Hello again JB – thank you again for some decent constructive feedback. If only everyone would work through the detail likewise.
Here are my thoughts on the points you raised:
<< In a previous post you said that a witness at one of the Inquiries had said that the odds of a control jam in two channels were "billions to one" - please tell me who said this as I would like to see the basis for his calculations, especially given the AAIB comments on this area. They certainly do not dismiss a major control jam as "nonsense" - indeed they accept that it could not be ruled out - what do you base your judgement on?>>
I believe it was Flt Lt Ian MacFarlane who was a proponent of the control jam idea at the FAI. Steuart Campbell’s book, p67-69 on the questioning of Flt Lt Ian MacFarlane by Mr Dunlop at the FAI summarises quite well, I think:
<<Mac Farlane was questioned at length by Dunlop about his alternative explanation … After he had agreed that all the control channels were separate and designed to be very secure, it was pointed out to him that the expected rate of failure of a single channel was not more than once in a million flying hours. He was also informed that the probability of a failure of two channels simultaneously was one in 1,000 billion flying hours. He did not dispute these estimates and also accepted that the probability of both pitch and roll channels jamming simultaneously and then un-jamming simultaneously was “one in a million, billion, billion flying hours”. … It turned out that MacFarlane was unaware that Boeing had, from December 1994, discounted the probability of a loose article jamming the flight controls and had withdrawn a previous requirement for visual checks. … When it was pointed out that he had described his scenario as “probable”, he retracted that view and described it instead as “possible”.
<< Now if, as you contend, the crew planned to land on the field by the lighthouse, why did they change waypoints. They had a VFR flight plan and were VMC when they changed waypoints - at that point as current MOD operators have pointed out they would have expected an immediate course change, indeed geography would have dictated this as well - after the WP change the HSI would, I assume, have been showing the demanded course to WP B, but, of course, they continued on essentially the same course. Why would they take such an inconsistent set of actions?>>
Contemporary systems that interacted with personnel locator beacons fed the range and bearing to the HSI – perhaps this was the case here.
Perhaps someone familiar with the operation of such a system as fitted to the HC2 Chinooks in 1995 could comment (don’t hold your breath waiting for a reply on this one!).
In 47Ds each pilot could select a different source for nav data to his own HSI but if such information went via the CDU as some kind of temporary waypoint that had to be manually selected (as opposed to some hardwired alternative), then neither pilot could call upon waypoints A or B (‘coz there’s only one TANS CDU).
Had there been a PRC112 up the hill (near where the memorial now stands) when it was expected to be at the suggested LZ (the “green triangle” that I have described previously), everything fits (eg the 035M heading and the overshoot that I suggest was the problem).
Regarding the current discussion on weather minima:
If they thought they knew where the Mull was and (rightly) assumed that it had a thin coating of mist it would have been reasonable to assume that
Nothing would be flying out of it AND
THEREFORE they could be as close to it as they liked without breaking the VFR rules as it could (rightly) be regarded as a solid object – a terrain feature which they could in a low level flight approach closely – a fixed object – until the last few seconds, they could have had excellent vis in all directions including towards the Mull as far as the top of the ground hugging mist.
I say again, it was the visual judgment of distance off that was the problem; at a low level (lower than the orographic cloud at 8-900 ft as I believe was the case) they would have been able to see the lower slopes of the Mull from a considerable distance (all the way across from NI, probably) but sight of the actual ground/vegetation/features would have been sporadic as the thin, streaking mist obscured detail/texture.
Their problem was not one of flying inside cloud but one of avoiding a fixed, fuzzy object to which, for whatever reason, they appeared to intentionally approach closely.
This mist is a common phenomenon there, as I have tried to describe at length previously – with that strong wind, it would have followed the slope for several hundred feet until it merged with the orographic cloud formed as the bulk of the air mass reached its dew point – and it would have been unlikely to have been more than 50 ft thick. This is my opinion of the local weather based upon years of sea trials in the NW of Scotland, much coastal navigation, low level flights in those areas in similar conditions, much hill walking in those areas, and several visits to the Mull itself.
Strong on-shore wind against uniform slope in near/moderately saturated conditions (as was the case that day) guarantees such upslope mist – it is the lowest layer of the air mass that is compressed and goes faster than the bulk of the air mass (a phenomenon known as “speed-up”) suffering a reduction in pressure and therefore temperature and thus reaches its dew point well before the height that the bulk of the air mass rising up the hill does.
If you still don’t get it, check out research for wind farms.