Exhaust System 101: Inspection & Maintenance

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Thin metals and fluctuating temperatures can literally “exhaust” your airplane’s exhaust system. Read on to learn how an aircraft exhaust system is constructed and get several practical tips to identify common trouble spots and prevent unnecessary damage.

Aircraft exhaust systems require detailed inspections and regular maintenance because of the highly corrosive and hot environment in which they operate. Cracks and leaking connections can cause significant amounts of damage to other engine parts, as well as possibly exposing airplane occupants to carbon monoxide. Understanding how to promptly identify exhaust leaks and repair them correctly can help owners and mechanics avoid expensive and/or dangerous problems down the road. (While owners can perform several of the visual inspections outlined in this article, consult your mechanic before performing any disassembly, assembly or repair. —Ed.)

Exhaust system anatomy and construction

Most exhaust risers and mufflers are made of stainless steel or Inconel. (For those who aren’t familiar, Inconel is a trademarked name for a superalloy used in high temperature applications. —Ed.) Some older exhaust systems also contained parts that were made of carbon steel. The pipes and muffler are manufactured with fairly thin walls to help keep the exhaust system as light as possible.

The thin-walled construction and the use of special metals in the construction of the exhaust system can make weld repairs difficult to accomplish in the field. Most items are typically sent to specialty shops that have jigs to prevent warping and have the correct replacement metals and welding rods. 

The single pipes that connect the muffler to each individual cylinder are called risers. A set of pipes that connect each cylinder to the muffler, but which are joined together into one section before reaching the muffler, is referred to as a stack. The outlet from the muffler is the tailpipe. 

Each connection of one exhaust component to another can be accomplished by welding, by use of clamps, or by slip joints. 

Welding creates a strong, but rigid and immovable joint.

Clamped connections utilize clamp halves of various sizes. The clamp halves are connected with nuts and bolts that occasionally corrode over time and require replacement. The clamps can leak and require repositioning or retightening.

Slip joint connections have one pipe inserted a few inches into the adjoining pipe. The mating surfaces fit closely together, but allow for some movement. These connections make the exhaust system removal and installation easier and permit some movement to relieve operating stresses. 

It is important to lubricate all joints using a high-temperature MIL-PRF-907F qualified anti-sieze compound such as Locktite C5-A (up to 1,796 F) or Jet-Lube Nikal (up to 2,600 F).

 

 

Heat and corrosion

The exhaust system’s job is to remove all the hot exhaust gases from each cylinder head exhaust port and deposit them overboard. Exhaust components heat up quickly, but they also cool down rapidly at shutdown. Extreme temperatures and rapid temperature changes create an environment that produces metal fatigue and cracks. 

Connections such as slip joints, cylinder exhaust flange attachments and clamps can all deteriorate and begin to leak over time. Exhaust gas is very corrosive and can do severe damage to any metal component that is exposed to a leaking area. 

 

Muffler inspection

All heat deflectors and muffler shrouds should be removed when inspecting exhaust components. 

Bulges or wrinkles on the muffler are signs of overheating and metal fatigue. The sidewalls and lower sections of mufflers are prone to deteriorate and become thin. Areas where thinning is suspected can be probed with an awl to see if it punches through the material.

A bright light or an inspection camera can be used to check the internal structure of mufflers. Straight tailpipes provide a fairly large opening that allows access for a bright flashlight and inspection mirror. 

Exhaust systems with tailpipes that have bends or that are some distance away from the muffler itself can still be inspected internally by using an inspection camera with a small-diameter, long, flexible shaft.

Broken baffles that are loose in the muffler can be an immediate danger because they can partially cover the exhaust outlet. Jagged or distorted baffles can create hotspots that can cause premature metal fatigue.

 

 

Detecting exhaust leaks

Some exhaust leaks can be found by a visual inspection. Most exhaust leaks leave a gray or black sooty residue around the leaking area. Some leaks leave a yellow-tinted stain on the exhaust system itself. However, not all exhaust leaks leave a stained area behind. 

