M20J - Exhaust Leak

[Shadrach]

On 1/1/2026 at 11:18 AM, WheelPantsOff said:

Sorry if this posts twice.  I don't believe the first response didn't get posted.  I'm highly confident that the crack didn't suddenly develop a month and less than 10 hours after the annual.  Either way, I can't prove it one way or the other.  Based upon the removal, I would also say that this was the first time in years that the exhaust/muffler was removed.  In 30 years and overseeing hundreds of inspections, and the last few of these at MSC's, I don't think the muffler/exhaust was ever removed and the inspection goes something like, 'the muffler's there, no squawks, inspection complete.'  I do see in the checklist that it's a required item.

In my experience, exhaust cracks do occur suddenly. The deterioration of the the area where the crack appears may happen slowly but the actual crack and subsequent leak typically happen in an instant. A small crack can propagate rapidly.  An "after-fire" event (what some are calling a back-fire), is the ignition of raw fuel in the exhaust and can compromise a muffler instantly.

BTW, a back-fire is actually the ignition of fuel in the induction system and can cause damage to the intake and airbox and other components ahead of the combustion chamber.

[EricJ]

1 hour ago, skykrawler said:

Good idea?   It's mandatory isn't it?

Stacks, yes.   Muffler, no.

 

[N201MKTurbo]

2 hours ago, WheelPantsOff said:

Sorry if this posts twice.  I don't believe the first response didn't get posted.  I'm highly confident that the crack didn't suddenly develop a month and less than 10 hours after the annual.  Either way, I can't prove it one way or the other.  Based upon the removal, I would also say that this was the first time in years that the exhaust/muffler was removed.  In 30 years and overseeing hundreds of inspections, and the last few of these at MSC's, I don't think the muffler/exhaust was ever removed and the inspection goes something like, 'the muffler's there, no squawks, inspection complete.'  I do see in the checklist that it's a required item.

The exhaust doesn’t need to be removed. The heater muff (cuff) should be removed so the muffler can can be inspected for cracks. Everything else can be inspected from the outside except the flame tubes inside the muffler. They can be inspected by removing the tail pipe, or with a borescope.

[47U]

The borescope is your friend… 

However, if I had not removed the #3 exhaust riser, I wouldn’t have discovered this chunk missing from the exhaust valve guide.

 

[EricJ]

4 hours ago, 47U said:

The borescope is your friend… 

However, if I had not removed the #3 exhaust riser, I wouldn’t have discovered this chunk missing from the exhaust valve guide.

 

A little less likely to stick that way. 

[skykrawler]

23 hours ago, EricJ said:

Stacks, yes.   Muffler, no.

 

Don't really believe you.  I can tell you the NTSB has big concerns regarding this topic (cracks in the muffler).  They talk about it in their presentations at IA seminars.

Of course they have no authority.

[EricJ]

1 hour ago, skykrawler said:

Don't really believe you.  I can tell you the NTSB has big concerns regarding this topic (cracks in the muffler).  They talk about it in their presentations at IA seminars.

Of course they have no authority.

FAR 43 Appendix D lists the required items in an annual inspection.   Section (d) covers engines.   Only item (8) has to do specifically with the exhaust and it says:

(8) Exhaust stacks—for cracks, defects, and improper attachment.

The muffler isn't mentioned anywhere.

So, no, inspecting the muffler isn't required.   IMHO it's a bad idea to skip it, but isn't strictly required.   Many things that people think are required to be done in an annual inspection are actually not required, and perhaps just common practice instead.

A few things that are actually required are often not done.    The very first item in Appendix D is:

(a) Each person performing an annual or 100-hour inspection shall, before that inspection, remove or open all necessary inspection plates, access doors, fairing, and cowling. He shall thoroughly clean the aircraft and aircraft engine.

So if you ever get your airplane back from an annual inspection and it's not been "thoroughly cleaned", including the engine, the annual inspection is actually not complete.   It also says that the inspector has to do the cleaning.   I suspect that rarely happens.   So the conclusion is that many, many GA airplanes are flying illegally.   

[Shadrach]

1 hour ago, skykrawler said:

Don't really believe you.  I can tell you the NTSB has big concerns regarding this topic (cracks in the muffler).  They talk about it in their presentations at IA seminars.

