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    Re: Kollsman Aircraft Sextants
    From: David Pike
    Date: 2021 Dec 9, 01:59 -0800


    Chasing submarines in a Bristol Freighter, could you keep up with them?

    During basic training we had personal Hughes MkIXBM sextants.  Before completing the Course, we had to hand in 100 (I think) calculations and shots which we had completed on the ground in our own time.  All completed the calcs.  I’m not sure how many completed the shots.  I know I did.  I took them through the open door of our accommodation hut; it was winter after all.  For in-flight astro the RAF had retained a couple of Valetta T4s with six astrodomes each, which were used purely for our first sextant familiarisation trip.  The RAF had learned some years before that you couldn’t have six students and an instructor all trying to navigate the same aeroplane at once.  Assessed astro trips were flown in the Varsity with one astrodome.

    Advanced training was completed in the Dominie using a Smiths Kelvin Hughes (SKH) periscopic bubble Mk2 bubble sextant.

    Then, after a few holding jobs and courses came the Vulcan.  The Vulcan BMk2 had either a SKH Mk 2 or Mk2b sextant depending upon which happened to be currently fitted.  The SKH Mk2 sextant had a bubble reference (and you had to make your own bubble), and the SKH Mk2b had a pendulous reference, with a graticule which was always present so long as power was applied and the sextant wound up.  I never saw a SKH Mk 2a (a 2b with a swivel eyepiece); a 2c (a later version of a 2a); or a 2T ((twilight), a Mk2 fitted with polarising filters to obtain heading checks in polar regions) in a Vulcan.

    The Vulcan had to have two sextant mountings, one port and one starboard, because the pilot’s blister, which also contained the life-raft, sat directly over the front of the rear crew compartment.  Enthusiastic crews like ours could draw a second sextant before a celestial sortie so that they didn’t have to keep moving the sextant from port to starboard during 7-shot sandwich fixing.  
    That meant you could get the fix over sooner (13 minutes).  Given the choice, most Navigators would choose a Mk2b, because you didn’t have to keep fiddling to achieve your ideal bubble.

    Using the sextant involved the whole crew.  The left-hand Pilot flew the aircraft.  The choice of how much autopilot was used was Pilot business depending on the airframe number and the weather.  The right-hand Pilot monitored the speed change, because the RH ASI was the easiest of the two ASIs to read accurately.  The Nav-Radar completed the pre-fix calculations, looked after the sextants, and watched in awe as the Nav-Plotter put the fix on the chart.  The Nav-Plotter monitored the heading change on their very accurate heading readout, applied the acceleration corrections using a Mears Slide, and put the fix on the chart.  The Air Electronics Officer called the timing.  The Navigators would check each other’s calculations and dot in an MPP somewhere between the fix and the System position based upon Group Rules and the Nav-Radars assessment of his observation.  Finally, the Nav-Plotter decided whether to update the System to the MPP or leave it till next time.

    If there was a 6th person on board, they would sit in the Nav-Radars seat during the sextant work to save them being trampled by the Nav-Radar jumping from side to side of the cabin in the dark. DaveP

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