Back in 1982 BA were exploring polar routes to Japan using the then state of the art INS nav equipment. I was flight engineer on a 747 trip to Alaska, with the fleet chief pilot as captain, and various avionics engineers keeping a watch on the nav gear.
All went well as we approached the pole, CAVOK viz and white flat ice with bright sun. Then 30 secs from o/h the pole one of the INS units failed, shortly followed by the other two. Failure mode was due to 'degraded nav data' 'standby mode' Frantic efforts by the experts failed to convince the INS units that all directions from the pole are South. This was during the cold war and a Korean airliner had recently been shot down by the Russians. It was essential to not be heading towards Russian territory and we needed a reliable heading to steer. But the three compasses were useless, our two VORs were useless, and the two ADFs refused to pick up anything. I remember suggesting that the boy scouts taught me to point the hour hand of my watch at the Sun and read off North at 12 o clock. Four faces looked at me and someone said pi*** off eng! But the First officer dug out a set of astro sight reduction tables and after much pencil work came up with a bearing relative to the Sun which we took up. Two nerve racking hours later the ADFs picked up a faint signal from a Canadian beacon and we were able to get a position. Feeding the data into the INS sets got one of them back on line, and we began to relax a bit.
