Exhaust System Alert
Extra attention badly needed
By Greg Napert
ohn Sturch, general manager of Wall Colmonoy Corporation, says that approximately 20 to 30 percent of all exhaust systems sent to Wall Colmonoy are in such bad condition, that they are beyond repair — scrap.
Pay more attention
"Unlike automobile exhaust systems," claims Sturch, "which simply reduce noise and carry exhaust gases away from the vehicle, aircraft exhaust systems perform several important functions. The primary function of an exhaust system is to route exhaust gases away from the engine and fuselage while reducing noise. In addition, the exhaust system serves an important secondary function, indirectly supplying cabin and carburetor heat."
The dangers that result from operating an aircraft with a defective exhaust system include the risk of carbon monoxide poisoning, decrease in engine performance, and the risk of fire. Sturch points to the fact that numerous accidents have been attributed to exhaust system leaks.
With these kinds of risks involved, technicians should be much more alert to the rate of exhaust system deterioration and should increase inspection intervals to include inspection of the exhaust systems, inside and out.
According to Sturch, "exhaust systems are constantly exposed to very high temperatures and corrosive environments. Temperatures in excess of 1,400¡F are not uncommon, and when combined with the corrosive attack of burned and unburned hydrocarbons, it's no wonder these systems are subject to failure."
There is more to properly inspecting exhaust systems than simply taking a quick look at the outside of the muffler, header, or tailpipe. "We see over 4,000 - 5,000 parts a year and even with that experience, we cannot pick up a muffler, and look at the outside and determine whether it is good or bad. We have to examine the entire part inside and out to determine whether it is repairable or not," says Sturch.
The heat transfer area of the muffler is covered by a "collector" or shroud. Without disassembling the collector or shroud and thoroughly inspecting the actual body of the muffler, there is no way to know if exhaust gases are leaking into the heating system of the aircraft.
Sturch suggests that a technician use a borescope to inspect the internal condition of the muffler. Internal baffles that are used to distribute heat and create back pressure frequently deteriorate before the walls of the muffler do. Damaged baffles can also cause exhaust gases to be concentrated on one area of the muffler creating "hot spots" which can weaken or eventually burn through the metal. Also, explains Sturch, baffles can dislodge and cause restrictions to the exhaust system and even engine failure.
"There have been many inquiries," claims Sturch, "as to whether mufflers can be operated with baffles missing." He does not recommend operating with missing baffles. He claims that missing baffles can reduce the efficiency of the heating system by as much as 60 percent. Also, the baffles serve to create back pressure for the engine — part of the design as determined by the engine manufacturer. When a baffle is missing or badly damaged, it is a sign that the muffler has damage in other areas as well, and it is time for a complete overhaul.
Larry Dawley, owner of Dawley Aviation in Burlington, WI, says that he really needs to blast all of the carbon deposits off of the base metal in order to do a thorough inspection of the exhaust system. Additionally, he explains, the mufflers are all pressure tested for leaks with a soapy water solution to ensure there are no pinholes or pores in the metal that cannot be seen with the naked eye.
It's cracking for a reason
Dawley explains that one reason for cracking on exhaust systems is that the stacks aren't in alignment due to a previously bad repair or distortion of the system from some other means. "The technician will force it on and that places a preload on the system. It then goes through several heating and cooling cycles combined with vibration and then cracks in a short amount of time.
"If something isn't fitting right, you need to take care of the problem. Don't wedge it on there; have it fixed. If you force the stacks into place, you'll be setting it up for cracking by creating stresses on the system," Dawley explains.
Typically, repair facilities can adjust the stacks by cutting out the distorted pieces of the stacks and welding in new pieces.
Dawley continues, "I like to tell people that if the aircraft is in for an annual, why not take the exhaust off and send it to us. For a minimum charge we'll do a thorough inspection, and if there's nothing wrong with it, we'll send it back. We can and also yellow tag it and warranty it for a year."
No patch jobs please
It appears as though exhaust systems are easy to repair. Find the leak and plug it up. Well, it's not that simple. Exhaust systems are difficult if not impossible to repair, at least if you want the repair to last for any significant amount of time.
Exhaust systems are constructed of a fairly light gauge stainless-steel material. This material should be welded in a controlled environment using inert gas welding equipment. Doing otherwise results in weld inclusions, contamination of the weld from exhaust byproducts, inadequate bonding, and excessive build up of weld material. Not to mention that it takes practice to weld using inert gas welding equipment. "A contaminated weld," says Sturch, "is not going to hold up to any kind of thermal or vibratory stress. Therefore, we do not recommend doing these repairs yourself."
In many cases, explains Sturch, what ends up happening when a field repair is attempted, is that a repairable exhaust system is made nonrepairable. On some aircraft, this can be an expensive lesson at best, and depending on the type of aircraft, a replacement part may not be readily available.
Dawley agrees that small weld repairs are typically not worth the effort. "People weld on, and that just causes more of a hot spot, which causes more cracking. So you just keep chasing cracks. If the shell is cracking, it's cracking for a reason. It's either fatigued, thin, or pitting, and this usually means it's time to replace it," he says.
Dawley says that they will work with technicians, however, if they feel that only one area needs replacement. "For instance," he says, "if the end plate is cracking and everything else is in good shape, send it in to us and we will remove the end plate and replace it with a new one." On the flip side, Sturch at Wall Colmonoy says that any damage to an exhaust system component usually signifies it is time for the entire component to be overhauled or replaced. Sturch warns against repairs without reconditioning the entire muffler. "We will not do a repair without overhaul," says Sturch. "Customers will ask to have a small crack repaired, and we refuse." Sturch claims that this position is in the best interest of safety, and that repair without overhaul is just not practical in terms of liability.
Sturch explains that many repairs in the field are made simply by welding a patch over a damaged area of a muffler or exhaust component. Adding a patch without removing the base metal creates an area where air is trapped between the base metal and the patch. As a result, the base metal and the patch see different contraction and expansion rates, and failure of the patch will soon result.
Headers usually require fixtures to keep all flanges in alignment. He explains that many attempts at welding headers without the use of a fixture result in warped headers which have to be forced into position. This additional stress on the header can cause premature failure.
Keep it together
Larry Dawley says that sending all of the stacks along with the muffler can help expedite the job by allowing the repair station to do a more thorough job. "We prefer as the repair facility to have the technician ship the entire exhaust system (exhaust stacks and muffler), so that we can check alignment on everything. Often what happens is that the stacks are warped, and the muffler warps with it, and when we repair the muffler back to new specs, it doesn't fit. But that doesn't mean the muffler is wrong; it means the stacks need to be returned to their original shape. So with both components, we can restore the entire system the way it's suppose to be.
"We won't repair a muffler to fit a bent stack. We put the muffler back to original specs and then fix the stacks," Dawley explains. "That becomes a big problem for those who feel there's nothing wrong with their stacks. They have to understand that heating and cooling cycles change the way these things fit. The tubes can't be bent back into shape because there is too much stress in the metal. Also, there's really not enough material to be bending the tubes. The proper way to make the repair is to replace the tubes where they're bent. That way you don't weld any stresses into it," says Dawley.
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. April 1998
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.