FAA Safety Briefing: Exhaust System and increased fatalities

Dan Thomas said:
I did a prepurchase on a 172 that had had an annual maybe 15 hours prior. I found two of the four exhaust stacks cracked two-thirds of the way around, just under their cylinder flanges, both on the right side of the engine. If they had both let go in flight the muffler would have dropped a bit and let flame out to set stuff afire. Those cracks didn't occur in the previous 15 hours. Nobody was looking for them, that's all.

Exhaust failures in an airplane are far more serious than in your car.
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If the exhaust system has been welded up in a manner that caused heat distortion, it can impart a stress on it when bolted to the engine. Heat cycles and vibration during operation may concentrate forces enough to begin a crack in the unit as it tries to relax.

I've noticed this a few times when bolting the flange to the cylinder. The displacement and having to use force to align the flange with the port are obvious clues. The condition can cause failures in a short amount of time.
 
3393RP said:
If the exhaust system has been welded up in a manner that caused heat distortion, it can impart a stress on it when bolted to the engine. Heat cycles and vibration during operation may concentrate forces enough to begin a crack in the unit as it tries to relax.

I've noticed this a few times when bolting the flange to the cylinder. The displacement and having to use force to align the flange with the port are obvious clues. The condition can cause failures in a short amount of time.
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All welding causes heat distortion.

All welding causes internal stress.

'Having to for align the flange' now you are pre-stressing a long thin tube that will get hot and vibrate... me thinks that's an issue.

Good thing AC parts have a warranty on them lol
 
3393RP said:
If the exhaust system has been welded up in a manner that caused heat distortion, it can impart a stress on it when bolted to the engine. Heat cycles and vibration during operation may concentrate forces enough to begin a crack in the unit as it tries to relax.

I've noticed this a few times when bolting the flange to the cylinder. The displacement and having to use force to align the flange with the port are obvious clues. The condition can cause failures in a short amount of time.
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It depends on who is making the stuff. The ones I bought were assembled and welded in jigs and I never had any issues installing them. The bigger factor, earlier on, was the aftermarket stacks that had flanges with sharp, rough edges that had not been dressed. Any mechanic knows that a sharp, rough edge or burr is a place where stresses concentrate and can cause cracking. I could have dressed the outside edge of the flange, but the inside edge was still burred and had the pipe inserted into it. Nothing you can do there except make a big bunch of noise about it, which I did. The flanges would crack first, and the crack would travel through the weld and into the stack pipe and around it.

Some mechanics overtighten the flange nuts and put bending stress into the flange. Add some heat and vibration and you have a problem. It will crack, sooner or later. And whether it's OEM or aftermarket won't make a difference.
 
pfarber said:
All welding causes heat distortion.

All welding causes internal stress.

'Having to for align the flange' now you are pre-stressing a long thin tube that will get hot and vibrate... me thinks that's an issue.

Good thing AC parts have a warranty on them lol
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Welded assemblies typically get normalized before they are removed from the jig. Do you know what that is? Without Googling it?
 
pfarber said:
I don't think I've ever seen a small GA shop with a dunk tank big enough to submerge a exhaust system to look for bubbles
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The one and only one time I've seen such an inspection done, the mechanic pressurized the system with low pressure air (as has been mentioned, with a leaf blower or vacuum cleaner outlet) and sprayed soapy water all over everything and looked for the telltale bubbles.

Tim
 
Many systems have provisions that permit expansion / contraction that are

often overlooked or disabled .

Piper and Mooney slip joints in the pipes or muffler.

When these joints seize a crack elsewhere may result.

Periodic disassembly and wire brushing will prolong component life.

It appears that anti seize cokes in place and is not effective .

I’m a fan of Mouse Milk.

Consensus?
 
Is your AME aware of your difficulty?
 
Some of those I have seen with alignment issues had welded repairs, which most likely weren't jigged or checked after welding.

It's certainly not common, but it happens. As Dan says often, careful inspection reveals problems that are overlooked by the quickie annual. If there's a crack in the exhaust, further investigation should be performed to determine if there are underlying causes.
 
timrb said:
The one and only one time I've seen such an inspection done, the mechanic pressurized the system with low pressure air (as has been mentioned, with a leaf blower or vacuum cleaner outlet) and sprayed soapy water all over everything and looked for the telltale bubbles.

