WPR12LA233

The pilot had just completed an aerial application of sulfur dust over a vineyard when the hopper gate closing mechanism failed as he attempted to stop the flow. The sulfur continued to flow from the hopper, so he initiated a steep turn to keep the dust within the confines of the vineyard. As he rolled out of the turn, the plume of sulfur ignited and a fire began within the hopper. He performed a forced landing into a residential driveway and the airplane sustained substantial damage during the ground roll. The airplane was configured to minimize the hazards of inadvertent fire during sulfur dusting operations; however, the evasive flight maneuver performed by the pilot may have brought the uninterrupted flow of sulfur dust into contact with hot engine exhaust. An alternative ignition source may also have been provided by a spark from the failed gate mechanism. The gate mechanism sustained thermal damage, which prevented an accurate assessment of its failure mode.

HISTORY OF FLIGHT On May 27, 2012, about 0815 Pacific daylight time, a Grumman G-164A, N8380, experienced an in-flight fire to its chemical dispersal system near Wilton, California. The airplane was operated by Harding Flying Service, and was engaged in agricultural aerial application operations under the provisions of 14 Code of Federal Regulations Part 137. The certificated commercial pilot was not injured, the airplane sustained substantial damage during the accident sequence, and was consumed by post impact fire. The local flight departed from a private airstrip near Clarksburg about 0730. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that he had just completed a pass over a vineyard where he was applying sulfur dust. As he pulled the hopper gate lever to stop the flow, the lever went limp. The sulfur continued to flow from the hopper, so he initiated a steep turn to keep the dust within the confines of the vineyard. As he rolled out of the turn, the plume of sulfur ignited behind the airplane and he heard a popping sound from the hopper, which was now on fire. He performed a forced landing into a residential driveway, and during the ground roll, the main landing gear became embedded in soft dirt, and the airplane nosed over. The majority of the airplane's non-steel components were consumed by fire. The airplane's maintenance records were examined by an inspector from the Federal Aviation Administration. According to the records, the airplane had undergone an annual inspection, which was completed on January 16, 2012. At that time, the airplane had amassed 12,376 flight hours since its manufacture in 1971. The FAA inspector stated that the airplane was in compliance with Airworthiness Directive 48-34-02, which defines the inclusion of specific aircraft features designed to decrease the hazards from fire during dusting operations involving the use of sulfur. The gate mechanism was the original factory installed system. Examination of the hopper gate mechanism, located under the cabin floor, revealed that the hinge of the gate interconnect tube had separated from the gate lever. The hinge bolt remained in place, and attached to the remnants of the hinge. The mechanism had sustained thermal damage, which prevented an accurate assessment of its fracture features. Review of the FAA Service Difficulty Report and Airworthiness Directive databases did not reveal any documentation of prior instances of this failure.