I recently acquired the bones of the Maule M7-235B Amphib that crashed at Sun River, Oregon in 2019. Unexpectedly, the mufflers suspected to be one of the causes of the accident were included. I am surprised by the internal damage, with no external damage. It is clear the failure of the internal baffle can be a major problem for any power plant. The failure of two can be catastrophic.

I had a chance last month to broach this issue with Brent Maule while in Moultrie. They were concerned of course and a little relieved to find the mufflers on the plane were not stock originals, relieving them of liability. I have not dug into the log books, and donot have the engine or that log, so may never know the vendor or shop that did the work involved.

Just thought it important, and in remembrance of a fellow Oregon pilot who paid the ultimate sacrifice, to encourage everyone to be careful when using modified/remanufactured/ overhauled parts. Use quality vendors that have a solid reputation for your muffler repairs, especially for internal baffles.

Inspection of the mufflers, shrouds and flame traps should be a part of every annual. I've replaced mine several times over the years that I've owned my MX-7-180 as a result of annual inspections that discovered damage.

A timely and sobering post. I replaced my original mufflers with Atlee Dodge versions just a couple of months ago, after discovering during annual that both sides had damaged internal baffles, with broken pieces of the baffle contained loosely in each side. Just as Flybrian noted above, neither of these showed any externally visible damage. They had been run about 75 hours since last checked (including internally) at last year's annual.

These are most definitely a weak point on the Maule, and need to be monitored closely. The original mufflers have needed some kind of work (different every year) every single year since I've had it. Hopefully the Atlee Dodge ones will prove to be more reliable.

Does the Franklin have the same type of muffler? Mine have not been gone through in much depth.

I had a chance to read thru the other maintenance history documents on N118G and found the original muffler repair documentation.

It appears the muffler system was rebuilt in 2011 by a reputable midwest vendor. The flame tubes in both the Rt and Lt mufflers were replaced.

The aircraft was apparently hangar stored and had routine annuals.

It had less than 200 hours since the muffler system re-work.

It appears no expenses was spared on this aircraft over the years.

PLEASE remove your Pain in the Ass to get off tailpipes every Annual/100 Hr and check the baffles!

Last fall during a hundred hour inspection I used a bore scope to look up at the flame tube. I noticed some cracking and a few small pieces missing. I decided to wait a couple of months at annual and check again. So 30 hours later at annual inspection I find the whole flame tube is gone. Of course without the flame tube the can warps and it needs to be replaced. If you catch it early you can have several flame tubes installed before needing to replace the can.

The exhaust on the Franklin is the same type.

Good thought on the borescope, mine may just be long enough to get in there. I’m suspicious now because my lower left engine mount tube has taken enough heat to flake the paint off. Wondering if it’s been partially blocked at some point.

For those of you with the IO-360, be careful of the pilot's side exhaust pipe.

First, it is a very complex part with multiple bends on multiple plains.

Second, it snakes through the motor mount. If there is excess play it will rub the mount, necessitating repair... Which makes the opening even more narrow.

Third, this thing passes very close to the gas collator. I think there is a service bulletin directed to this if not an AD. Can't remember.

Fourth, due to the complex angles and close fit, inspection of the top of this pip is difficult. Annual before last I found a large hole rotted through the top of this pipe where it passed through the tunnel in the boot cowl.

FYI.

So I ran a borescope this evening to find this. The right side looks perfect, but the left had some damage seen below. What happens if these fail? Seems like they would just blow out? Surely I’m missing a failure mode here. Maybe they blow out and burn a hole through the muffler? Allows exhaust into the cabin if the right side fails? https://photos.app.goo.gl/7DBR8fxwKUvD3h8WA

I couldn’t pull up the photo, but I assume you’re referring to a broken flame tube. That’s the tube that runs longitudinally through the middle of the muffler and has a bunch of holes drilled in it.

The official story is that the flame tube protects the outer skin of the muffler by preventing the initial super-hot exhaust gases from directly contacting it as they come out of the cylinder.
Failure of the flame tube can lead to failure of the outer skin of the muffler, allowing hot exhaust to contact flammable items in the engine compartment, causing a fire. This is quite serious. Another failure mode involves broken pieces of the flame tube blocking the muffler outlet, causing reduced power, sometimes severely.

I have known someone to use mufflers on which the flame tube is completely gone for several years and roughly 1000 hours. Eventually the wall of one of the mufflers was discovered on annual to have a bulge developing, so both mufflers were replaced before wall failure. Operating with flame tubes missing is obviously a gamble, and it is not recommended. Also, it would not be worth even thinking about doing if there were still any pieces of the flame tube left that could still break off and cause a blockage. Usually the pieces blow out, but sometimes there is one big enough to plug the outlet. I had that happen once on a PA-12; this reduced power by about 30%.

Bottom line: check your mufflers often, and repair or replace when compromised.

I couldn’t pull up the photo, but I assume you’re referring to a broken flame tube. That’s the tube that runs longitudinally through the middle of the muffler and has a bunch of holes drilled in it.

The official story is that the flame tube protects the outer skin of the muffler by preventing the initial super-hot exhaust gases from directly contacting it as they come out of the cylinder.
Failure of the flame tube can lead to failure of the outer skin of the muffler, allowing hot exhaust to contact flammable items in the engine compartment, causing a fire. This is quite serious. Another failure mode involves broken pieces of the flame tube blocking the muffler outlet, causing reduced power, sometimes severely.

I have known someone to use mufflers on which the flame tube is completely gone for several years and roughly 1000 hours. Eventually the wall of one of the mufflers was discovered on annual to have a bulge developing, so both mufflers were replaced before wall failure. Operating with flame tubes missing is obviously a gamble, and it is not recommended. Also, it would not be worth even thinking about doing if there were still any pieces of the flame tube left that could still break off and cause a blockage. Usually the pieces blow out, but sometimes there is one big enough to plug the outlet. I had that happen once on a PA-12; this reduced power by about 30%.

Bottom line: check your mufflers often, and repair or replace when compromised.

Yes, the flame tube has a clear sag in the tube under the #1 cylinder. The rest of it looked fine. I can’t see that failure is eminent, but it warrants further inspection and possible replacement at annual. I have a JPI900 so I should see if any blockage is created. I just need to be sure a fire is not possible.

This is a problem on other aircraft as well, the Bellanca 7CBC have an AD on inspection of the flame tubes,

This is a problem on other aircraft as well, the Bellanca 7CBC have an AD on inspection of the flame tubes,

This is a problem on other aircraft as well, the Bellanca 7CBC have an AD on inspection of the flame tubes,