What sort and size of parts?

A very effective way of cleaning aluminium parts is to sand-blast (or bead-blast) them but the finish almost always looks ‘frosted’ so somewhat different from originally machined surfaces…..although possibly ideal for some castings?

I would avoid anything but the finest wire brush wheels (tiny Dremel types for small work) but some form of polishing wheel on a bench grinder would be a relatively cheap and quick method for larger parts but would prove difficult to get into intricate areas.

I would assume…on a combat patrol…that the guns would have been cocked and tested very soon after takeoff anyway…just because the pilot was relaxing and bored doesn’t necessarily mean the gunner was.

I will concede that it is possible that the guns were cocked and that the pilot had turned the bezel to ‘fire’ although it still seems unlikely given the relaxed tone of the anecdote and the fact that they were flying in an area where they were unlikely to be bounced by a brace of Messerschmitts!

The interrupter mechanism on the Defiant was a contact pad on the turret and a ring of contacts on the outer ring it revolved in; for the bits they DIDN’T want to fire at….there was simply a break, a cutaway, in the ring of contact pads.

That sounds like an exceptionally ‘fail-safe’ method; if there are simply no contacts where the turret wasn’t intended to fire then surely there would be no possibility of ‘arcing’ when the pilot inadvertently pressed his firing button. What then could be the likely failure mode?

Crucially the interrupter mechanism has to fail and it has to fail in a specific way because if it is still working the guns cannot fire in the way that the anecdote tells us that they did. In the case of the anecdote the interrupter mechanism has to fail so that there is a ‘short-circuit’ between the ‘relay solenoids’ and the pilot’s firing button (or some other part of the firing circuit). The only other way the guns could fire is if there is a live connection to the relay solenoids or the gun firing solenoids and then the guns (or one or two of the guns) would fire but it would fire without the pilot inadvertently pressing his firing button!

The more I look at this the less likely I think it is that it happened in the way the anecdote says it did.

…did the Defiant have interruptor gear for the propellor…

No, I’m certain that the defiant only had ‘interrupter gear’ to prevent the guns firing into the propeller arc when elevated less than 19° and traversed forward.

Although the Defiant had a retracting fairing behind the cockpit I’m also fairly sure that there was some system that would prevent the traverse of the turret over the forward arc with the guns at less than 19° elevation (as they wouldn’t clear the fairing even when it was retracted).

All part of the service! 🙂

Well, logically the fact that condensers HAD to be fitted would indicate there was a need for them…

Capacitors on switches are fitted for a number of reasons but I don’t think the primary reason is to prevent an open switch spontaneously becoming closed through ‘arcing’ (again electrical theory is not my strong point).

Personally I think the possibility of problems with the switches is a bit of a distraction from the anecdote; the real issue would be some failure of the interrupter system that would allow the guns to fire in the position that is suggested (when the pilot accidentally pushed his firing button).

But that still leaves my two questions unanswered; the safety bezel on the firing button would need to be set to fire and the guns would need to be deliberately cocked…..all while the crew are relaxing over the Irish Sea.

I can’t prove anything but probability is certainly on my side.

Sorry to be so picky, I was sure it was a typo but I didn’t want anybody to get the wrong end of the stick given the way these sorts of threads polarize opinion.

I’m actually coming at this from 100% the other direction – that a firing for any reason – pilot clumsiness or mechanical/electrical fault could indeed have caused the incident.

It’s Clarke’s statement that has been put up to contradict these possibilities…

Actually what you mean is ‘pilot clumsiness and mechanical/electrical fault’ because both have to happen at the same time for the guns to fire as described in the anecdote!

I’m prepared to set aside Clarke’s statement for now because there is simply no evidence for it to be true but going back to an earlier statement of yours:

This is the one issue where a physical fault COULD have caused the incident – arcing over the switch, something like that….but ANOTHER possibility is more interesting….

Look again at the details in there on the free/engaged switch. One of the reasons I’m also trying to get a full copy of AP1569C on the BP turrets from Hendon is to find out if there’s any possibility that when the combination of switches – master switch and free/engaged switch – is set wrong – the guns could be “live” for the pilot…when the gunner MIGHT think they weren’t. If there’s any part of all that mega clumsiness that would combine to bypass the electrical interrupter!

A possibility of human error?

Just for the sake of argument let’s assume that the pilot can position himself so that his sleeves are a target for the guns (‘facing forward and at zero elevation’) as this is difficult to argue without actually trying it in a Defiant. And let’s also assume that the pilot can accidentally press his firing button within its safety bezel (or whatever it is called) on a free and easily-moved (anybody?) control-column.

That brings us to the argument of some sort of breakdown or design fault with the ‘mega clumsiness’ of the firing circuits (that you have kindly provided).

Personally I think that the firing circuits have been carefully designed with safety in mind; four switches (pilot’s, pilot-off-gunner, free-engaged and interrupter) all have to be positioned correctly to operate the ‘relay solenoids’; if any one of them isn’t closed the guns will not fire. Also when these four switches are closed the primary circuit activates a second circuit to operate the gun firing solenoids. This tells me (and electrical theory isn’t my strong point) that the primary circuit is relatively low-powered so that would tend to reduce the risk (to zero) of ‘arcing’ over any of the switches.

There is always the remote possibility of a short-circuit from another positive wire activating one of the solenoids (and firing the guns without anybody pressing anything) but that would have to happen exactly at the moment the guns supposedly fired in the anecdote. A much more likely earth short would simply blow the fuse (when the firing button was pressed).

Anyway even assuming that some combination did inadvertently provide power to the gun firing solenoids there are still two major problems that I see with the anecdote as it stands:

Firstly, the pilot would have to move the safety bezel on his control column to ‘fire’ (in a situation where he is relaxing over the Irish Sea).

Secondly, the gunner would have had to have manually ‘cocked’ the guns so that they would fire when the gun firing solenoids were activated (again in a situation where he is relaxing over the Irish Sea).

Actually I’m not certain about that second point so I’d be glad to hear from anybody who knows for sure.

Anyway none of this supposition proves anything…..but it is interesting to speculate!

Welcome to the forum by the way…..we’re not usually this argumentative (honest :rolleyes:) and it is nothing personal…

…stick to your guns! 🙂

What is the collective noun for a group of pedants, anyway? :diablo:

A forum.