Some aircraft require substantial pressure test to pass. Always consult the aircraft maintenance manual to ensure that the mufflers are pressure-tested to the required point. Some only need 2-3 psi and some as high as 15 psi.

One of the most thorough ways to inspect an exhaust system for leaks is by use of a shop vacuum, some duct tape and a bottle of soapy water. 

To perform this inspection, the shop vacuum hose is inserted into a cold exhaust tailpipe and thoroughly sealed using tape. The vacuum switch and/or hose attachment on the vacuum should be set to “blow,” so that air blows out the hose rather than being pulled into the vacuum. 

Once the vacuum cleaner is turned on, the air blown in from the vacuum will pressurize the exhaust system enough to check for leaks. Slip joints, clamps and the welds around the muffler itself should all be sprayed with the soapy solution. Leakage will be immediately evident as the soap solution will begin to bubble. 

If it is possible to remove the components from the aircraft, a tank of water can be also be used to check for leaks.

The flange attachments on the cylinders are prone to leaking, especially on the cylinder flange attachments that have only two studs. Most Lycoming engines and a few Continental engines have the two-stud attachment. These flanges are elliptically-shaped, with a hole on each of the small sides. The flange on these connections is inserted over two studs on the cylinder port and drawn up tight with nuts. 

A gasket is used to seal the gap between the flange and the cylinder port. Over time, the flanges can become warped and get bent upward on the ends, leaving a gap in the center, which allows exhaust gas to leak past the gasket. 

Leaks in these areas can be detected by pressurizing the system as described above or by using a small feeler gauge to check for gaps in the gasket’s mating surfaces. 

If the flanges are warped, use of a spiral-wound gasket makes the problem worse. 

 

 

Heat damage and corrosion

As previously mentioned, leaks in exhaust systems should be immediately repaired, not only because of the danger they pose from possible carbon monoxide exposure or fire hazards, but also because the hot corrosive exhaust gas may cause rapid damage to anything that it blows on. This may include components near the exhaust, like the engine mount tubes shown in the picture (top, this page). 

  

 

Cylinder exhaust ports and exhaust flanges

The effects of exhaust heat and corrosion are especially pronounced on the cylinder flange attachments. (Refer to “Detecting exhaust leaks” on Page 30.
—Ed.) The cylinder port attachment for the exhaust flange is aluminum. The flat aluminum surface has two or more threaded inserts with steel studs installed. 

The aluminum degrades and pits quickly when leaking exhaust gaskets allow exhaust gas to blow out between the cylinder head and gasket. The gap and the exhaust leak will get larger with subsequent use if left unchecked. 

If the leak is not addressed, extreme pitting can occur to the point that resurfacing the pitted area on the cylinder is required. This is a laborious repair that involves removing the uneven metal to recreate a flat sealing surface. Surfaces that are severely eroded can’t be fixed and the cylinder must be replaced. 

There is no high-temperature silicone or sealant that will help to seal the gap. Proper sealing requires mating surfaces to be in contact with the exhaust gasket all the way around the gasket seal. Silicone is not effective as a sealing agent because it can’t withstand the extremely high exhaust gas temperatures.

In addition to causing erosion of the cylinder surface, exhaust leaks in the cylinder port area allow cold air to flow directly into the cylinder port through the gap. The cold air in the hot aluminum exhaust port causes the hot aluminum surface to crack. 

These cracks are not usually detectable during a cylinder compression test because they occur in the exhaust port area outside of the exhaust valve. When the exhaust valve is closed, this area is sealed from the combustion chamber. 

There is most likely a crack present in a cylinder exhaust port that has had a leaking exhaust gasket for several hours of operation, whether or not you can see the crack by visually inspecting the cylinder exhaust port. 

