Arkroyal
Thanks for your input (still working on that trip but am running a bit late in the season!). Here's some food for thought:
From Boeing document which analyses the flight in great technical detail and which I recommend that you download and read in full
8-7D20-DSS-0306, Enclosure 4
Dated: June 18, 2002
<<In light of the actual flight path that was followed, however, this discussion becomes moot. For, as shown in the previous discussions, instead of turning left to follow the directed course of ~12 degrees True to Waypoint B, the flight crew elected to turn right on a course of 26 degrees True. At the aircraft position corresponding to the last steeringcommand calculation, the cross-track error to the directed flight path would have been ~0.16nm. This turn should not be considered as being the consequence of allowing the flight path to drift, as there was a clear aircraft heading change made, and the heading change was into the wind, rather than with the wind, which would have been more likely if the turn was purely due to drift. Consequently, the real issue that should be addressed with respect to the aircraft horizontal flight path is why this right turn was made.
A possible answer can be determined from the setting of the TACAN Control Unit (CU), this was set to Channel 107x, which is the channel for the TACAN beacon at RAF Macrihanish. If the flight path of the aircraft is extrapolated from the position of the
aircraft corresponding to the last steering command calculation, along the course it was then following, the flight path arrives at RAF Macrihanish, although not on a direct course to the TACAN beacon. This information provides a possible explanation for the
right turn, but does not explain why this turn was made. It is possible that the flight crew had determined from the existing weather conditions that continuation of the flight plan
under VFR rules was no longer possible, and that they were transitioning to IFR conditions using the Macrihanish beacon for direction. Certainly, making this turn placed the aircraft flight path across the highest points of the local terrain, with the sectional map showing the terrain rising to ~1500ft before it descends at Macrihanish. >>
Don’t you think that this suggests the possibility that they had a useful signal?
Boeing puts the altitude during the approach at better than 600ft.
What could put this argument to bed – or develop it – would be if someone retraced the approach in a light aircraft equipped with DME and found out for sure.
Better would be the word of a RAF pilot who did this (regular) run.
The simplistic rule for DME is that you need line of sight for a useful signal – but, having had some experience as a radio engineer with the CAA, with regard to the topography in this case and that a “strong” signal is transmitted by the ground equipment, a severely attenuated but possibly still useful signal may have been obtained; this would certainly be marginal but why doesn’t someone clear this up? It is, after all, a nice area for a jaunt for anyone who flies in the general area anyway.
A little story for you:
When I went over to the UK purposefully at the time of the FAI, I spoke to the lighthouse keeper. We were standing part way up the slope between the lighthouse and the crash site at a point where below us we could see the sea/lighthouse but around us and above there was thick mist. A helicopter came in very close and turned very near us – we could feel it but we could not see it. “They do that all the time” he said; “They turn over that big rock” he said pointing to a prominent feature right on the shoreline. Now, I don't not believe that you come in that close in those conditions without using some reliable primary aid.
The TANS would not have been trusted and ZD576 did not have radar.
Ask SAR helicopter pilots what navaid they would use under these circumstances and you will see my point.