Meadowbank
One of your questions (on radar recording) is answered in the following section – the others at the end.
SVFR
The crew were OK to use DME near the Mull if so agreed in Special VFR.
This flight was a classic for SVFR: because of the icing limitation, IMC was not on and the difficult part (for VFR) was localized (at the Mull) – conditions so prevalent that a routine SVFR practice would have been expected.
Special VFR is a concession granted by ATC not to go IMC in a planned VFR flight where VFR conditions cannot be met for the whole flight but, with suitable arrangements by ATC and the pilot, the pilot wishes to proceed VFR (very common for helicopters who do not wish to fly IMC in cloud).
I believe that in an SVFR flight, use of a local beacon would have been allowed if agreed with ATC; however, ATC would probably have monitored progress in the area of concern (yes, I have found a credible reference to the aircraft track on military radar right up to the point of impact) and would probably have allocated an appropriate squawk code to identify the SVFR aircraft and remind them of what it was doing nav wise in that area (the possible origin of 7760 on ZD576’s IFF).
DME transponders, in normal circumstances, give very accurate range and would have been a logical choice to use in navigating an isolated hazard – if there wasn’t one at the Mull helipad, then there should have been, such would have been the utility of it for a/c regularly doing that route (closing to handrail up the coast).
It is more common for helicopters to use SVFR – so perhaps you contributors with (much) fixed wing experience can be forgiven for not having brought this option up earlier.
It is amazing that this option was not discussed at any of the inquiries.
Meadowbank – you wrote:
<<I agree that their change of waypoint tends to indicate that no emergency was ongoing. However, your assumption that a turn of 3 deg to the right shows control is incorrect. The aircraft may have been diverging in pitch and/or oscillating in roll. Incidentally, what are you using as your information source for the 3 deg right turn? There was no ADR and it seems that there is, indeed, no radar recording.>>
If you work back from the impact, allowing a little for that last evasive turn, a straight line from waypoint change at the speed the a/c had been holding for the flight up to that point gets you there – put another way, if at waypoint change the a/c turned right 3 deg and maintained that heading (in a straight line) and speed it gets to the crash site at exactly the right time; if, as you suggest, the a/c could have been “diverging in pitch and/or oscillating in roll” – bearing in mind the nature of the beast – this would surely have resulted in a significant loss of velocity (along that line) and possibly a net divergence from track – this would have required the a/c to have done two things to get to the final position (at the exact time):
Make up the average speed exactly (unlikely as the a/c was already at the top end of its cruise speed);
Do a dog leg to get back on that line exactly (I suggest odds against this happening by chance – and I don’t think the pilots would have been too fussed about re-establishing that path (closing with high ground) if they had been experiencing any difficulty).
Further, an air traffic controller reported that it just went straight (and he specifically stated that he had seen the RECORDINGS).
So I say it is more likely that the a/c just stayed on the heading that it selected at waypoint change when it was under control.
That there is said to be no radar recording and it was not referred to in the inquiries is interesting as it certainly did exist immediately after the crash; together with the radio call anomaly (either the last call unanswered or the recordings after that point no longer available) this rather suggests censorship of some kind for whatever reason.
You wrote:
<<Your repeated reference to a transponder being used for a DME readout is wide of the mark and a number of correspondents (with Special Forces helo experience) have already explained this (look back at recent posts from, amongst others, Tandemrotor).>>
Well, many SAR as well as military helos had been so equipped at the time – I am not imagining the equipment, tens of thousands of PRC112 types had been manufactured for the US and other NATO countries by 1994 – check the manufacturers blurb – better, talk to SAR helo pilots or 7 Sqn.
I repeat, DME transponders, in normal circumstances, give very accurate range and would have been a logical choice to use in navigating an isolated hazard – if there wasn’t one at the Mull helipad, then there should have been, such would have been the utility of it for a/c regularly doing that route (closing to handrail up the coast).
HC2s were so equipped to get range AND (approximate) bearing to portable DME (embedded in PRC112 types) of which there were probably several on the Mull that day on the persons of the SEALS who were all over the crash site at the time (and who are equipped with PRC112s or equivalent).
That steer of 3 deg to the right at way point change was entirely consistent with ZD576 finishing off the approach to the coast referring to a portable DME inland of the lighthouse proper if they did not see the lighthouse itself (or were not that concerned as they trusted their readout).
Whether this system was used or not, its existence and possible availability should have been brought up at the inquiries so that its use could have been determined by specific questions in a formal setting – instead, we have had the sin of omission.
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