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    Re: A lesson in a photo
    From: David Pike
    Date: 2016 Aug 30, 01:30 -0700

    Frank.  I was aware of the sine tangent similarities for small angles (and wasn’t there also something similar about sines and radians?) but I chose tangent, because base is easier to spell than hypotenuse.  It’s also the first trig function taught in English schools, so it’s the one you always remember to work the others out from.  My Dad even trusted me to take his B. Cooke & Son sextant to school so we could measure the height of trees using tangents. 

    I was also aware of the radian ratio thing, but not in that sort of detail.  All we said was if a segment of circumference = to radius was 57 degrees, then one degree gave a segment circumference of 1/57 of the radius.  I.e. if you were one off to the left or right in 57, you were one degree off, except 57 isn’t good for mental arithmetic, so we called it the one in 60 rule.  The units didn’t matter as long as they were the same.  You could use it for nm in the air or for feet on the ground.  The Vulcan had a very accurate heading reference system (HRS), but it had to be lined up accurately on the ground.  A Vulcan was also 60 feet from nose wheel to tail.   Therefore, the true bearings of all the lines on the concrete the pilots had to follow to place the aircraft centrally in their pans were measured with something known as a precision indicator of meridian (PIM) and recorded. Then all you had to do was assess how far the nose wheels were off the line, how far the tail was off the line and you could work out the aircraft’s true heading.  DaveP

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