Ormeside28 (#10579),
..."Soon after settling in at Aldergrove the Vulcan crashed at London airport. It had been supported on its trip to Australia and New Zealand"...
This will be the 1st October 1956. I pray in aid my p.207, #4137 (and two later Posts on this Thread; this is inordinately long, but four men died in the course of it, so I hope you [and the Moderator] will indulge me):
...Danny sees an Enemy in the Camp.
(This is taking me away from Sleap and my main thread, but that's my own fault, I started it, so I must tell the whole story)....D.
Of course the challenge was taken up; Providence put a hidden gremlin into the mix. After each "run", Tracker spins his little wheel down ready for the next customer. He would appear at the limit of range, say 7-8 miles. Director would have the aircraft down to 1500 ft at that point, at circuit speed, cockpit checks complete and three greens for landing (or whatever), ready for his handover to Talkdown at 6-7 miles.
But at 7 miles Glide Path is at 2100 ft, so Tracker's blip is so far down under that even at maximum depression he can't get his line onto the blip until, with the reducing range, the blip plods along (right to left) till it approaches the sloping GlidePath. * Then, as the range continues to close, he winds up steadily to keep his line exactly over the blip. That's all he has to do.
* ILS Glidepath needle (noted by one who has only ever "flown" them in the Link) behaves in a similar way.
Talkdown has one eye on his own blip, and the other on the Errormeter. When his blip first appears at limit of range, E/mtr needle will show at bottom stop. Then as range closes it rises steadily until at 50 ft below G/Path, Talkdown will say: "Do not acknowledge further instructions, you are five miles from touch down, commence descent at your normal rate of descent" - and the game's on, to end (you both hope) 2½ minutes later with the rubber on the runway and another Satisfied Customer.
Our Gremlin bides his time.
He is crafty beyond belief. Unerringly he goes for the weak point - the Tracker's cursor. (Surely not !) After each run, Tracker spins the little wheel down. Sometimes (repeatedly ?) it hits the stop with a bit of a bump. The pea bulb in the cursor may be jerked just a wee bit out of position. This may introduce a danger out of all proportion to its insignificant cause.
The light is, as it were, "contained" inside the cursor, but behaves in strange ways. With the pea bulb dead centre, the scribed line should get all the illumination, the top and bottom edges remain unlit and invisible (or at least faint) over the tube. But if it's out of position on Tracker's cursor? A case can arise (and cases had arisen at Sleap) in which the top or bottom edge of the Cursor was so much more brightly illuminated than the scribed line that it could easily be mistaken for it. It was not uncommon for two lines to appear at the same time; Tracker could interpret these as top edge over scribed, and so follow the lower, whereas what he was actually seeing was scribed over bottom, and he was following that.
Suppose this went unnoticed in the darkened Truck, what would be the effect ? Tracker would wait for his next blip as before. His false line is lower than the true one, so the blip would meet it sooner (by about half a mile, but that need not worry him, the aircraft is not always exactly at 1500 ft when it appears). Talkdown's E/mtr would bestir itself a bit early, no cause for alarm, the system should easily accommodate a bit of deviation if it is properly set-up.
Now the situation would develop with all the inevitability of a Greek tragedy. Everything would look and sound absolutely normal to any observer. The aircraft continues on the G/path - E/mtr says so. Talkdown is happy, Pilot is happy.
But he's on a false glide path - about 150 ft below the true one. If he continues on it, head down in cockpit on instruments until told to "Look ahead for the Runway" (as a good Pilot should be), he will touch down among the approach lights about a half-mile short of the threshold. And no one can work out why.
Ring any bells, anyone ?
From an armchair, it is easy to pick out "Why didn't"s. Why didn't Tracker see that his line no longer met ground at touchdown, but half a mile before, while Talkdown is saying "you're on the glidepath ?" (but Tracker's orders are to keep the line on the blip, nothing more, all the time - he is to mind his own business, and nobody else's).
