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. I talked to Tom Heid, president of Aerospace Welding Minneapolis, Inc. (AWI) to learn some of these tips. Heid is an A&P that is very 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-sieze 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.
Since many mechanics don't have the TIG welding equipment, expertise, or comfort level to do an exhaust system repair, sending it out for repair is a common practice. There are repair stations like AWI that specialize in exhaust system repairs. As an alternative, some mechanics choose to take the exhaust part to their local welding shop to have it repaired. If you are having a local shop do the repair or if you are tackling the repair yourself, there are several things you need to know to help ensure you get a good repair.
Alignment. Proper alignment is important when repairing an exhaust system component. Most repairs need to be done in a jig in order for the component to fit properly during re-installation. Not using a jig can cause improper alignment, setting up stress after installation that can damage the part.
Lack of experience. Many welders don't have experience working with aircraft exhaust systems. Exhaust systems are comprised mostly of 321 stainless or 601 or 625 inconel. There aren't many other things that are made out of these alloys, and most general welders don't have the experience of working with them. Even if the welder has welded stainless before, it is not the same as 321 stainless, which requires a specific rod and techniques. Using the wrong rods coupled with wrong procedures will result in a weak joint.
Proper cleaning. Thorough cleaning of the part is critical. The outside of an exhaust system component is typically dirty with oil and other deposits on it. But just cleaning the exterior of the part is inadequate. The inside of the part is full of carbon deposits left behind from burnt fuel and fuel additives. As soon as you start welding, the crack opens up from the heat and that contamination from inside the part is pulled right through into the weld puddle creating a weak weld. So the part needs to be thoroughly cleaned inside and out before welding.
Purging. A final tip for welding is to ensure the part is purged when welding. Purging is the process of providing a separate source of argon to the inside of the part. This pushes all of the air out of the part and creates a pure argon atmosphere inside the part. This pure argon atmosphere helps pull the weld puddle through the crack during the welding process, and the resulting weld is as clean on the inside as it is on the outside. Not purging will cause oxidation of the weld puddle on the inside, creating a rough, jagged bead. Not only is this a weaker weld joint, but the jagged edges will disrupt the gas flow creating hot spots that will set up spots for future failure.
Stainless vs. Inconel
Another thing that mechanics need to be aware of is that exhaust system components can be manufactured of either stainless or inconel. These two materials are similar in appearance and can be difficult to differentiate without chemical tests or destructive (grinding) analysis. It is important to realize that these two materials have different characteristics that affect the visual indications of a pending failure.
Over time, stainless steel tends to deteriorate. The molecules of the metal start to break down and the metal starts to stretch, bulge, and deform. This is a good visual indication that the part is close to failure, and requires repair or replacement.
Because of this susceptibility of stainless to weakening and bulging over time, some engineers decided to go with a stronger metal that could withstand the heat and prevent these bulging failures. They chose inconel, a metal in the stainless-steel family that has more nickel and chrome in it, allowing it to withstand higher heat. Typically, inconel does not bulge and deform like stainless does. But what it does do is pit out from the inside. The metal properties of inconel aren't very compatible with the mineral deposits that are left behind in AvGas. This causes severe pitting, almost like a cancer. Heid notes that 95 to 98 percent of the inconel parts that come through their door are severely pitted out.
The bottom line is that as a mechanic you may not know what material you are inspecting. Just because there are no cracks or bulges on the system, doesn't mean that there aren't any defects. You could be looking at an inconel part that is pitting from the inside.
Proper exhaust system inspection is critical to safe operation. By knowing proper inspection procedures and ensuring your parts are being properly repaired, you can do your part to ensure the aircraft doesn't experience an exhaust system failure.