Exhaust Failure
July 1990
"Dear RAF,
I thought I would write to report an exhaust failure on our Defiant that could have been quite serious. This involved the front engine with about 200 hours on it. The exhaust was a unit purchased from Wag Aero. It is a standard wide deck exhaust for a Grumman Tiger. The failure occurred at two places on the unit. One spot was on the exhaust stud coming from the right rear cylinder. It was a total fatigue fracture about 1/2" below the weld to the flange. The other failure spot was on a lower left juncture of the combined pipes as they went into the muffler. I could not determine which crack was primary and which was secondary, but I suspect one of them caused the other. What was interesting was that the cylinder near the site of the failure had been pulled by a repair facility when an intake valve cracked. I did not oversee the repair since it was on a standard engine and muffler combination.
After a discussion with Aero Fabricators who repaired the muffler, I came to the following conclusions:When the cylinder was pulled, they probably did not loosen the entire muffler from all the other cylinders. When the cylinder was replaced, the muffler was sprung back into place in a stressed condition and was bolted. into place. Aero Fabricators suggested that when the exhaust system was reinstalled after repair that it be loosely bolted into place and then heated by running the engine until it was good and hot. In this hot state, the cylinder bolts and sleeve clamps are then tightened to appropriate torque.
This exhaust system was only about 200 hours old. Since this was a certified muffler on a standard engine, things point strongly to an error in installation procedures. This caution might be relevant to other exhaust systems that are somewhat rigid between multiple cylinders. We are also going to be balancing both engines in the near future since both starter ring gears were not part of the engines when we bought them.
What was really scary was that we had a fuel line failure on the same flight on the same engine, within only 1 hour of each other. The fuel line failure by the way was one of fancy expensive lifetime custom made all stainless steel lines that come from Aircraft Spruce, It appears that the failure was a combination of a poor weld on the stainless steel tube and vibration failure. I am considering replacing them with good old rubber Aeroquip rubber lines that you periodically throw away. At least I never saw a rubber line fatigue. Did you ever notice that it is all that metal on our fiberglass airplanes that seems to break all the time? I think I am ready for fiberglass engine mounts and ceramic engines.
John Steichen