Some good material from Tandemrotor .
I have been in correspondence with the MoD with a hypothesis of my own for the cause of the accident. I don't wish to go into the hypothesis itself here, but part of the latest response that I have received is worth sharing for general discussion.
"It is acknowledged by all involved in this case that the aircraft was under the full control of the pilots at the time that the waypoint change was effected. By that time, the negligent act had already been committed. The pilots had already committed to a flight path which would take the helicopter too fast and too low, into cloud. In doing so, they failed in their duty of care, and it is this that is the basis of the finding of negligence."
This is the crux of the matter and, notwithstanding our individual preferred explanations for the crash, it is to show that no negligence can be proved up until this point that would unhinge the case for the prosecution and render a reversal of the finding compulsory. I shall therefore start the ball rolling again.
As Brian Dixon points out, we do not know the speed of the aircraft at the waypoint change. Mr Holbrook (the yachtsman) estimated that, when he saw the aircraft, it was travelling at 80-100 kts, but other estimates suggest that it must have been flying faster than that in order for it to achieve the impact speed estimated by the AAIB (whose estimate is, with all respect to the excellent work they do, an estimate). If Mr Holbrook's evidence could be verified (which it can't) this would already cast doubt on the negligence finding as the AMs contend that the aircraft should have been slowed down prior to the waypoint change. However, I suggest that this slowdown may not have been necessary. If the aircraft was flying at an indicated airspeed of 120-140 kts, resulting, with the tailwind component, in a higher groundspeed of the order of 140-160 kts. As the waypoint change was made some 0.81 nm from the lighthouse, it is reasonable to assume (all parties seem to be agreed on this) that the crew had identified the end of the Mull prior to this action. The Air Marshals’ view is that the crew should already have slowed down prior to this point, but what we cannot know is how much time elapsed between the crew identifying the Mull and the selection of the new waypoint. It is entirely possible that the crew had already had the Mull in sight for a considerable period of time, that they had been considering their options for a minute or more and therefore had no need to slow down as the Air Marshals suggest they should have done. At first sight it may seem unlikely that the crew had this much time to think, but all that would be needed, at a groundspeed of 150 kts, is an airborne visibility of 3.5nm (approximately 6.5 km) for this to have been possible. Given that Mr Holbrook had been able to discern breakers on the shoreline from his position at sea level, visibility away from the Mull, where the clouds were shrouding the hills, is likely to have been perfectly satisfactory for flying at a groundspeed of about 2.5 nm/minute (150-160 kts). A previous poster has even indicated that an eyewitness could see the Mull from the coast of Northern Ireland at the time. My own experience of low-flying around the West Coast of Scotland is that, generally speaking, when outside of any cloud one is flying in good visibility.
The fact that ZD576 must have been flying at about this speed at the time of impact and had therefore not slowed down prior to the waypoint change is a strong argument that the airborne visibility was perfectly adequate, rather than evidence that the crew had been negligent. It is certainly illogical to surmise that the crew had already "committed to a flightpath which would take the helicopter too fast and too low, into cloud."
Alternatively, the Air Marshals suggested the crew should already have decided to climb, but this would have been foolish, not only because of the icing conditions to be expected in the clouds in that position, but also because to turn early towards the next waypoint and to fly up the coast in VFR would have been so much easier.
The final alternative suggested by AM Day is that the crew should already have turned away from the Mull. I suggest that this is exactly what they attempted to do, but for some unknown reason (whether it be FADEC, UFCM, control jam or even meteorite strike, sudden appearance of another conflicting aircraft or whatever) they were prevented from doing so.
In all probability, the crew had no need to slow down because they were in visual contact with the coast of the Mull and only needed to make a 7 degree turn towards the next waypoint – a matter of certainly less than 5 seconds’ manoeuvring. I challenge the Air Marshals (or their 'supporters') to come up with an explanation as to why the crew would select the next waypoint if they had decided to fly "too fast and too low, into cloud", rather than make the small turn to the left.
To claim evidence that they were already negligent by the time they selected the waypoint change is nothing short of preposterous. There is simply no such evidence other than the circumstantial hypothesis that, having flown into the hill, they must have got there by simply carrying on, under full control, in a straight line! I accept that this remains a possibility, but that is all that it is - a possibility, and a fairly unlikely one at that! Certainly, it is not what happened "beyond any doubt whatsoever".