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On February 21, 2012, about 1440 Pacific standard time, a Haycraft Sport Hornet, N72PD, experienced an in-flight fire near Mariposa-Yosemite Airport, Mariposa, California. The airplane was substantially damaged during an off-airport landing and consumed by fire. The owner was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The sport pilot and sole passenger were not injured. The local personal flight departed from Mariposa about 1340. Visual meteorological conditions prevailed, and no flight plan had been filed.
In a written statement, the pilot reported that after taking photographs of property, he began to return back to the airport. With the airport about 5 miles to the north, he noticed the smell of smoke in the cockpit. He maneuvered the airplane directly toward the airport and noted that all the engine temperature and pressure cockpit gauges indicated normal operation. The smoke intensity increased and after about 15 seconds, he opted to perform a forced landing into a pasture below. During the landing roll, the nose wheel separated from the airplane and slid to a stop. The pilot and passenger egressed and watched the airplane burn. The pilot further stated that he first observed the fire underneath the engine area.
The Haycraft Sport Hornet single-engine amateur built airplane, serial number 0054, was completed in 2008. The airplane was equipped with the originally installed Rotax 912ULS engine. The pilot stated that the airframe and engine had accumulated a total time of 97 hours. The last conditional inspection was dated as having been completed August 07, 2011. The airplane had accrued approximately 10 hours since that inspection.
TESTS AND RESEARCH
The airplane was configured in a pusher-type style, with the engine mounted above and aft of the cockpit. The airframe fabric covering was burned from its steel tubular structure with the exception of panels that remained covering the left wing and several panels on the outboard right wing. The engine, a Rotax 912ULS, remained affixed to the three-bladed propeller. The engine had sustained thermal damage and the firewall was melted, with the only identifiable pieces being molten material clumped in the wreckage.
A postaccident examination revealed that the predominant area of thermal deformation was around the No. 4 cylinder (right side) and concentrated around its forward inboard section where several cylinder fins were bent. Examination of the bottom of the engine revealed that the right side was white in coloration and the left side was black, consistent with more thermal exposure and hotter temperatures reached on the right side. The housing on the exhaust pushrod of the No. 4 cylinder was thermally destroyed exposing the charred pushrod.
According to the Rotax representative, the exhaust system installed on the engine was manufactured by Titan Aircraft. There was no evidence that there was heat shielding between the exhaust system and engine. The distance between the exhaust pipe and the ignition module was about 3 inches; the module was consumed by fire as was the ignition harness. In the area that had sustained the greatest thermal damage (above the No. 4 cylinder) was where the following components were routed near an exhaust pipe: fuel and cylinder head coolant lines, an ignition harness, and a Bing Carburetor (mounted via a rubber carburetor socket).
According to the Rotax Installation Manual section 11, “Exhaust system,” the “shape and execution of the exhaust system is determined essentially by the free space available in the aircraft.” A caution notes to, “fit heat shields near carburetors or as required,” and “because of high temperatures occurring, provide suitable protection against unintentional contact.”
The Rotax representative stated that there is no defined maximum or minimum distance the components need to be from the exhaust system, but the installer must take into account that a certain amount of back pressure will occur during takeoff, which will affect that distance. He noted that keeping the exhaust system closer to the engine is most desirable as long as the installer does not exceed the minimum bend radius on the exhaust pipe and that the proper heat shielding is used.
The Rotax representative further stated that the exhaust system reaches temperatures of 1,560 to 1,616 degrees Fahrenheit, as explained in the limits section of the Installation Manual. The ignition modules have a maximum ambient temperature of 176 degrees Fahrenheit, and the representative stated that if there isn’t proper heat shielding to protect the modules and fuel lines from the exhaust heat, the engine may be susceptible to a fire.
Titan Aircraft, the exhaust manufacturer, issued an advisory in July 2010 for another aircraft equipped with Rotax 912 series engines, stating that an ignition module failure can occur due to heat from the exhaust. The advisory states, “Installation of exhaust/header wrap on the muffler and outlet pipes or fabrication of a heat shield to cover the ignition module is required.”