Of course they have no authority.

What an interesting way of conveying that you don’t understand the difference between what is safe and what is legal. And that one does not necessarily contribute to the other.

[WheelPantsOff]

12 minutes ago, EricJ said:

So if you ever get your airplane back from an annual inspection and it's not been "thoroughly cleaned", including the engine, the annual inspection is actually not complete.   It also says that the inspector has to do the cleaning.   I suspect that rarely happens.   So the conclusion is that many, many GA airplanes are flying illegally.   

Even though few people care about this from a regulatory compliance perspective, this is an excellent thing to keep an eye out for during vetting the shop and when you pick the airplane up.  And it doesn't matter if you're using a CRS, MSC, or independent shop.  I don't believe that there's any meaningful difference anymore.

[N201MKTurbo]

15 minutes ago, EricJ said:

FAR 43 Appendix D lists the required items in an annual inspection.   Section (d) covers engines.   Only item (8) has to do specifically with the exhaust and it says:

(8) Exhaust stacks—for cracks, defects, and improper attachment.

The muffler isn't mentioned anywhere.

So, no, inspecting the muffler isn't required.   IMHO it's a bad idea to skip it, but isn't strictly required.   Many things that people think are required to be done in an annual inspection are actually not required, and perhaps just common practice instead.

A few things that are actually required are often not done.    The very first item in Appendix D is:

(a) Each person performing an annual or 100-hour inspection shall, before that inspection, remove or open all necessary inspection plates, access doors, fairing, and cowling. He shall thoroughly clean the aircraft and aircraft engine.

So if you ever get your airplane back from an annual inspection and it's not been "thoroughly cleaned", including the engine, the annual inspection is actually not complete.   It also says that the inspector has to do the cleaning.   I suspect that rarely happens.   So the conclusion is that many, many GA airplanes are flying illegally.   

I usually F that up. I wash the engine when I’m done, so I don’t have to do it twice.

[skykrawler]

1 hour ago, Shadrach said:

What an interesting way of conveying that you don’t understand the difference between what is safe and what is legal. And that one does not necessarily contribute to the other.

Am I being antagonistic?  I don't think a I am.  I concede, its not in Appendix D.    Some airplanes don't have mufflers. Appendix  D also says nothing about turbochargers either.  Are they to just be ignored?  Another poster showed muffler in the Mooney 100hr maintenance inspection.   If an accident follows a shoddy inspection signed off as airworthy it can get real legal.  The original poster apparently was the recipient of some of this type maintenance.

You guys should be advocating for these checks. Not making excuses to not do them.

AMT Handbook (not regulatory)

During the inspection of an exhaust system, close attention
should be given to all external surfaces of the exhaust system
for cracks, dents, or missing parts. This also applies to welds,
clamps, supports, support attachment lugs, bracing, slip joints,
stack flanges, gaskets, and flexible couplings. Each bend
should be examined, as well as areas adjacent to welds. Any
dented areas or low spots in the system should be inspected
for thinning and pitting due to internal erosion by combustion
products or accumulated moisture. An ice pick or similar
pointed instrument is useful in probing suspected areas.
The system should be disassembled as necessary to inspect
internal baffles or diffusers. If a component of the exhaust
system is inaccessible for a thorough visual inspection or is
hidden by nonremovable parts, it should be removed and
checked for possible leaks. This can often be accomplished
best by plugging the openings of the component, applying
a suitable internal pressure (approximately 2 psi), and
submerging it in water. Any leaks cause bubbles that
can readily be detected. The procedures required for an
installation inspection are also performed during most regular
inspections. Daily inspection of the exhaust system usually
consists of checking the exposed exhaust system for cracks,
scaling, excessive leakage, and loose clamps.

Muffler and Heat Exchanger Failures

Approximately half of all muffler and heat exchanger failures
can be traced to cracks or ruptures in the heat exchanger
surfaces used for cabin and carburetor heat sources. Failures
in the heat exchanger surface (usually in the outer wall) allow
exhaust gases to escape directly into the cabin heat system.
These failures, in most cases, are caused by thermal and
vibration fatigue cracking in areas of stress concentration.
Failure of the spot-welds, which attach the heat transfer pins,
can result in exhaust gas leakage. In addition to a carbon
monoxide hazard, failure of heat exchanger surfaces can
permit exhaust gases to be drawn into the engine induction
system, causing engine overheating and power loss

 

[N201MKTurbo]

I don’t think anybody is saying they shouldn’t be done. But if they are not done, you probably won’t get any satisfaction from the FAA. You would have to sue them. But what were your losses? At this point, probably nothing. 
 

