Adventures in aviation celestial navigation
To the editor:
What many voyagers don't seem to appreciate about celestial navigation, besides its obvious back-up benefits, is the intellectual pleasure it gives the navigator in knowing how to plot a line from a celestial sight. I know from experience how good that feels.
I set out to sea as a cadet in the Canadian merchant marine in 1946. In between chipping and painting I spent my time in the chart room gleaning what I could about chart work and the arcane ritual of sextants and chronometers. Little was actually taught to me, as it was by tradition left to the cadet to ask and sort things out for himself.
In 1950 I changed professions and became a pilot in the Royal Canadian Air Force. I did my tour of flying fighters and instructing. In 1956, unhappy at the prospect of flying a desk, I left the air force and started with United Airlines and by 1963 was flying DC-8s on the San Francisco-to-Hawaii route.
It was a pleasant way to earn a living: once airborne only the autopilot and navigator really worked -- we pilots spent the time drinking coffee, making position reports, and looking out the window. This tended to be a bore, so once we settled down in cruise I would get out of my seat and look over the navigator's shoulder and ask questions. In time I got to sit at his table and fiddle with his navigation paraphernalia. Most of it was WW II-vintage stuff, though the periscope bubble sextant was the latest model.
And it was then that I began to appreciate the enormous differences between marine and aviation celestial work. On a boat the navigator takes an altitude, notes the time, and works the tables for a line of position. Essentially simple stuff, and even if you are slow it's very unlikely there will be more than a few miles between the sighting and the plotting.
Flying at eight nautical miles a minute is another story, and only half of the story at that, because for a lot of good reasons, to do with inertia and altitude, a marine sextant won't work in high-altitude, high-speed flight. Errors of 60 or more miles are predictable if you do try to use a marine model. To compensate, the aerial navigator uses a bubble averaging sextant. The bubble replaces the horizon, and the averaging clockwork device, usually operated over a period of two minutes, balances out the inertial errors.
Besides all that, the periscopic sextant, inserted through the overhead of the cockpit, has to be pointed in the right direction and set at the right angle to locate the celestial body, as its field of view is very small. This means that the sextant has to be pre-programmed for each shot; as each one takes about five minutes to program, two minutes to shoot, and 10 minutes to fiddle with the averaging device, calculate, and plot, it means that, by the time you put pencil to chart, the line you draw is where you were 80 or so miles behind you. The net effect is that the flight navigator spends a lot of time advancing and retarding lines of position to find out where he is from where he was. Tight, neat triangular fixes that I used to see in my merchant marine days became something of the past.
It was also about this time that Inertial Navigation Systems (INS) started coming on the scene, and United declared its intention of phasing out the professional navigator in favor of these black boxes. And it was because of this and the fact that I had no ticket to reflect on my years at sea that I challenged myself to master the art of flight navigation and get an FAA Flight Navigator's ticket before the craft and position was just aeronautical history. Using the Air Force Air Navigation Manuals and the Navy's HO 216, I set to work and eventually took the battery of written exams.
The air regulations, meteorology, weight and balance, and performance exams were three hours each, and because they were already part of my pilot curriculum presented no problem. The other three exams, each lasting five hours, where as big an intellectual challenge as I've ever experienced. To begin with, the FAA examiner presented we with the exam book and a three-by-three-foot piece of paper without as much as a speck on it. Supposing it to be a chart, I took it back to him and asked where the lat and long lines were. He told me to read the first question, and that's when I really broke into a sweat. It required me to create my own Mercator chart for latitude 35° N and longitude 180°. I was stumped, and sat for five minutes before forcing my mind to remember something I had picked up in the Air Force about each succeeding latitude line being equivalent to the next increasing angle. That part solved, the exam then got worse, as all the following navigation problems had me crisscrossing the dateline with each follow-up question dependent on the accuracy of the previous problem. To top it off, the finale required a square search for a downed aircraft with the winds, of course, being variable depending on position and altitude, so that by the end of the five hours I was thoroughly wiped out.
The next exam challenged my theoretical knowledge of grid navigation: the sort of thing used when flying over the poles where longitude lines crowd into each other and magnetic variation and dip make the magnetic compass useless and aircraft have to fly by gyroscopic compass. The next exam had me pressure pattern flying, where a correlation between a radio altimeter and pressure altitude can, so they say, give you a pretty accurate line of position. This was only theory to me, but I hacked away at it the best I could.
Anyway, I passed this horrendous intellectual torture and took the results to the chief fight navigator. He was as surprised as I was that I had passed because it had been years since anyone had taken the test and nobody seemed to know what was on it anymore. A flight test date was scheduled, and both he and an FAA examiner came along to hover over me on my rating ride between San Francisco and Honolulu. As part of the test a fix from three celestial bodies was required, and as it was a day flight I spent the previous day pre-calculating where the sun, the vaguely visible moon, and the almost invisible Venus would be so that I would be able to set the azimuth and altitude into the sextant before each shot and not waste time scanning for them.
After we got airborne and cleared the departure corridors I took over and just about blew it, but totally. My first required action was to give the captain a card with the magnetic heading to the first waypoint with an ETA. The old-time navigators had warned me never to let go of that card till I had doubly checked the variation and deviation arithmetic, because if it was wrong it would mean an automatic failure for the whole test. The captain put out his hand to grab the slip, but for some instinctive reason I would not let him take it way from me. A potential tug of war nearly was averted when I said: let me double check it. And sure enough I had added instead of subtracted. Everyone, both pilots and examiners, breathed sighs of relief, for they had noticed the error and none of them wanted me to fail.
The three-LOP celestial fix, something I had never done in flight before, went surprisingly well, thanks to the preflight planning. I used the second shot as the master line and advanced one line and retarded the other.
There were other challenges to overcome, but I knew I had it made when we passed the halfway point and the FAA examiner cheerfully quipped, "I hope you don't miss the islands because we don't have enough fuel for China."
My navigator's ticket is now buried with a lot of my other aeronautica in a cardboard box in the garage. I never did use it professionally, but I'm more proud of my navigator's ticket than any other of my FAA licenses
I don't venture too far from land in my boat anymore, and my sextant sits on a shelf, but I have the pleasure of being a celestial navigator, and learning how to be one was the most intellectually satisfying thing I ever did.