Flight Navigator

[Bubi352]

Did anyone do the FAA flight navigator written test? If yes, how did you study for it? I just bought a fully functional A12 bubble sextant and Polhemus celestial computer. I know there is no application to it but the subject is very interesting.

[Linktrained]

The Earth is a sphere, like an orange, roughly.
Look up at the sky overhead. And find a star that is directly overhead. That is it has an Altitude of 90 degrees. Note the Time. Look up in the Air Almanac to see what is that star's Sub-stellar position is ( or was) for that Time, and that is where you were, at that Time.
If the Altitude is not 90 degrees, lets say 88, then you have a circular position line of 120 nm (2 x 60 minutes of Altitude) centred on that Sub-stellar point.
Most Altitudes will be much less than 90 degrees and the Sub-stellar point will be off the chart. So use an Assumed position to work out what Altitude and Azimuth you would find for your star at that Time. ( I used HO249 with its preselected six stars for a Time and a whole degree of Latitude.) When you measure with your sextant, if your reading is greater than that you Assumed, then you draw a part of the circle as a position line which is nearer to the Sub-stellar Point.


There are additional corrections, for another time !
I held a UK F/N licence but the basics must remain valid, even after many decades.

[Linktrained]

A Flight Navigator, equipped with a Drift Sight, a means of checking the compass and a sextant, were required for flights of 1000 nm over water or 1500nm over land. .


The earlier exams seemed to assume that calculations would be done using spherical or spheroidal trigonometry. ( Adding machines etc. were not around. Perhaps the examiners were Extra Masters, formally sea going, when speed of the vessel would have been different ?) Later on they appeared to accept a more practical approach, allowing the use of pre-computed tables, like the Hydrographic Office's HO249.


Training was available at Britain's Air University, or some correspondence courses in the UK.... Some 60 years ago !


I used a Mk9 BM sextant though an Astrodome, on unpressurised aircraft. Or a Hughes or other types of periscopic sextants on pressurised aircraft. With the periscopic type it was possible to get Sun,Moon,Venus position lines in broad daylight, for a Fix.



Before Periscopic sextants became available, somebody lost their Navigator though a pressurised Astrodome. It was said that a monkey chain was subsequently used to keep the Navigators on board.

[Linktrained]

Both Peri- and Dome types averaged 60 (?) readings over one or two minutes, to allow for aircraft movements. The Astrodome was calibrated, and one hoped not to use the lower readings, except at dawn or dusk. (DR or Dome refraction was one of the correction to be applied together with AR for atmospheric refraction.)


TIME is important because the Earth rotates. A minute late and the sub-stellar position will have moved 15 minutes of longitude to the west. Which is 15nm at the Equator, nothing at the Poles.. and... you can work it out for where you are !

A periscopic type has a limited field of view, so you have to pre-compute. A sight every four minutes allows one to plot the three star fix within a couple of minutes of the final star-shot.


Your A12 Sextant may allow you to find LUNAR time, what people HAD to use, before chronometers became affordable.


( Sir Cloudesley Shovell led four Royal Naval ships onto rocks some distance to the west of here, because he did not know his Longitude. Some 2 000 lives were lost, in 1707... )


More recently the Secretary of the Royal Institute of Navigation sailed, solo, in Jasper, across the Atlantic, using Lunar Time.
Have fun!


LT

[Linktrained]

It was usual to take THREE star shots at something like 120 degrees apart as this should tend to compensated for a sextant having gone out of calibration - been knocked at some time, for example. The THREE position lines ought to meet at one point, but often form a small triangle, called a COCKED HAT, after the kind of hat once worn !


You must have tried finding your position, where ever you may be, just for practice and to get the feel of using your sextant.


The Long Range Desert Group used similar techniques to find their ways across the Sahara in the 1940s. ( An emergency fuel dump was at 24N 24E, IIRC !)

Francis Chichester navigated across the Tasman Sea to Norfolk Island using Astro homing, taking sun sights from an open cockpit float-plane, in the late 1930s, using a sextant similar to yours. He had pre computed the sun's ALTITUDES for his flight time and used a sextant not very different to yours.


I listed earlier, the minimum Navigational requirements for longer flights. Those were what were available for us for world-wide flights until the late 1960s when LORAN was fitted. But that only covered the Atlantic and Pacific.


LT