CarltonBrowne the FO
Because the Macrihanish TACAN was not LOS (Line of Sight) to the Chinook on this leg and therefore unusable.
Arkroyal
Most people think of DME in terms of the co-located VOR/DME (civil) or TACAN (mil) ground equipment – the big array is for the VOR, the DME does not require a big array and, with modern electronics, the ground transponder of a DME system can be incorporated into a hand held communicator such as for example the PRC-112 (tens of thousands of equivalent units have been manufactured for the US alone).
Most people think of an IFF system as an aircraft transponder which, when interrogated by a signal from another aircraft or ATC ground equipment, replies with a coded signal giving information on the type of flight, the aircrafts altitude, etc, but many modern airborne IFF equipments (such as for example the AN/ARS-6) have the ability (when selected in the right mode, “3a”) to interrogate any of the portable types (eg as above) within LOS range, giving an accurate range to such units.
These are typically fitted in SAR and Support Helicopters (SH) for the purposes of (I name but a few – not my imagination but extracted from manufacturers blurb:
Location of downed aircrew;
Location of special forces elements for re-supply or extraction;
Marking of isolated navigation hazards in operational areas …..
Why not just GPS, I hear you say? – in case GPS is jammed/ DME is relative and accurate/ it only transmits when correctly interrogated thus remaining covert as much as possible, etc..
You say:
<<I've said this before, but if the crew were using ANYTHING other than the Mk 1 eyeball for terrain separation, they were indeed negligent.>>
With respect, this is the simplistic VFR vs IFR argument - I have tried my best in past postings to explain the scenario where, in such common conditions that prevailed at the Mull, a special arrangement MAY have been accepted to help helicopters (suitably equipped and with trained crew as was the case in ZD576) in this one awkward turn that would otherwise have complicated a walk-in-the-park/pleasant low level VFR flight on a regularly used leg.
And:
<<I don't beleive that thay were.>>
I say that this is a moot point at this stage as the existence of such a system to the crew should have been mentioned at the inquiries. Further, that mode 3a WAS selected strongly suggests that they were using it. Confirmation of the squawk code as allocated by ATC Aldergrove, or as detected by them (Aldergrove ATC had been tracking them and so would have been able to confirm the code that they had set PRIOR to the impact) could settle this issue once and for all – for example, in some regions 7760 specifically alerts ATC or other a/c that the aircraft in question is working off such a ground transponder in local operations such as SAR exercises.
It is of note that queries at the Lords inquiry on the meaning of the squawk code were answered in generalizations – it would seem rational for the squawk code apparently (to ATC) used to have been stated.
The crew should not be held negligent if this was an accepted practice for such flights on this leg unless the ground equipment was known to be secure, unaltered, and in its correct location at the time (and these factors should have been covered in the inquiries if such a system was used).
My initial interest in this case (10 years ago) was to remind authorities to check these factors before evidence to the contrary was lost – not to push it as THE cause but the checking could not wait while the rest of the investigations continued.
THAT TURN
Let me try and explain better.
If you were flying VFR at low level over water or a featureless patch of desert, you would glance regularly at your altimeter for reassurance, surely? – a basic, trusted instrument, the altimeter. Now, if you were descending to low level over such terrain (VFR still), you would trust your altimeter over your visual judgment – as you approached the (featureless) ground you would refer to your altimeter to support your judgment of the right moment to level out?
The Mull in those conditions would have been a grey, featureless, irregular surface which was being approached at high speed and it was desired to make a turn up the coast close in; imagine if a crew member was watching a nice digital reading of range to go on a trusted system – he could even have been counting it down for the pilot; how strongly would this influence the pilot’s visual judgment?
No matter that the judgment should only have been visual, think of the influence of that countdown in his interpretation of what he sees.
And what if the ground transponder had been moved/substituted/altered etc.?