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I've just been to view an aircraft that had potential for a purchase. For a range of reasons, it's a no.

On inspection, one of the main structural alu tubes supporting the empennage has a major crack running its length. I very definitely checked it was a crack; there is a clear gap where the tubing has split, running around 50 cm from the joint with the vert stabiliser. If only I could post photos, it would be clearer than my explanation.

I asked the owner about this and they were not concerned, suggesting it was probably just speed tape (which it definitely was not).

Although it's a no from me, I'm worried that I was right to be concerned and I do not want the owner (or future next owner) being involved in an accident if my concerns are valid. They of course might not be (and experience on this forum rather says so!).

So, what's the best way of nudging the owner on this? Very happy to chat to him later and raise it as a concern, but if they are not interested, should I just leave it?

More generally, what might cause a crack in one of the longitudinal tubes like this?
Last edited by flyingearly on Fri May 21, 2021 9:12 am, edited 1 time in total.
Aluminium tube, rather sounds like a microlight. If it were me I'd contact the BMAA (or LAA or CAA if overseen by them) and let them know my concerns, in writing. Straight away.

If it is a crack in a major structural member, and the owner is daft enough to laugh it off, then he needs protecting from himself (as does the primary school he'll be flying over).
Rob P wrote:Permit or certified?
Rob P
I don't think that makes a difference, Rob.

If the OP is sufficiently concerned (and it sounds so) I would mention to the owner/seller first of all. If said owner ignored advice, then do exactly what Kemble Pitts suggests. In aviation, if there is doubt: there is no doubt.

Better to embarrassed by a false alarm, than have to live forever knowing you could have saved a life.

Flyingearly: feel free to contact me by pm

Rob L
Sadly not everyone in aviation will take damage or faults on the chin and will attempt to sell on an a/c in a dangerous condition to prevent their financial loss.

A few years back in the LAA mag IIRC a new owner decided to re-skin a lightweight a/c. On removal of the existing fabric some major structural tubes had significant bulges and splits.

Of course it's possible that the vendor genuinely didn't know, but I kind of doubt it.
Glad that's sorted.
It's very unlikely that a tube would crack along its length - they just aren't stressed that way (well high pressure gas pipes are, but not tubes used as structural members in a frame construction). Cracks appear usually from stress raisers like rivet holes - this is a known issue on the C42 fuse tube for example.
Use of prop tape to stop abrasion is common practice in rag and tube aeroplanes.
More generally, though, I agree with @Kemble Pitts that contacting the tech office of the relevant airworthiness organisation is the right thing to do if the owner is clearly being silly.
Cracks, anywhere, in aluminium can be caused by a builder, or modifier, marking out such tubing with a pencil rather that a low chloride marker such as a “Sharpie”. It would be the first thing I’d look out for if checking over a potential purchase. If found I’d reject it out of hand.
In addition, whilst there may well be minimal “ in flight” stresses in aluminium tubing, there could be latent manufacturing stresses to propagate cracking.
Carbon and aluminium are at opposite ends of the galvanic corrosion scale. You will notice too that aluminium ferrules on carbon fibre propellor blades will corrode (will turn powdery).
It is not sufficient, once the pencil contamination has taken place, to wipe it off - the carbon is simply rubbed into the aluminium grain.
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