warranted being explored as a possibility. The thing is, it could have been verified or dismissed by a few pertinent questions in the right forum, questions like:
Was there a beacon there on that day for evaluation or had it been positioned there for some time?;
What was the squawk code before impact (as per radar recordings) and what was its meaning to the air traffic controllers? (The code found set in the wreck was 7760 which may indicate that they were referring to a ground transponder.);
If there was no beacon for the judgment of that regular turn in such common conditions, what was the accepted practice on this leg? (Don’t start, this was a regular route and I don’t believe that the problem of closing with the Mull would not have been discussed.)