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.
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