In my case, the hole was facing the oil pan, so did not show up as a discoloration of the cowling. There was some soot on the oil pan, but it was difficult to see, and the hole was not visible from the outside view with the cowling open for a normal pre flight inspection. I have adjusted my pre flight inspection to feel the inside of the exhaust manifold.
We had a hole develop in our exhaust (discovered during preflight inspection – since we have a cowling that opens, I saw discolouration of the lower cowling that wasn’t there before, so I stuck my hand in to feel around the exhaust and found a hole where there hadn’t been one before). The cowling is metal, but the heat had caused the paint on the inside to erode and I could feel a slight bubbling of the paint on the outside of the cowling.
I think I heard the bit of exhaust departing, although I didn’t realise it at the time – I heard a slight change in engine note after the glider I was towing released, but I had been easing the throttle out at the time so I didn’t think much of it till I found the hole the next time we went to fly.
That’s amazing.
I would have expected rapid cowling damage in such a case. The TB20 has quite a lot of room around the engine but the exhaust pipework is around 5-10cm from the lower cowling.
It also risks CO ingress into the ventilation system. The exhaust only has to burn through one of the pipes coming back from the muffler jacket.
As I keep saying, these systems should be made in Inconel and then they would last as long as the aircraft. But for some reason exhaust makers don’t like doing it. There is nothing actually difficult about it but one hears all kinds of crap like the Russian invasion of Ukraine causing Inconel price increases to $50/kg (which is utterly immaterial looking at how much new exhaust pipes cost – about 100x that figure). I think a more likely reason is that exhaust makers buy in pre-bent sections from outside, and these don’t exist in Inconel, perhaps…
I had an exhaust manifold issue like the A36 one shown by quatrelle in my Bonanza. Mine was on the inside, so not able to see on pre-flight. I had complained to my AI for some time that I would hear popping or backfire sound when I would close the throttle on landing. One day, doing some takeoffs and landings, a fellow pilot told me, my engine sounded really bad. Other than the popping noise when I closed the throttle, I did not notice any other indications. So the next flight I went with another pilot friend to listen to me fly the pattern. We did a thorough preflight before the pattern flight, and he put his hand inside the manifold and said, you are not flying today. Putting my hand along the inside of the LH manifold, I could feel a large hole, probably more than 2 square inches. I don’t know how it had been missed, for probably two annuals. The manifold could have been repaired, but I bought a new one because I did not like the thin metal on the pipes. With the new manifold, no more popping or backfire noises when I close the throttle. Fortunately, there was no heat damage on the engine as I hadn’t flown much that year because of the COVID pandemic.
Yes. AWI even advertised that specific part, by the Socata P/N, on their website.
They have fixtures and if you have a fixture then you can make pipes like this all day for $500. I think the issue was that these fixtures are developed over time, as customers send in their old exhausts.
AWI originally wanted the entire exhaust so they could get it right, which makes sense, but that is still not enough information because you can build a “perfect” exhaust with the muffler in a wide range of positions. I would not send them the whole exhaust because if the parcel got lost then my plane would have been grounded potentially for ever.
It’s been an interesting exercise and should serve as a warning to others to acquire spare exhaust parts as and when they can. In particular, the Y sections are most likely to fail.
That’s not an easy result to achieve. Some kind of fixture would be required, pulled off the original. I don’t know how they do it for $500.
Looks really nice.
Here is an update:
Firstly, the whole Socata exhaust system is stainless steel, not inconel, so the whole “GTs have an inconel exhaust” is a total Socata lie which their people repeated countless times over the 20 years I’ve had the plane. I asked Socata about this and never got a reply.
The other bit is that the welded up exhaust pipe lasted 1.5 years.
Unfortunately, I wasn’t proactive on this and didn’t use that time to get hold of a proper spare, which is what I usually do. And, on the last Annual, Dec 2022, a lump was found:
and by that time the end which goes into the muffler didn’t look too great either
I embarked on a search for a replacement pipe, mentioned here, which took a lot longer than expected. No European contact got anywhere near delivering. Specialist aviation welders were basically not interested, one UK company was but at a price of about 4k, Socata wanted 2.5k for theirs but with an indeterminate lead time of “maybe Feb or March” which, knowing Socata, means very little. In the US, there is Acorn and AWI (both now same company) and mentioned here. Acorn could not do anything. AWI quoted $500 and on a 2nd try produced a usable pipe (within a few mm of the original) in stainless (and thicker than the Socata pipe on which some derisory comments were made re the very thin metal used) so that was installed
It’s been an interesting experience, especially discovering that of the firms advertising exhaust “repair” (which, given lack of STC or PMA is done by utilising some tiny portion of the old exhaust in the new one, so it is classed as a “repair”) none actually have a template derived from an original Socata exhaust. Of course the dimensions of an IO540 and where the exhaust ports come out is known, but you need two other key bits of data: the dimensions and position of the muffler (for which Socata probably charge about 10k, so there won’t be a lot of them kicking around) and this has to be done right so the main exhaust pipe comes out exactly right relative to the cowling hole
Some people expressed incredulity that an IO540 could generate enough EGT to soften the steel. Even an acetylene+propane blowlamp – the hottest thing you can easily get short of oxy-acetylene – struggles to do that. But AWI said they have seen this plenty of times, once the metal gets thin. And the Socata metal is thin when brand new 
All done
The verdict is that the “whole exhaust on a GT is inconel” is a con. This has been repeated by Socata reps on their US owners group many times (I have a number of screenshots).
Well, maybe it is not a complete con and some “very late” GTs are actually all Inconel but mine, 2121, and fairly close to end of all piston production in ~2002, is definitely stainless on the pipes, and the silencer (muffler) is Inconel.
Not bad condition given ~2500hrs airborne time, all at peak EGT.
I got this welded up.
The amazing thing is that it is stainless steel. Socata heavily promoted the GT aircraft as having Inconel exhaust but clearly this is not the case…
The guy reckons I should get the exhaust replaced at some point, especially that section.
Silvaire wrote:
The weld should not be ground smooth.
…on the inside it should: it must not be left with rough protrusions. On the outside I agree with you.
