I hope everyone had as good a Christmas as I did – magic!
Chinook240
I understood from references to the "steer meter" output that the ADI in the HC2 was fully an Attitude Director Indicator but I did not know that there was not a way of driving the HSI from the Flight Management System (FMS, Racal 252 in this case). At first sight it seems strange that the RAF would choose not to have this option as the components were there in similar architecture to the 47D which could display GPS data on the HSI as could, for example, the commercial version of Racal's RNAV-2. Perhaps someone on this forum could explain why?
I also understood Chugalug's point about the necessity of navigating visually at low level, relying upon eyeball and maps – correct me if I'm wrong, but GPS was not to be used as the primary navigation even in civil aviation for some time after this event – but there was no restriction to its use to save pilot workload for keeping to track on a long route (fuel economy and time) provided that VFR were complied with (ie that you could see the ground below and around for a minimum distance).
They had very much done a bee line from Aldergrove to waypoint change, the last 20 miles over the sea without a distinctive feature on the Mull to aim for, and so I say that they would surely have had to have had some practical course deviation indication for the HP – it seems that this would have been on the AI (in the form of steer indicators) rather than the HSI – I do not believe it would have been practical or necessary to have had the NHP looking at the CDU and saying "Left a bit, right a bit" for 20 miles nor would it have been practical to have had the HP trying to read the CDU for himself. Recalling what Group Captain A D Pulford said to the committee (27 Sept 2001) on spatial disorientation:
<< … What we are looking at here when considering the possibilities of spatial disorientation, when flying over the sea in what I would term a "goldfish bowl" (this is when the poor visibility and mist and weather conditions you are flying in tend to merge with the grey sea underneath) the body is deprived of visual cues and you are then very dependent upon the instruments that you have in front of you to inform the brain that actually up is up and down is down. … I think here we were looking at the possibility, as a contributing factor, where certainly the flying pilot—the handling pilot—would have had his spare capacity diminished by the fact that he was having to concentrate on the instruments to ensure that he kept the wings level and the aircraft flying. ...>>
To which I add, “and keep on track” - which they had done over 20 miles of sea with the cloud and mist obscured Mull ahead.
The steer directions on the AI would definitely have been processed data as opposed to raw therefore the potential for error would have been compounded as I have been trying to explain in previous posts. Anyway, I have made the point for a long time that it would have been wrong to rely upon the GPS as close in as waypoint change – they would only have been using it for the route flying and, the crux of my arguments, had to have had a local reference of some kind to have approached so closely in the prevailing conditions, nevermind turn in and reduce power.
Do you agree that the pilots would nevertheless have set their HSI course selectors on the course that they were interested in? That the course deviation (from the GPS computed track) was displayed to the pilots on the AI rather than the HSI does not detract from the significance that I have put on the HSI course selector settings in my analysis with respect to the planned and actual tracks.