We crossed the country to Shrewsbury and settled ourselves in Porthill. Daily, I had to rise early, cross the little footbridge over the Severn, up the "Quarry" to St.Chad's at the top, and down into town for the bus to Shawbury. Tranship to the crew bus to Sleap, where our trusty"Bendix" clonked and whirred ready for us.
If you want a complete description of the "Stephenson's Rocket" of GCAs, you cannot do better than Google up:
http://www.rquirk.com/cdnradar/cor/chapter21.pdf (double click).
by Whitehead and Batch, (link kindly found for me by
Molemot in #4011 p.201, and as amended by my #4022 p.202).
So sit yourself down in front of my CRT (I have the Azimuth tube, the tracker, sitting at my left, has the Elevation one). First thing, I am
not in front of my tube ! In those days, it was feared that the emanations from a CRT might be dangerous, as the visible spectrum was mixed with X-rays and all sorts of nasties, which might limit - or indeed dash - all your hopes of posterity.
A crafty solution had been found. Now, IIRC , the tube was mounted
vertically, shining directly downwards through an half-silvered mirror set at 45°. Apparently the light (which is all I was interested in) was reflected back to me, the nasties went straight through harmlessly (or were stopped by the lead glass in the mirror ?)
(You may well ask why this information was not shared with the Great British Public, who in those days were glued to their "Goggle-Boxes" all evening till all that was left was the little spot in the middle. But it seems that, as they didn't have their noses an inch from the tube - which we did - they were reckoned to be safe. At least, no reduction in the birth-rate was observed that might be attributed to this cause).
My nose wasn't exactly on the mirror, but on the "Cursor" just above it. The boffins could give me a Plan Position Indicator (or a 40° segment of one), but hadn't yet worked out how to superimpose an electronic centreline to run between the "goalposts" of the Touchdown Markers. That came on later types.
So we had to do it the hard way. The Cursor was very like a transparent perspex school ruler, some 8in long and 2in wide. Down the centre of this was scribed a line. Mounted vertically close over your mirror, it was pivoted from the top, you adjusted it so that the top of the line was over the point of origin of the timebase.
But your truck is about 300 ft from the runway centre. You pivoted your Cursor to put your line over the Offset Marker, (level with the Touchdown Markers, but the same distance from the Runway as the Truck), and locked it. Now you've a line exactly aligned with your runway. The last bit was easy. The mounting incorporated a parallel-ruler sort of thing. You moved the locked line across until it ran between the Touchdown Markers. There's your Centreline. Job done, bring 'em on ?
No, not quite. It had to be dark in the Truck (remember when we had to draw the curtains to see the old "Box"?), and if your "tube" was bright it was sometimes hard to see the scribed line. No problem, a tiny pea-bulb in the top of the cursors would flood them with light and illuminate the line, so you could easily see it against the mirror.
Now that's "Talkdown" set up. What about "Tracker". This was much simpler. The elevation "sweep" was vastly magnified, so that an actual sweep of some 8° appeared on the tube as about 60°. Therefore your 3° glidepath would show as 22.5°. The cursor now was fixed at that angle, but could be moved sideways until the scribed line intersected the Touchdown Markers (viewed from the side). This was your Glidepath. Problem: how are Tracker's observations to be communicated to Talkdown while he is talking non-stop (as he must) and is glued to the blip on his own screen ?
Enter Heath Robinson. Tracker's cursor could move bodily up and down (while still maintaining the 22° slope). He had a little brass handwheel to do this. And the whole of his task was to twiddle this to keep his scribed line on the approaching blip. As he did so, a linkage moved a needle on a dial just to the left of Talkdown's tube. "Errormeter" was hardly an inspired name, but a needle showed "on glidepath", and, at 100 ft markings, up to 300 ft above and 200 ft (I think) below glidepath.
So now, if the cursors are set up perfectly, and the talkdown is perfect, and tracker is perfect, and the blips stay on the lines,
what can possibly go wrong ?
It is never wise to offer Providence a challenge like that, see the next Thrilling Instalment.
G'day, folks,
Danny42C
You never know.
P.S. : My apologies to
MPN11 and
HughGw01, and any others whom I may have inadvertently misled. My tale of woe starts on
p.113 here, not p.115 as mistakenly stated. (call it a Senior Moment - or "brain fade" ?)