But Prestwick heard the call and were surely handling the area, not Mac tower?
And approaching up the coast, the high ground to the right and the all too common mist would not have often allowed a VFR low level helo to safely see the aerodrome to its right – surely this is a classic situation for Special VFR?
The Machrihanish MATZ is immediately to the north, only minutes away, and would have had to have been considered at the time in question. Surely it would have been a sensible practice to get low flying helos on this leg to standardize their turning point and accurately estimate their ETA to cross the runway approach by having a reliable reference (purely visual could have resulted in different turning points away from the Mull depending upon the conditions resulting in inaccurate ETAs and perhaps not so square crossings) – hence the utility of a local DME beacon.
This is, I suggest, subtly different to using the DME for terrain avoidance – it was being used to get the a/c close in for a standard leg; while visual reference was the rule for safety, with the difficult scene of land whose detail was obscured, a clear numeric readout from a trusted system would have had a strong bias on their visual judgment. Had such a readout been wrong, they may have got too close to the mist before they had clear indications (eg, actually seeing the shoreline pass beneath them, entering the mist, or hearing a radar altimeter warning – all of which would have been possible at the time of their final desperate evasive turn) to effect their intended turn away in time – which is what I believe happened.