The exhaust port is typically covered in hardened exhaust deposits. The only way to remove the deposits is by use of a media blast material—chemical cleaners won’t cut through it—and this requires cylinder removal. 

An untreated gasket leak that causes an exhaust port to crack requires cylinder replacement. Replacing the cylinder is an expensive repair that is preventable by detecting and correcting exhaust leaks quickly.

The flanges on the exhaust risers themselves can be resurfaced by holding them flat on a belt sander and removing the high material until the flange has a perfectly flat surface again. This can only be done if the flange material on the riser is thick enough to allow some removal of the material without weakening the flange. If too much material is removed, the thin flange will become warped quickly and begin leaking again. It may also develop a crack. 

 

Hold-down studs

Leaking exhaust gaskets also corrode the hold-down studs to which the exhaust flange is attached. The threads can become damaged and cause stripped nuts that won’t tighten properly or hold torque. 

The studs also can vibrate and loosen in the threaded cylinder inserts. In this case, an oversize stud or new insert may be needed to hold the stud in place.

 

 

Exhaust gaskets

There are two main types of exhaust gaskets that are used on the cylinder flange attachments. 

The most popular and the longest-lasting are spiral-wound gaskets, also called “no-blo” gaskets. These gaskets are made of a thick carbon steel outer area surrounding an inner sealing area. The seal itself is made of layers of alternating stainless steel and asbestos. Spiral-wound gaskets are less prone to erosion and are more effective than other types of gaskets—provided the mating surfaces on the cylinder and exhaust flange are flat and not pitted. Some mechanics will reuse or reinstall spiral-wound gaskets when they are found to be in good condition.

The other gasket types (sometimes called “blo-proof” gaskets) are thinner and made of stainless steel or copper. These softer and more flexible gaskets are installed in pairs. They don’t last as long in service as spiral-wound gaskets, but they are more pliable and work better to help seal slightly uneven surfaces. The drawback with these coupled gaskets is that they will eventually begin leaking and so must be replaced periodically. In addition, if the exhaust system is removed for any reason, these gaskets may not be reused. 

 

Cracks and slip joint leaks

Leaks due to cracks or excessive leaks around slip joints can only be fixed by removing the parts and having them replaced or repaired. It’s best to send repair work to specialty shops that have the jigs to hold the parts in place as they are welded to prevent deformation. These specialized shops also have the correct repair material, which ensures a long-lasting weld repair. 

Some manufacturers recommend assembling slip joint connections with a small layer of ultra-high temperature anti-seize compound to help ensure a smooth disassembly later on down the road. 

 

Maintenance and operating tips

Here is a list of best practices to help preserve your airplane’s exhaust system.

• Pencils shouldn’t be used to mark exhaust components during maintenance because the graphite can weaken the metal as the exhaust heats up in use and cause a crack.

• Occasionally during an engine runup and magneto check, a pilot may accidentally turn the ignition switch all the way to “off” instead of selecting one magneto. If that happens, the engine starts to die. Most pilots will realize what has happened and will try to suddenly turn the switch back to “on.” This action can cause a severe and potentially damaging after-fire in the exhaust system. If the ignition switch is accidentally shut off during a magneto check, it’s best to let the engine shut down and then restart it. 

• As with all engine components, keeping the engine as cool as possible on climbout and as warm as possible on descent helps to minimize sudden temperature changes and extends the service life of exhaust components.

 

Conclusion

Detailed inspections completed at regular, close intervals can save aircraft owners money in the long run by catching problems before they do too much damage. Basic visual checks of the exhaust system should be a part of every preflight inspection; more detailed inspections should be performed regularly with the help of a qualified mechanic. 

Know your FAR/AIM and check with your mechanic before starting any work.

Jacqueline Shipe grew up in an aviation home. She soloed at age 16, has her CFII and ATP certificate and also obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. Shipe has also logged over 5,000 hours of flight instruction time. Send question or comments to .