WPR21LA250

The experimental, amateur-built airplane was retrofitted with an experimental exhaust system that comprised a titanium exhaust header and aluminum muffler. On the first flight with the new exhaust system, the engine lost total power just after takeoff. The pilot performed a forced landing during which the airplane impacted trees, resulting in substantial damage. Examination of the engine revealed that the muffler inner tube failed due to creep fatigue and fractured into several fragments. Components of the muffler included a perforated inner tube, surrounded by fiberglass packing material held in place by five clamps; an outer shell; end caps; and an inlet and exhaust tube, all constructed out of 6061-T6, an aluminum alloy not suited for use above 350°F to 400°F. Any use near or above that temperature will cause additional heat treatment and changes to the properties and performance of the material. The observation of the creep separation indicates that the material was exposed to much higher temperatures than this, likely between 600°F and 1000°F. It is likely that, when exposed to the hot exhaust gasses during takeoff, the fragments, clamps, and fiberglass packing material migrated aft and collapsed the remaining section of the inner tube, which plugged the exhaust port and resulted in a total loss of power. A postaccident test run of the engine without the muffler revealed no anomalies.

On June 28, 2021, about 1030 Pacific daylight time, an experimental, amateur-built, CCX-2000 airplane, N10TA, was substantially damaged when it was involved in an accident near Hood River, Oregon. The pilot was not injured. The airplane was operated as a Title 14, Code of Federal Regulations Part 91 personal flight. The pilot was the director of maintenance for a maintenance facility contracted to perform work on the airplane. He reported that, at the request of the owner, the airplane was retrofitted with an experimental exhaust system designed to quiet the engine. The replacement exhaust system comprised a titanium exhaust header and an aluminum muffler. The pilot stated that, following the installation of the exhaust, an uneventful full-engine-power ground run was conducted. After the ground run, the pilot initiated the airplane’s first flight with the new exhaust system. Shortly after takeoff, the engine lost total power, and the pilot performed a forced landing to a field. During the landing roll, the airplane impacted trees before it came to rest upright in the field resulting in substantial damage to the rear spar carrythrough. A video recording of the takeoff revealed, about 24 seconds after takeoff, metallic shiny debris trailing from the airplane, as seen in figure 1. Figure 1: Airplane during takeoff. According to the manufacturer, the exhaust system was similar to a Carbon Cub design but with a header constructed out of titanium and a “Swiss style” muffler constructed out of aluminum. The manufacturer stated that the company had no trends with an aluminum muffler and stated this to the company requesting the design. The aluminum muffler was examined by the NTSB Materials Laboratory. Examination revealed that the muffler, as it was manufactured, comprised an inner cylindrical perforated hollow tube (approximately 2.5-inch diameter) surrounded by fiberglass packing material. The packing material was secured to the tube outer diameter by five band clamps and wire. This assembly was inserted into an approximate 3.5-inch diameter by 36-inch-long tube, which served as the outer shell of the muffler body. The packing material volume was sized to fill the volume between and nominally center the inner tube and outer shell. Designs of this type are sometimes referred to as “Swiss style” mufflers. End caps, an inlet tube, and an exhaust tube were then welded onto the assembly. The muffler inner tube, muffler shell, and inlet and exhaust tube were identified as 6061-T6, an aluminum alloy. The muffler inlet and exhaust tubes were sectioned from the muffler body. At the inlet end, the inner tube was fractured 360° around its circumference approximately 3.5 to 4.0 inches from the inlet end and the fiberglass packing had fallen inward, obstructing the view further inside the muffler. At the exhaust end, the inner tube had collapsed inward in three nominally symmetric segments, pinching the outlet end shut. (see figures 2 and 3.) Figure 2. Image ofthe muffler body inlet, showing the fractured perforated inner tube and the fiberglass packing that had fallen inward. Figure 3. Image of the muffler body outlet, showing the collapsed inner tube. The fracture features of the inner tube were consistent with a failure that started in tension near the 7 o’clock position and transitioned to bending as it progressed around both sides of the tube and terminated near the 1 o’clock position. The fractured ligaments, when viewed under magnification, had a specular appearance with no apparent crack growth features that would be typical of fatigue or overstress fractures. The ligament fractures were examined in a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (EDS). The observations were consistent with a high-temperature creep separation/fracture. According to the Federal Aviation Administration, a postaccident test run of the engine without the aluminum muffler revealed that the engine started easily and responded well to throttle and propeller control adjustments. There were no issues noted during the engine run, and no mechanical malfunctions or failures that would have precluded normal operation.