If you are concerned about these inspections, tell the inspector that these inspections must be done and ask for proof that they were.

[N201MKTurbo]

BTW, I always do these inspections. I have got the poisoning, I know how important they are.

[Shadrach]

15 minutes ago, skykrawler said:

Am I being antagonistic?  I don't think a I am.  I concede, its not in Appendix D.    Some airplanes don't have mufflers. Appendix  D also says nothing about turbochargers either.  Are they to just be ignored?  Another poster showed muffler in the Mooney 100hr maintenance inspection.   If an accident follows a shoddy inspection signed off as airworthy it can get real legal.  The original poster apparently was the recipient of some of this type maintenance.

You guys should be advocating for these checks. Not making excuses to not do them.

AMT Handbook (not regulatory)

During the inspection of an exhaust system, close attention
should be given to all external surfaces of the exhaust system
for cracks, dents, or missing parts. This also applies to welds,
clamps, supports, support attachment lugs, bracing, slip joints,
stack flanges, gaskets, and flexible couplings. Each bend
should be examined, as well as areas adjacent to welds. Any
dented areas or low spots in the system should be inspected
for thinning and pitting due to internal erosion by combustion
products or accumulated moisture. An ice pick or similar
pointed instrument is useful in probing suspected areas.
The system should be disassembled as necessary to inspect
internal baffles or diffusers. If a component of the exhaust
system is inaccessible for a thorough visual inspection or is
hidden by nonremovable parts, it should be removed and
checked for possible leaks. This can often be accomplished
best by plugging the openings of the component, applying
a suitable internal pressure (approximately 2 psi), and
submerging it in water. Any leaks cause bubbles that
can readily be detected. The procedures required for an
installation inspection are also performed during most regular
inspections. Daily inspection of the exhaust system usually
consists of checking the exposed exhaust system for cracks,
scaling, excessive leakage, and loose clamps.

Muffler and Heat Exchanger Failures

Approximately half of all muffler and heat exchanger failures
can be traced to cracks or ruptures in the heat exchanger
surfaces used for cabin and carburetor heat sources. Failures
in the heat exchanger surface (usually in the outer wall) allow
exhaust gases to escape directly into the cabin heat system.
These failures, in most cases, are caused by thermal and
vibration fatigue cracking in areas of stress concentration.
Failure of the spot-welds, which attach the heat transfer pins,
can result in exhaust gas leakage. In addition to a carbon
monoxide hazard, failure of heat exchanger surfaces can
permit exhaust gases to be drawn into the engine induction
system, causing engine overheating and power loss

 

Not only do you seem antagonistic, but you are ascribing a motive of your own fabrication to those that are simply telling you what is regulatory. 

[WheelPantsOff]

3 hours ago, skykrawler said:

Am I being antagonistic?  I don't think a I am.  I concede, its not in Appendix D.    Some airplanes don't have mufflers. Appendix  D also says nothing about turbochargers either.  Are they to just be ignored?  Another poster showed muffler in the Mooney 100hr maintenance inspection.   If an accident follows a shoddy inspection signed off as airworthy it can get real legal.  The original poster apparently was the recipient of some of this type maintenance.

You guys should be advocating for these checks. Not making excuses to not do them.

Because I was referenced, I'll respond directly.  I do not believe I was the recipient of a 'shoddy inspection,' and more important than my feelings is my proof that I can't say I was.  How many airplanes develop squawks or issues during or immediately after maintenance?  Do you blame all of them on the shop either not performing the inspection correctly or willfully inflating the bill? 

So what am I going to do about it?  I'm going to have the muffler repaired or replaced and the rest of the exhaust inspected and repaired as necessary and life goes on.

Can I prove that something was or was not done relative to the muffler?  No.  All that can be proven is what is on the logbook sticker.

Can I prove that the cracks developed in the handful of hours post-annual?  No.

