BernieC (#9846),
Welcome aboard !
Can't quote you official chapter and verse, but this lengthy extract from my Post (Page No.134. #2680) may cast a little light on the subject.
I'm surprised that no one has already jumped in with an answer to this, as
AFAIK, this was the standard safety mechanism for all medium sized H.E. bombs during the war. Never was in Bomber Command myself, but I understand that the 4,000 lb "cookie" was too fat for any safety device, so they were "live" from the moment the fuses were fitted until the armourers removed them after landing.
Any ex-armourer within earshot ?
...Hang-ups are rare, but can be very dangerous. If you have one, you try to get rid of it safely by chucking the aircraft about over water or open country. If it still refuses to budge, you have a difficult decision. In theory, if the switches are "safe", the thing should be harmless and you can land with it - or even crash with it, (as I proved the following year}. But it ain't necessarily so.
Shortly after I left Khumbirgram on posting to 8 Sqdn. a crew was killed there when a wing hang-up dropped on landing and exploded when it hit the runway. I think 8 Sqdn. were still "working up" far back in Bengal at the time, so details of the affair were sketchy and took some time to reach us.
It is difficult to imagine how this came to happen. Did the pilot not know he had a hang up? Impossible, you'd say, from what I've been telling you about the hang-up check a few posts ago.
Confession is good for the soul ! The whole of my tale about the jail sortie is perfectly true. But it's actually a composite of my first VV strikes (where in truth we just formed up and went home after bombing) and later ones when this mandatory check had been introduced. (It seemed neater for me to tell the two parts of story in one piece, as it were, as I didn't intend to tell it again - mea culpa!)
So in the early days, not only did we not do any checks, but a practice had sprung up whereby the bomb switches were left "live" after we'd bombed until landing and switch-off. There was some method in this madness.
When a bomb drops, a loop in a wire "fusing link" is held back in the rack by a solenoid bolt which closes when the rack is switched to "live". The other (two) ends of this wire "link" run through holes in a sort of "safety cap", and locks this onto the end of the bomb fuse on which it is loosely threaded. (Same way as a split-pin locks a nut).
This cap protects the detonator inside from accidental impact, (but not from idiots with hammers and chisels!). Incidentally, there are two fuses to a bomb, nose and tail.
It is amazing what blows this cap can survive and still do its job. If a bomb is dropped "safe", the solenoid bolt stays open, the fusing link goes off with the bomb, so the cap stays on the fuse, still held by the wire. It can now go down 20,000 ft into the ground and (should) not go off.
But if the bomb has gone "live", the wire is held back in the rack; the cap has lost its locking. "Windmill" vanes are machined round its circumference, it is loose on the thread, the airflow spins it off in a moment, away we go.
That is rather a cumbersome explanation, but it brings us to the point. If you return the switches to "safe" after dropping your bombs, the solenoids withdraw, the links fall out and are lost. Why should that matter?
Because, if you come back with no links, there is at least a possibility (worse, even suspicion) that you have stupidly "bombed safe". With your links "all present and correct", you're in the clear. Also, you don't need new links for the next lot of bombs (there may even have been a shortage of links - it's exactly the sort of small, cheap, insignificant thing we would be short of), and it's one less job for the armourers. Leave the links in (switches "live").
Good idea? So we thought. And now we can see what might happen. Suppose you have an unnoticed hang-up, it falls off as you land. That's it ! How could it come to be unnoticed on a wing when the pilot rejoined the formation? Only if he were the last man, and it was on an outside wing, it might be possible that no other pilot would notice it. But then, couldn't a gunner on an aircraft ahead, looking back, spot it?
Supposing he did know, he would certainly have done his best to get rid of it, failed and concluded that a landing was safe - it wasn't!
The switches would have gone back to safe, of course, but the trouble with a hang-up is that you never know just how things are in the rack. The bolt may have jammed in the closed position (rear door in my very old car jammed a few months ago, very similar mechanism; main agent estimate £500 [ouch!]; friendly auto-elec chap down road: 3hrs @ £20 = £60 - fixed). And how securely is the claw still holding your bomb? You don't know.
In this way we lost two good men. In fact, it was a risk too far (my log tells me I've done it myself on one occasion, I was lucky). Really, the only sensible thing to do was to bale out and let the aircraft go - there were plenty more where it came from, and a crew is worth more than an aircraft.
I believe that it was in consequence of this accident that hang-up checks became the rule.
One curious little thing: the front fuse cap spun off well clear of the aircraft and was lost (I can see some museum director in the future trying to puzzle out what this little round thing, dug up by a treasure-hunter, might be).
But the tail fuse safety device took the shape of a little sheet-metal butterfly-shaped thing (I've no idea how it worked). On quite a few occasions, an aircraft would come back with this thing embedded in a flap. It was too small to do any real damage, but the flap had to be patched after you pulled it out. It was a nuisance....
Cheers, Danny42C.