Tuesday, January 26, 2010

Preflight Inspection Tip - Inspect the muffler

Recently dealt with an old Cessna 180, with 470. Owner has flown 60 hours since buying aircraft 5 months ago. Prior to purchase, the sellers engineer stated that entire left bank cylinders went cold on one occasion only, and after failure to diagnose a reason, it cleared itself after running at 2000 RPM.
The condition reoccured for the buyer, this weekend. This time it was the right bank that went cold, and idle RPM was 1000 RPM. The current owner, during a subsequent run-up attained 2000 RPM, at which point the engine smooted out and all returned to normal. I invstigated today, and found the right muffler flame tube missing. There are obvious signs of very recent failure of the cone. I do feel the exhaust obstruction prevent the entire right bank to breath properly and fire. This same thing likely occured sixty hours ago with the left muffler, but the engineer at that timefailed to identify it. Both mufflers are now just open top to bottom. In reading so many reports etc. on the web it's obvious that this is a little known condition with huge consequences when not inspected and maintained properly. I'd like to suggest that it should have a spot as a possibility in unexplained rough engine operation or loss of power. Below are just a couple of the links that I passed on to this pilot (new customer) regarding his mufflersystem.

Following was copied from Archer Bravo http://www.archerbravo.com/bravotips.php

April 13th, 2006

Don't Get Muffled
When doing your pre-flight walk around, don't forget about your muffler! Here's how to check it: use a flashlight and look all the way up inside the exhaust pipe. You should then see the muffler. If its internal baffling is shaped evenly (cylindrical or conic shaped), with evenly sized holes, it's in good shape.
If it appears warped and distorted, this means the flame cone is fatigued, and it will no longer do its job of vaporizing gas.
Eventually, the parts might begin to shed, and come out of the tailpipe. This can block it, decrease performance, and increase the chance of fire and/or carbon monoxide poisoning. Yes, this is the worst case scenario, but certainly the last thing you want. By the way, some mufflers weren't created with a baffle, so don't panic if you don’t see one initially.
One other important note: if you are in-flight and experience loss of RPM, or rough engine - and your systems check reveals no other problems (magnetos, carb ice, etc), do an inspection of that tailpipe and muffler upon landing.
Submitted by: Flight Instructor Mariellen Couppee

Following was copied from http://www.patentstorm.us/patents/5496975/description.html

One type of present day small aircraft muffler currently used has a series of cones that are tack welded to small diameter rods. This assembly is placed within a housing that has an inlet and an outlet. Although this muffler design works well to muffle noise, it has a tendency to fail. Vibration and heat often cause one or more of the cones to break free of the welds. In many cases, the outlet side cone will invert and become lodged in the outlet of the muffler causing immediate power loss due to extreme back pressure in the introduction system. If this power loss occurs during takeoff, a crash is highly likely.

Following was copied from http://www.supremecourt.ohio.gov/rod/docs/pdf/8/2009/2009-ohio-5365.pdf

The engine logbook to the aircraft contains an entry dated August
28, 1987 that states “replaced muffler,” which was entered by Edward Ramsey,
an FAA licensed airframe and powerplant (“A&P”) mechanic. Appellants,
through their experts, maintain that the logbook entry establishes that the original
muffler was replaced with a new muffler. Appellants’ theory of liability is that the
aircraft experienced a sudden loss of engine power when the muffler’s flame tube
separated and blocked the exhaust.

Another excellent reference,

AC 91-59 A,
Inspection and Care of General Aviation Aircraft Exhaust Systems
see page 2, 4 b for the following

As an example, we received a safety recommendation concerning the internal failure of a muffler on a Cessna 207 airplane. The cone, which deflects or helps distribute exhaust gasses inside the muffler, came loose and blocked the exhaust gas outlet resulting in engine power loss during takeoff. This mufflerconfiguration is common or similar to many other general aviation airplane mufflers.

Elsewhere in this same AC,

(3) Partial or full engine power loss caused by loose baffles, cones, or diffusers on mufflers and heat exchangers that partially or completely block the exhaust gas outlet flow. This condition may occur intermittently if internal components are loose within the muffler and move around during subsequent flights.

Regards John, as always I continue to push your toolbox as an excellent resource for anyone involved in aircraft.


  1. Anonymous12:15 AM

    Wow, very cool idea and definitely creative... Adding your feed now. Thanks!

  2. Anonymous2:26 PM

    Fantastic info in all these articles. I read about accidient investigations and the findings all the time. This makes me want my PPL and a job in/around aviation even more, I have the bug. Thank you so much for sharing your knowledge its very much appreciated and Im sure it has saved lives by making people aware.