Exhaust system alert

Exhaust System Alert Extra attention badly needed By Greg Napert April 1998 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...

"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.

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