Grumman Acft Eng Cor-Schweizer G-164A N8380 accident near Wilton, California

Probable cause

The failure of the hopper gate mechanism and subsequent sulfur fire during a low-level maneuver.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for WPR12LA233 for the full record.