Why didn't Talkdown query the aircraft height when it looked to be meeting G/path half a mile too soon ? (why should he, talk-down would last about 15 seconds longer, but that was all).
IMHO, it could all so easily have been avoided. That was the real tragedy.
More about that next time, to round off a story in which four good men had died.
(I must make it clear that my attribution of this cause to the accident in question is entirely subjective - I cannot prove it, but I firmly believe it, and it was a belief shared by all the Sleap instructional body at the time).
Goodnight once more, chaps. Danny42C.
For the want of a nail.....
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Danny42C #4147
Danny has a Sad Story to tell.
The trouble was that this snag was so easy to fix. Off with the cursors onto the bench, out with a half-round file. File a "deckel-edge" along both sides of the cursors. Now you can't confuse it with the central line. The "mod" was so quick and easy that no one saw any need to put it up for official adoption.
The word quickly got round all the MPN-1s in the RAF and you just did it. After the Sleap GCA School, which first recognised the fault, the two MPN-1s I later worked (Strubby and Gatow) had this "mod" done before I got to them. But there was an MPN-1 which (
AFAIK) hadn't. And I believe it wasn't a "Bendix", but was from another maker, but exactly to the same pattern.
This was the one at Heathrow. They had ILS, of course, and I would think that 99% of their traffic preferred this. Not that their GCA was idle, far from it. It was used (on the "belt and braces" principle) to monitor the ILS approaches. If the approaching aircraft were coming in too far adrift, they'd give Approach a shout. They'd done thousands of such "dry runs" over the years this way, but relatively few "real" ones. And their cursors had not been "modded".
I do not know why this was so. Their GCA was operated by the MCA, or the MoA or the BoT, or whatever. Either the RAF had not told them about this, or they had pigeonholed the advice (as being Not Invented Here ?).
Our gremlin waited.....One Day.
The day came on 1st October, 1956. The Vulcan which had been out to NZ and back on a flag-waving mission had behaved perfectly; our friends had been heartened and our foes dismayed. It was returning home now in a blaze of glory. The co-pilot was Air Marshal Sir Harry Broadhurst, newly appointed CinC of Bomber Command.
Mindful of the enormous cost of the new Vulcans to the hard-pressed taxpayer, he had recently issued a fiat: "
There are to be no more flying accidents in Bomber Command". A poster was issued round his Stations to that effect (this I would not believe until I saw one).
"
Ah, luckless speech and bootless boast
For which he paid full dear". (Cowper: John Gilpin's Ride)
The original plan had been that it should return without fuss to Lyneham. You never know, it might have disgraced itself, and be coming home under a cloud. But now all doubts were stilled, here was a fine photo opportuity for the Government to seize. It would come in to Heathrow and get the full red-carpet treatment, and be welcomed by the Great and the Good. All the freeworld's air attachés, their Press and cameramen were invited; this triumph of British aeronautical engineering would be displayed for all to see.
The Vulcan had ILS, of course, but this had to be re-tuned to each of the airfield's ILS it might need to use en route. This was then not a matter of merely punching a button or twiddling a knob. Separate discrete crystals had to be manually fitted at every stage. Of course, they had set out with a full kit of crystals for all the airfields on their itinerary, plus likely diversions. But it had never planned to use Heathrow: they didn't have the crystals for that, and for some reason (short of time ?) they couldn't get them now.
No problem, we'll use the GCA if the weather's bad. And it was, and they did.
Google will tell you what happened.
("Vulcan Crash Heathrow" will start you: there is interesting meat in all the links in the list, but I found it helpful to start with:
"VULCAN AIRCRAFT CRASH (REPORT) - Hansard 1803-2005").
and read in conjunction with my previous Post #4126 p.207, in particular:
("he will touch down among the approach lights about a half-mile short of the
threshold. And no one can work out why").