Am I comfortable and confident in the shop's abilities, knowledge, regulatory compliance?  Yes.

Can I prove that something was performed incorrectly?  No.

Safety and regulatory compliance do not always see eye to eye.  Look at all the NTSB recommendations that die on the vine because due to a variety of factors.  The NTSB has no regulatory authority and strictly focus investigations, probable cause, and recommendations.  I do believe your heart is in the right place when you say what should be included for inspection, but that shouldn't be mistaken for what must be included in an inspection.  I can say that if a mechanic were to sit down and go over what was performed and then make recommendations above and beyond requirements, but that rarely happens in a completely comprehensive situation because the list would be never-ending for 40+ year old airplanes.  And, at some point, once you independently vet a shop or mechanic, you have to trust that they're doing the work correctly.  I did speak with the mechanic afterward and he explained what he did for the annual and I have to take him at his word.  I will continue to use him in the future because nothing has proven to me that the outcome is his fault.

Edited January 2 by WheelPantsOff
Grammar

[takair]

Depending on where the crack originated, even with the heat muff opened, it can be difficult to see. There is just very little gap and it’s hard to get enough hands, lights, mirrors, etc in there. Hell, I’ve seen cracks that were external that were difficult to see and decipher. Scratches in heated stainless often look like cracks. It is important that mechanics try to look for damage, but I think the best defense these days is the previously discussed CO detector.

[N201MKTurbo]

4 minutes ago, takair said:

Depending on where the crack originated, even with the heat muff opened, it can be difficult to see. There is just very little gap and it’s hard to get enough hands, lights, mirrors, etc in there. Hell, I’ve seen cracks that were external that were difficult to see and decipher. Scratches in heated stainless often look like cracks. It is important that mechanics try to look for damage, but I think the best defense these days is the previously discussed CO detector.

I look for exhaust stains on the inside of the muff.

[EricJ]

4 hours ago, skykrawler said:

Am I being antagonistic?  I don't think a I am.  I concede, its not in Appendix D.    Some airplanes don't have mufflers. Appendix  D also says nothing about turbochargers either.  Are they to just be ignored?  Another poster showed muffler in the Mooney 100hr maintenance inspection.   If an accident follows a shoddy inspection signed off as airworthy it can get real legal.  The original poster apparently was the recipient of some of this type maintenance.

You guys should be advocating for these checks. Not making excuses to not do them.

You asked a specific question:
  

On 1/1/2026 at 9:14 AM, skykrawler said:

Good idea?   It's mandatory isn't it?

To which I provided an accurate answer, it is not mandatory. 

Then you said:
 

6 hours ago, skykrawler said:

Don't really believe you.  I can tell you the NTSB has big concerns regarding this topic (cracks in the muffler).  They talk about it in their presentations at IA seminars.

Of course they have no authority.

So I cited the actual regulation.   As somebody else pointed out, there are differences between what's safe, what's good practice, and what's actually legally required.   When you asked what's "mandatory", you got an accurate answer.    You can learn the differences or not, it's up to you.

4 hours ago, skykrawler said:

Some airplanes don't have mufflers. Appendix  D also says nothing about turbochargers either.  Are they to just be ignored?

Did anybody suggest that?    I don't think so.    But if you asked whether it was "mandatory" to specifically inspect a turbocharger, it isn't.

But there's a catch-all under part (d) (engines) in FAR 43 Appendix D.   Item 10 says:

(10) All systems—for improper installation, poor general condition, defects, and insecure attachment.

So everything should get at least that level of inspection.   Many, if not most, would not find that adequate or appropriate for many things, like mufflers.

4 hours ago, skykrawler said:

You guys should be advocating for these checks. Not making excuses to not do them.

I think we have been avocating for good inspections.   I haven't seen anybody suggest an excuse to not do them.    On the contrary, people have been saying it's good to do them and bad not to. 

I also think it's not a good idea to misunderstand or misquote regulations, too.   Knowing the difference between what's required and what's not is extremely useful in making sure that things you want done get done.   If you assume something will get done because you think "it's required" and it's not, you may wind up with a safety level that is less than what you thought or wanted.   This forum is pretty good about helping sort out that kind of thing.   So when you ask whether something is "mandatory" and you get an accurate answer, that should be useful to you.    You're welcome.