For those non nav ticket holders, very briefly, an astro fix was done by assuming where you would be when you took the fix and using that position you would precalculate what the altitude (ie angle above the horizon) of three stars would be if you were there from details provided from the Air Almanac and your star table books. You then shot the three stars at the alloted time and compared the actual altitude with what you had precomputed. If the actual altitude was more than you had precalculated, the actual position must be nearer the star than the assumed position by the difference (in minutes of degree expressed as NMs (IIRC!!! it was 45 years ago)help me here someone ). And if less the actual position must be further from the star than the assumed. You then plotted the three lines on your map with ref to your assumed position and hopefully they crossed in a small cocked hat, the centre of which was where you were at the precomputed time.
BOAC taught you how to do this in the 'astro simulator' in Braincrank...this consisted of a plywood box fixed to the ceiling of the class room containing a small ight inside as a star with a sextant mount complete with lever to open the small circular hatch through the hull to the air at 35000' . A three position star fix would be taken by a student and his data used by all the other students to update their assumed position calculations on their chart plot.
In the classroom, the sextant had to be pushed upwards against gravity as you pulled the opening lever as Braincrank was not at 35000' so there was no diff pressure to 'suck' the sextant up. On the aircraft, the sextant had to be more or less 'hung on' to allow a slow docking against the 8.2 diff.
On my first rip as nav under instruction to YYZ, my instructor didn't tell me this (it wasn't ExSp33db1rd I hasten to add, although I did come across him later in the course - and very good he was too!! (That's a beer you owe me if I get to NZ again 'A'.) so when I was invited to 'stick up the sextant' to do the compass check shortly after reaching cruise, I pushed against the gravity I had been used to at braincrank. The sextant shot up to the roof, sucked by the diff, smashed against the plate on the hull and was followed by a tinkling sound of broken optical equipment. It didn't work for the rest of the crossing. Company was called and a spare from JFK sent to YYZ for the return crossing. Company at JFK, said this was the second time in a week they had had to do this - what was going on?
I never found out who the member of my course who had done the same was.
Last edited by Hobo; 17th Nov 2014 at 20:23.