Post mortem and wrap-up and my comments (for what they're worth) next time,
Cheerio to all, Danny42C.
.....the horse was lost.
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12th Aug 2013, 17:35 #4161
Danny Sums Up and Delivers Judgment.
Before I start pontificating on this accident, and to avert accusations of the dreaded "sciolism", I must emphasise what I told Chugalug many moons ago, when this my tale was yet in its infancy. That is, I am not, and in no way hold myself out to be, any form of authority on this (or most other) subjects. It is nearly all hearsay and therefore not evidence.
I merely retell what I heard, or was told at the time (and the accuracy of my memory of even that cannot be guaranteed), or read in the newspapers. The only thing I am certain about is my description of the Truck interior, the CRT tubes and the Cursors (and the Funny Things which Happened on the Way to the Theatre). With that disclaimer firmly in place, I'll begin.
Reading the "Statement in the House", and the comment on the Dr.Touch report (did it ever see the light of day, or is it under some 50-year wrap ?), it seems to me that nobody had the problem by the throat. "Tracker" merits only one passing mention in the Parliamentary Report. There seems to have been some inconclusive references to what Talkdown said and when he said it (were there no tape recordings then, and was nobody monitoring his transmissions ? Was there no transcript ?).
Talkdown is totally reliant on his Tracker for Glidepath information. If there is anything wrong with that, go straight for the Tracker. I was told at Shawbury (and I think my Course lasted a week or two after the incident) that RAF Shawbury had tentatively offered the bottom-edge-of-cursor hypothesis to the CoI. But this was dismissed on the specious ground that the Tracker in question was highly experienced, having clocked up thousands of runs: it was inconceivable that such a person could commit so simple an error.
If this response from the CoI be true (and it rings true), then I can only say that it would not be the first time in the history of aviation, and it will not be the last, that such a thing has happened.
And now we have to take a look at the Heathrow MPN-1 (must have been that, as it had a Tracker) and how it was operated. From what I was told, it did almost all its "runs" in the ILS-tracking mode. Actual GCA approaches were few and far between, as naturally all the civil traffic inbound would go for the well-used and trusted ILS with which all its pilots were familiar. I would guess that the odd "full" GCA would only be on request from a RAF visitor, and even then only if ILS was not available for some reason (as was the case with our Vulcan).
It probably follows that they were well out of practice on the real thing. It made little difference to talkdown; he would be quite familiar with the ILS-following blips coming in at a slightly offset angle. But it was different for Tracker. If he fell into the bottom-cursor-edge trap (which, if the "deckle" had not been done, was more than likely), then the first time the E/Mtr reported "150 ft above glidepath", it would be passed via Approach to the incoming Captain, who would indignantly deny it, telling Approach that its ILS G/path must be "up the wall". The mechs would be hastily summoned to check the supposedly incorrect ILS - for this is very serious for Heathrow. Recriminations follow when it was found to be a false alarm.
When they got that sorted out, and the cursor error quickly discovered, Tracker would have his ears firmly pinned back. But in a "real" run there is no such "check and balance". We know what had been demonstrated in practice: now we had had the real thing.
All this begs a host of questions. Was there a radar alt on the panel ? If so, who was watching it ? Why were they in that pickle at all ? What about "Minimum Approach Heights" (or whatever we called them then ?). It is hard to avoid the conclusion that in the end it was a fatal case of get-home-at-all-costs-itis that was the proximate cause of the accident. The bottom-edge-of- cursor gremlin just tipped the balance, and sealed their fate. Without it, they might just have brought it off.
And I'm convinced that that's the way it was. And now it's nearly 57 years ago, Sir Harry is dead, S/Ldr Howard (the Captain) would be older than I, so he's almost certainly dead. Now the last crew of XA897 are together once more. R.I.P.
As for me: back to Shawbury and Sleap next time. Goodnight, all, Danny42C .
"
Res
ume normal navigation"
........................................................THE END.