Yet more on maps......
Boac ,Dalek and others.....
Guys............. I understand entirely that they may normally only have used the Supertans as a rough guide , there have been several quotes about choosing a place with vertical extent as a waypoint , so that it can be recognised from a distance......but what happens when the visibility ahead is becoming diminished and you cannot actually recognise the waypoint ...????....you slow down a little maybe and continue on the same heading for a short time..????.......
.......even using it as a rough guide is dangerous so close to a high place........in the event they do appear to have flown almost directly towards waypoint 'A' but drifted a little to the right of it ..... .....all the actions of the crew are consistent with them confidently tracking in towards what they thought was a safe waypoint , with good adequate margins all round .......so where did they think it was...??????????...
..in view of the terrain it would have been safer to have chosen a point offshore in order to look for the lighthouse from a safe distance( yes I know its easy in hindsight , perhaps that is where they thought it was)........
........I think that some contributors do not have a feel for the terrain around the Mull lighthouse..........to put it into context............
..the coast is pretty much north/south , they are approaching at an shallow angle , sea on their left , land on the right.......if they creep east they are closer to the land..........so the longitude setting is more critical than latitude......
...the waypoint 'A' ......as plotted..... is inland ,some 670 feet east of the lighthouse , and is at a point 323 feet above sea level...........that is nearly 100 feet above the top of the lighthouse ....(and is actually in a gully where a stream runs down the shelf to the cliff edge ).............surrounding ground is even higher eg. 350 feet just north and east of the waypoint ........climbing to 400 feet further to the north as you become east abeam of the lighthouse......
....but then the GPS signal had a quite considerable error (in my view)........and so was guiding them to a point a further 490 feet east and at an elevation of 500 feet above sea level...........
......so you can see the choice of the position of waypoint 'A' becomes critical.............the difference between safe and not-safe.........or go and no-go is very small , in aviation or any other language.......
...if they had chosen a longitude value of '5 deg.48.5minutes' , the waypoint would have been offshore , and had they flown in the same manner should have survived..............
..this degree of accuracy should have been possible , as it is documented that they had 1/4 Mil maps available which in those days were OS maps with additional aviation information over-printed.........a measurement of 1/2 minute of longitude is about 2 mm or 1/12 inch at that latitude..........
.....(.I have a' 1/4 inch to the mile ' OS map of the area , which is the map the 1/4 Mil was based apon , and the lighthouse does appear to be close to , but on the seaward side of , the '5deg 48min' longitude line....interestingly the latitude is not accurate on this map , as the scale is set about 1/2 minute of latitude (1/2 nautical mile)..further north than it really is.......but it is the 1970 revision so may have been corrected by now.....???????????......)
......if the weather had been better any mis-plotting might never have been an issue .......
.....if they had been navigating using the doppler source they would probably have realised in time that they were close to shore ( waypoint 'A' would have been on the shoreline just south of the lighthouse..)......
.......if the GPS error had been in the opposite sense..(direction) , it would have led them to a waypoint 'A' which was also on the shoreline much the same as the doppler source , just a little further out on the rocks...............
regards Robin.....