A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Feb 27, 10:08 -0800
Tony Oz you wrote:
By the way, what would those pilots do if the Sun was not visible from the cabin on their current course?
I suspect this is very much belt and two pairs of braces post about 2000. As the Captain said, there are other methods of resetting to magnetic. In fact, if they were previously in true or grid, they must have been checking their heading all along. In any case, if they felt uncertain about going to ‘Compass’ i.e using the earths field to monitor the gyro, they would probably still be in ‘gyro’ mode anyway even if they shortly intended to start calling headings in magnetic below 60N.
Notice the Boeing 737-200 has been in service a long time and from well before GPS became available. I saw them operating into and out of Goose Bay in the early 70s. It would be interesting to know what aircraft the airline had before the 737-200. Perhaps it had an astrodome or a periscopic sextant mounting, which make astro heading checks more practical. Perhaps it was felt at the time the aircraft were ordered that a couple of astro compass mountings would be very useful. A periscopic sextant mounting in the roof would have been better. With the Smiths Kelvin Hughes series of periscopic sextants you could use the normal model for heading checks when the Sun was above the horizon and at night and the model 2T, which had polarising filters, for heading checks during the long periods near the Poles when the Sun is only just below the horizon. Interestingly, with the 2T, you didn’t point the sextant at where the Sun ought to be; you pointed it well above the horizon, because apparently polarised Sunlight is reflected down from the upper atmosphere. Don’t ask me any more about the 2T. My answer will be ‘By magic’. Q. What do you think the T stood for? DaveP