Pulse1
Try not to confuse the “Heading Bug” (HB) with the “Course Selector” (CS).
Both would be on the Horizontal Situation Indicator (HSI) on a 47D Chinook (and therefore presumably on an HC2 Chinook).
The HB can be used in a couple of ways: say, your are flying VFR – you can turn the HB knob until it is on the heading that you desire and it will be a simple reminder – if you have an idea of how many degrees the wind is blowing you off track, you can add that to your heading, increment or decrement the HB’s position on the rose accordingly, and that will be a simple reminder for you of the direction to steer to keep you efficiently on a straight line for your desired track/route.
Further, when the autopilot is engaged it will keep the a/c heading on the heading that the HB is on.
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The CS comprises an arrow and parallel bars (referred to commonly as “Track Bars”) and/or a centre line (the bars/centre line are the “Course Deviation Indicator” - CDI).
The CS arrow is moved around the compass rose by turning a knob on the HSI and the heading to which the arrow is pointing on the compass rose is repeated as a three digit number in a window in the instrument, a bit like the old mechanical odometers in cars (this number was preserved in ZD576 so we know what the CS was set to on the handling pilot’s HSI).
The CS is used for navigation tracking – that is, put simply, keeping you right on the line/route/track that you wish to follow over the ground as determined by whatever navigation system you are using – in 47Ds, the Mode Selector panel for the HSI gives a choice of the CS working off the GPS/Doppler nav computer (eg a SuperTANS) or a Radio Nav aid (VOR, TACAN, etc – whatever your nav radio is tuned to).
For example, if you are familiar with VOR operation you know that you can get a precise heading from/to a VOR beacon; if you were doing the old VOR navigation (in pre GPS days the most common method for aerodrome to aerodrome nav) you would have your nav radio tuned to the aerodrome of departure’s VOR, set the CS to the desired track away from that aerodrome, and the CDI will tell you if you are off track, and you steer towards the CDI to get back on track, etc..
The way a nav computer inputs the CS is more relevant to this case: if you want to go to a waypoint, the GPS/Doppler computer will calculate the required heading and also calculate continuously your adherence/deviation from that track.
Even if you are flying VFR, a navigation system prevents you getting lost and keeps you on an efficient course!
By the time ZD576 had reached the critical area it had already covered almost 40 miles (VFR); this included many miles over the Antrim hills where conditions were not very good and several miles over the sea while the nearest landmass ahead was shrouded in cloud and mist; yet they got to the position of waypoint change (about a mile from the coast) which was exactly on the 027 radial (magnetic bearing) from Aldergrove – the very radial that they had declared to ATC that they would be following.
Speed/distance/time calculations by Boeing’s Mr Mitchel deduced that they had to have been very much on track the whole way to that point – in a helicopter with a strong wind blowing.
It would seem obvious that, up until this position of waypoint change, the handling pilot would have had 027 on his CS.
(The non-handling pilot had 028 on his and he wasn’t the driver.)
After the waypoint was changed in the nav computer, the a/c turned right and made a track corresponding to a magnetic heading of 035 to the crash site – this was the setting found on the handling pilot’s CS.
The nature of the knob makes it unlikely that it could have been displaced by so much in the impact and besides the a/c had followed that track on its final leg.
This implies that the turn was intentional and, as I have tried to explain, there must have been a navigation system that could determine that track or surely the pilot would not have bothered turning the CS knob to that heading – remember, if you needed a simple reminder (unlikely in close manouevring) you could always use the heading bug.
Now there was no known suitable radio navaid in that direction, and no waypoint was stored in the SuperTANS that would have been of any use to the CS on that heading – a bit of an enigma?
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Anyway, I hope you see my point as to the relevance of the CS setting.
I believe that the combination of their dumping the (still useful) waypoint A from the SuperTANS and the selection of a new course on the CS suggests that they had some other navaid to work off – the only one I can think of is a PRC112 but I am told that this was not possible.
If the system had been under trial/demonstration/exercise on this occasion, I would suggest that it looks like they were expecting it to have been at the landing area but it was actually up the hill.
Any other suggestions welcome.