Exhaust System ChecklistApril 1998
First remove all muffler and stack shrouds and shields to permit full inspection.
Look for leaks
• Examine surfaces adjacent to exhaust system components for telltale signs of exhaust soot. They reflect leakage points.
• Look for gray, red, or black gas stains at welds, clamps, flanges, etc.
• Check that no part of the system is being chafed by cowling, cables, or other parts.
TIP: To verify a suspected leak, connect the exhaust outlet of a vacuum cleaner to the tailpipe. Seal around with duct tape. Apply soapy water to talcum powder.
Check for loose connections or binding of slip joints.
Examine bends and low spots for thinning and pitting.
TIP: Use an ice pick or awl to probe for weak spots.
Pins and fins
Look for missing or damaged heat transfer pins or fins - they can create a hole.
Inspect all surfaces for bulging and distortion, and for patches of small cracks.
TIP: Do not mark any surfaces with lead pencils or any carbon-containing markers (they will cause cracks).
Look inside mufflers for broken baffles and tubes. ¼ They can restrict the outlet causing power loss.
Inspect carefully all internal surfaces that lie hidden under external gussets or stiffeners.
TIP: Run a borescope into the tailpipe or overboard to inspect internal conditions.
• Install only correct parts and don't force-fit.
• Do not reuse gaskets. Properly align connecting parts. Tighten nuts evenly to proper torque value (per OEM specs).
Beware of overhaul abusesMany methods for repairing exhaust system are not up to par.
John Sturch, general manager for Wall Colmonoy in Oklahoma City, OK, says it's very difficult to sit back and observe other repair facilities make substandard repairs on exhaust systems and get away with it. Wall Colmonoy is one of the largest manufacturers of original equipment (OEM), parts manufacturer approved (PMA), and overhauled exhaust systems in the country. The company works closely to supply many of the original exhaust system components to aircraft manufacturers such as Beechcraft, Cessna, and Piper Corporation. It started overhauling exhaust systems in 1975 and eventually purchased Hanlon and Wilson, who at one time had about 90 percent of Cessna's business for both normally aspirated and turbocharged aircraft.
Sturch explains that incorrect or inadequate repair schemes seem to be a persistent problem in the exhaust system overhaul business. He says, "One of the biggest problems we have with many of the repairs being made out in the field today is that many of the design improvements that are made by us in conjunction with the aircraft manufacturer are not being incorporated."
Sturch explains that the result is that many of the repairs do not satisfy the requirements of the design and should not be considered legal. These abuses include anything from using the wrong material for the repair, to manufacturing products that are not even close to the original design.
Don't replace Inconel® with stainless steel
He continues, "One of the biggest problems is the use of the wrong materials. Inconel is used in many of the high-heat areas of the exhaust system, and it's very difficult to tell the difference between it and stainless steel. Unfortunately, many in the field aren't taking the time to determine what they should be using or are taking shortcuts and replacing the Inconel with stainless. Inconel is used for a reason. It holds up much better at higher temperatures. 321 Stainless is really only good for 1,450 to 1,500 degrees continuous temperature. Inconel goes up to 1,650 degrees continuous temperature. That extra 150 or so degrees provides a significant margin of protection for the exhaust system. Replacing it with stainless could mean a failure of the exhaust system and potentially endangering someone's life," he explains.
"One area where we see the wrong material being used quite often is on the internal cones on the Beechcraft Bonanza muffler. These cones are made from Inconel. They were originally made from stainless, but there were several failures, and Beech decided to swap to Inconel. We know this because we're OEM for this exhaust. Unfortunately, we see these being replaced with stainless, and stainless just doesn't hold up in this particular case. It may not be that they are intentionally replacing these with stainless — it may just be that they don't know. Beech wouldn't go to the extra expense of using Inconel if it didn't have to, but if the cone does blow out, it can plug up the exhaust system, and the aircraft wouldn't operate. Baffles breaking loose and plugging up the exhaust system have been a problem for a long time.
"Some repair stations," he says, "just don't know how to work with Inconel and so avoid it. One of the major factors in locating a good source for overhauling your exhaust system is to find someone who is comfortable at working with Inconel," he says. "Inconel is more difficult to work with, and the material cost is much higher. Depending on what grade of Inconel you're talking about, it can cost as much as four times more for the materials. Inconel is a bit harder than stainless to form; plus, if you don't heat treat the welds properly, they will crack. We use certified welders in our facility, and on every repair we make, we Ônormalize' the weld to eliminate stresses by heat treating all repairs in our furnaces."
Does the shop have the right tools?
Another problem to be aware of, explains Sturch, is "the older aircraft get, the more rare some aircraft types are becoming. This becomes a problem for many repair stations because they don't have the jigs and equipment needed to repair these somewhat unique systems. It's very important to have the proper tooling and jigs for these systems in order to do the correct repairs. Find out if the overhaul center you're dealing with has the equipment to work on your particular system before you send it to them"
New or overhauled?
Sturch says there is also a real problem in the industry related to those overhaulers that are building virtually new exhaust systems and calling them "overhauled." "Some shops are replacing up to 98 percent of the exhaust system and still call it overhauled. They essentially take a small piece of our exhaust system and our tag and completely replace the exhaust system and call it an overhauled part. Yet none of the system goes through the quality control and engineering that our systems go through. We spend a lot of time making these things so they last, and someone comes along and replaces the entire exhaust system with substandard parts and gets away with it. It's not just unfair, it's dangerous.
"I'm not saying that all overhaulers are bad. I have no problem with those who are coming in and doing it right. However, unlike some other types of products, a significant portion of the exhaust system needs to be replaced at overhaul, and not many people are taking the time to go out and get PMAs for the exhaust components that they're replacing. When we PMA something, we have to go in and prove that the parts are identical to the original. And it's important that they are identical because the manufacturer goes through a great deal of pains to determine what works. The back pressure has to be correct, as well as the material and the area that the system occupies. Some overhaul facilities are simply changing the design at their own discretion, and technically the airplane is no longer in conformance with the type certificate. All of the overhauls that we do use PMA'd components in the overhaul process."
Sturch points to specific reoccurrences of parts appearing that are not in conformance with the type design. "Some of the internal cones that are being replaced in the field are also not being made correctly. They have fewer holes and are shaped differently. Ultimately, these cones will fail, and they often adversely affect the performance of the engine. Another example is a case and the pins that are used for heat transfer. Again, we find people making their own heat transfer cases, and they may work, but some of the ones we've seen have far fewer heat transfer pins, and the result will be poor heat transfer and an inadequate heating system," he says.
"We've also improved our slip joints on many of the systems we manufacture as new. And we also see replacement parts being made that don't conform to the type design here as well. Finally, exhaust flange ends are frequently being welded on without the use of a jig, and they don't align properly. The bolts are then torqued down and stresses are placed on the system. The exhaust then cracks in numerous places, and the technician who installed them wonders why. I also see numerous gaskets being used to take up space which should not be happening. If you notice the stack is not sitting against all the cylinders, either the repair was made incorrectly, or you have a cylinder exhaust flange that is ground out of specs. In either case, you should not force fit the exhaust system," says Sturch.
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