Master of None
SFFP dobbed you in as the expert in this area when he wrote:
<<Because I was never a Chinook avionics tech I cannot answer your question however Master of None in posts #3313 and #3326 DID answer your question and qualified his answer with "As someone who has operated and instructed on the RAF Chinook HC2" but you chose to ignore him as is your want.
Why you continue to ask questions then ignore the answers given, even when they come from such experts in their field is beyond me >>
So I suppose I had better address what you have written:
I ignored your post #3313 as, at first reading, I thought it was inadequate and confusing; however, going through it again, I recognise that it was just very concise and someone experienced with the system should have understood your point clearly – I recognise that point now and I apologise for not responding earlier – the problem is one of confusing the course selector as a whole and the Course Deviation Indicator/Bar (CDI/CDB).
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Your post #3326, again giving a confusing (to the layperson) description, showed that you have misread or misunderstood what I have written, the bit about there being no VOR or TACAN of use particularly illustrates this as it was a fundamental point that I was making (agreement!).
I reproduce those posts of yours below and follow them with a brief summary of how I believe the instruments work so that readers can compare and perhaps get a clearer understanding from the two versions – and perhaps you could contribute further understanding:
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Post # 3313
<<Walter on the Chinook HC2 the CDB can be set by either pilot to any hdg regardless of any other outside stimuli (infact external sources cannot move the CDB), he can also manipulate the other pilots CDB. That the CDBs were on different hdgs would suggest to me that something may have nudged it, say some turbulence?.. or crashing at speed into a large cumulo-granite!>>
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Post # 3326
<<As someone who has operated and instructed on the RAF Chinook HC2 and not just gleaned information from other uninformed opinions I'm confident that there is no embaressment on my side. The CDBs can only be moved by the pilots. They can be used to display cross track information from a variety of sources including a navigation computer, but are often used just to display to the Handling Pilot a desired heading set by the Non Handling Pilot. The reasons the CDB in this case was indicating what it was will never be known but it cannot have been 'dragged' around there by an external influence. If the HP/NHP had reason to doubt the nav computer they may have been looking for a known point to 'zero' it on, please note the use of the phrase 'may have'. The only other info the CDB can work on are TACAN or VOR indications, neither of which were situated on the Mull that day, at least they weren't when I was.>>
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My version:
I recommend readers without an aviation background to get familiar with the functions and uses of Horizontal Situation Indicators (HSIs) by reading around – there are plenty of well illustrated descriptions available on the web – and perhaps play around with a PC flight simulator doing a bit of VOR nav – this would make it easier to follow the arguments.
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COURSE SELECTOR DESCRIPTION
The course pointer has a head and tail, physically attached to each other within the instrument housing, and when they rotate about the instrument face, one moves with the other. It is moved around the face of the instrument, and therefore relative to the compass rose, by turning the knob on the lower right face of the HSI. The position of the pointer relative to the rose is repeated digitally in the top right corner of the HSI – this is the Course Indicator.
(Briefly) the rose is driven by the gyro compass and moves around as the a/c heading changes – the course pointer position on the rose is the desired course that is fed back to the nav computer/aid/whatever which returns the drive for the Course Deviation Indicator (CDI) / Bar (CDB), a bar which lies between the head and tail of the pointer and which indicates in what sense the a/c is on or off track and the alignment with the desired course.
The pointer head, CDI and the tail are collectively called the COURSE SELECTOR.
The course selector is used for navigation tracking in several modes including for example following a VOR radial or the track to a waypoint generated by a GPS/Doppler nav computer.
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SLAVING
While in the a/c type of interest here each pilot may independently select different navigational modes for display on his HSI, it is usual in route navigation for the CDI, just the bar in the middle that is, to be slaved to the co-pilot/navigator’s HSI, such that the handling pilot just has to follow the CDI to keep on the planned track.
How this is done on the 47D is that the navigator presses a button on his HSI mode select panel (CMD SEL) which does two things:
If the AFCS is set accordingly, his HSI heading bug will demand the heading (no interest here);
His CDI position and orientation with respect to the compass rose will be duplicated by the other HSIs CDI – regardless of what position the handling pilot has his course selector pointer (and course indicator), moving his knob around will move his course selector pointer (and indicator) but will not affect the orientation of the CDI wrt the compass rose on his HSI.
However, there seems to be an area of confusion here – perhaps the HC2 is different in this respect and I would appreciate being put right if the following assumption is wrong:
The type of HSI fitted to a 47D of the time was an electro mechanical device which would not apparently support a servo type movement of the course selector pointer by an external source – just the knob seems to be the only way of moving it (I believe MasterofNone was making this point) – this is quite different to the CDI bar which is driven externally and therefore it is easy to understand how this is slaved.
Inspection of cockpit photographs of a 47D and an HC2 show what appears to be the same type of instrument (HSI).
Thus I am assuming that, even in the case where CDIs are slaved, the individual course pointers (along with their digital indicators) can have different bearings to each other – and these two values were preserved in ZD576 (028 and 035, evidence rated “positive”).
Had they been just doing normal route flying, the position of the CDI bars should have been in the same position – their positions were preserved by way of marks caused by first impact and were not the same (left/nhp 0,9 div left, right hp 1,5div left, evidence “positive” and “not highly positive” respectively). So the track bars were (assuming 1 div is equivalent to 5 deg) 3 deg different.
I believe it is the latter, the difference in the CDI bar positions, that MasterofNone was describing – not the whole course selector – when he was suggesting perhaps the impact had caused those positions.
(It is interesting that, from memory, Group Captain Pulford [was it?] did not make this distinction either when answering questions put by Lord Toombs at an inquiry.)
One should consider that the difference in bar positions, while not huge, was not to be expected (if slaved) from impact because of the positions of those instruments in the same plane on the front panel – they should have experienced the same dynamic force at initial impact.
As I have pointed out in previous posts, it is hard to see that the hp’s HSI selector could have been moved from 027 to 035 by impact.
If you consider that the last leg (pos of waypoint change to crash site) was on a track of 035 mag this suggests that the hp’s course selection of 035 was deliberate – it is just too much of a coincidence to have been otherwise.
The combination of the CDI bars being different and the Course Selector values being different could reasonably suggest that the HSIs were not slaved. I do not know how this is achieved in an HC2 but the HC2 Chinook has a few more options on its Mode Select Panels (lots more buttons than the 47D), the choice of feeds coming via the FDIC (Flight Display Interface Controller), and so I would welcome some comment on this from those who should know.
Just for interest, in later HC2s that were equipped with ARS6 was the usual practice to have the hp’s HSI working from ARS6 input whilst the nhp kept an eye on area nav on his HSI?
I agree with MasterofNone that there were no VORs/TACANS within range on a bearing of 035 and I add that none of the stored waypoints in the SuperTANS could have generated such a bearing – points I have made before when suggesting that they could possibly have been referring to a PRC112 that was supposed to be on that LZ I have described.
While my suggestion of this particular equipment is speculative – the only thing I can think of that could have misled them – there is plenty of evidence that, when summed together, points to their intention to land or closely approach that landing area; this should be addressed by the Mull group as it strongly indicates an extra task, thus far undeclared, that surely clears the pilots of blame.