Walter
I genuinely never wanted to have this conversation, however it is now becoming inevitable. It may be time you found a resolution to the (rather speculative) theory that you have. However, I don't think you will find it here, since very few of us believe there is any mileage in offering 'alternative' theories. Perhaps it is time you expended your energy along other avenues of inquiry.
For the record (and off the top of my head) let me state the following.
1) As a retired SF Chinook pilot, I am not aware of any equipment fitted to Chinook aircraft, that would allow a DME type, range read-out, from the IFF/SSR system. I am not even sure why one would be required, in view of the accuracy of alternative navigation aids.
2) The AAIB ascribed 'confidence' levels to all instrument readings, and switch positions retrieved from the wreckage of the cockpit of ZD576. I cannot recall precise details, however (because I too was interested in the transponder code at one time) I know that the level of confidence associated with the selected code was low. In other words, the selected code 7760, was likely to have been affected by the mechanics of the impacts of the aircraft with the hillside, and therefore could not be relied upon as being set prior to the accident by any conscious act.
3) I do not believe that this crew would have proceeded at low level, in the vicinity of the Mull, unless the weather was unambiguously suitable for flight in accordance with the Visual Flight Rules. To suggest that they were using ANY instrument to prolong 'marginal' VFR, makes the hairs on the back of my neck stand up. It is simply NOT a runner!
4) You mention the author Dave Mcmullon. Dave is a personal friend of mine, and a very talented navigator (and now training Captain!) Neither he, nor the two crew members on board, would have sat idly by watching the two pilots, fly towards the Mull, counting down DME from a beacon. Dave has suggested to me, he would be waving the fire axe around in the cockpit to prevent it! I know him, I believe him!!
5) You seem to imply some kind of official 'cover up' of the use of this equipment (at least in the circumstances of this accident!) I simply cannot accept this. As cynical as I am about the whole affair, I know people involved in the BOI, not least, the president, and cannot accept either their complicity, or ignorance, in such a situation.
I remain a little bemused by the amount of bandwidth you seek to air your theory (though I have to respect your perseverence, and have recently thanked you for it in a pm)
I feel a little uncomfortable asking you this, but I must. Please accept, that the track you are on, is at odds with the track that we here are pursuing.
You say you are seeking the truth, and are not interested in the limited aim of clearing the pilots. You have a 'theory'.
We believe we already KNOW the truth, and that is crystal clear, and simply this:
Even on the 'official' version of the 'truth' natural justice does not allow these two pilots to be found guilty of ANYTHING!
Yours is a search for 'truth' ours is the search for 'justice.'
I respectfully suggest that you are becoming a very major distraction, from what is an honourable, justified, and tough search for justice for these two young men.
By all means keep us informed of any major developments, but I doubt you will find any further answers here.
Fare well.