Views:4 Author:Site Editor Publish Time: 2019-03-04 Origin:Site
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.