An aircraft’s exhaust system is critical to flight safety. Defective exhaust systems can lead to carbon monoxide poisoning, fire, or loss of engine performance. There are some tips that can ensure you are properly inspecting and maintaining these systems. AMT talked to Tom Heid, president of Aerospace Welding Minneapolis, Inc. (AWI) to learn some of these tips. Heid is an A&P that is familiar with exhaust system inspection and repair. Here are some pointers he shared during our conversation.
General inspection tips
Before inspecting the exhaust system, be sure to remove all shrouds and shields from the muffler and stacks to permit full inspection. Some mechanics get in a hurry and instead of removing the shroud, they will open it up, split it open manually, and just kind of look around in there and then close it back up. Heid has several examples of cracks and deformities in mufflers that wouldn’t have been caught if the shroud was only partially removed.
During inspection, you want to look for signs of leaks. Inspect the surface areas of components next to the exhaust system for signs of exhaust soot. Also look for signs of leaks on the exhaust system itself. Leaks will appear as a yellowish or orangish powdery residue. Any time you have that kind of discoloration in an area of an exhaust part, that is a good telltale sign that you have a leak. You want to pay particular attention around welds, clamps, and flanges.
Another way to find leaks is by performing a pressure test. Refer to your maintenance manual for detailed procedures of a pressure test. In general, to do a pressure test, you insert an air source such as a shop vac (in reverse mode) or regulated shop air in the tail pipe and pressurize the exhaust system to about 3 to 5 psi. Be careful not to overpressurize the system, as exhaust system and/or engine damage can occur. You can then spray a soap and water solution on all the joints and the system in general to make sure there are no cracks, pinholes, or any excessive leaks at the clamp or slip joints.
You also want to inspect all surfaces for metal fatigue. This will be indicated by bulges, distortions, or cracks. Examine bends in pipes for pitting and thinning of material. You can use an awl to probe material in suspected weak spots.
Use a flashlight to shine into pipes for inspection. You can also use a borescope to examine internal components.
Inspect for damaged or missing heat studs, fins, or other heat sink material. These defects can cause uneven heating of the muffler surface and lead to holes in the muffler can.
Look to see if the muffler has internal baffles or tubes. If the baffles are damaged or missing repair or replace the muffler. Broken baffles may become dislodged and restrict the outlet and cause power loss.
Inspect internal areas where possible for wear, pitting, cracks, and broken baffles. Corrosion may be occurring on a component that looks good externally.
AWI offers the following installation tips for exhaust systems.
- Don’t force fit any parts, cracking will occur and shorten component service life.
- Do not reuse gaskets.
- Make sure that all parts are properly aligned, first loosely mount on aircraft then tighten all connectors to OEM specifications, retighten after a hot run.
- Use an anti-seize compound rated to at least 1,400 F such as Bostik Never-Seeze or Loctite C5-A on all slip joints.
- Inspect all hardware and clamps for wear, pitting, or heat stress. Replace as necessary.
On a turbo 182, unlike other exhaust systems that have a turbo system installed, there is no support bracket for the turbo. All of the weight of the turbo rests on the exhaust header (or Y assembly as some people refer to them). This puts a lot of stress on that header. There have been several of these exhaust headers that crack and break. This can cause an in-flight fire, and it is an area that needs to be inspected carefully. It is an extreme safety factor.