Tim
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Yup. The tricks to do it right are two: that soap/water mix has to be really soapy or it won't make bubbles properly; and the pressure must be low. Really low. Or the air squirting out of leaks will just blow the soapy water away instead of making bubbles. That's why the Canadian AD wants a vacuum cleaner used. One is smart to have an old vacuum that has been thoroughly cleaned, hose and all, and never used for sucking up dirt, or you could blow dust and junk into the exhaust that could end up in a cylinder.
 
3393RP said:
Some of those I have seen with alignment issues had welded repairs, which most likely weren't jigged or checked after welding.

It's certainly not common, but it happens. As Dan says often, careful inspection reveals problems that are overlooked by the quickie annual. If there's a crack in the exhaust, further investigation should be performed to determine if there are underlying causes.
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I found cracking in some stacks that had reinforcing patches welded over the stack at the flange. Brand-new stuff was coming like that, with those patches. The idea, I suppose, was to offer a thicker section in the stack near the flange, to strengthen it against vibration. The problem, I believe, is that the stack, inside that patch, gets hotter than the patch, since the stack has flame in it and the patch is out in the cooling airflow off the head, so that the stack itself expands more than the patch, stressing the patch and cracking it near the welds that are all around its edges.

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pfarber said:
So the bottom line seems more to be that its just a bad idea to heat your cabin with the exhaust system.

I've seen a few of the cabin heat valves, on light GA almost none of them close fully or seal off 100% The inner 'cylinder' always seem to stay open .25in or so.

But the FAA seems to be worried more about internal defects and cracks more than CO2.

I don't think I've ever seen a small GA shop with a dunk tank big enough to submerge a exhaust system to look for bubbles
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My Sentry ADS-B receiver comes with a built in CO detector. I believe other brands do as well. Works great summer and winter. :)
 
pfarber said:
So the bottom line seems more to be that its just a bad idea to heat your cabin with the exhaust system.

I've seen a few of the cabin heat valves, on light GA almost none of them close fully or seal off 100% The inner 'cylinder' always seem to stay open .25in or so.

But the FAA seems to be worried more about internal defects and cracks more than CO2.

I don't think I've ever seen a small GA shop with a dunk tank big enough to submerge a exhaust system to look for bubbles
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Got a better idea for cabin heat? I put a converted Subaru in a Glastar and used the car's heater core. Liquid cooling is nice that way. Aircooled engines present difficulties. Cooling air off the cylinders is no good in cold weather; anyone that had an old VW bug in the winter knows that.

If cabin heat valves are maintained properly they can close pretty tight. But a lot of them, on 40-year-old airplanes, have never been rerigged or repaired. Or even inspected, it seems. Of course they won't work right.
 
Before water cooling, all Porsche 911's had the same heating system. Airbox over the exhaust manifold. Pretty sure older VW's had similar. Heat worked great and came up fast. Same potential for CO getting in.

While I agree with the need for checking this carefully, not sure there has been a rash of CO issues lately?
 
Hang 4 said:
Before water cooling, all Porsche 911's had the same heating system. Airbox over the exhaust manifold. Pretty sure older VW's had similar. Heat worked great and came up fast. Same potential for CO getting in.
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VWs had them too. One thing about most of the VWs that saw any winter use was that the heater channels were often so rotted out I'd be surprised if there was enough of a source of CO to be a problem.

https://vwparts.aircooled.net/Heat-Exchanger-Heater-Box-Right-043-255-108H-p/043-255-108h.htm
 

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I had to take my friends Cherokee exhaust all apart and get the locating holes welded shut and then reinstalled with new clamps. Someone had installed it with alot of stress due to misalignment. Mouse Milked the slip joints to free them up too.
 
Dan Thomas said:
That's a newer setup than my '62 Beetle had. Mine was cooling air, and it also had the optional combustion heater in the trunk. It liked to catch fire.
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Youre referring to the “stale air” system. It still passed air around the exhaust, just less so. I’m not as familiar with that system as I am the newer ones.

 
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