NTSB Factual Report

On June 28, 1994, at 1954 Pacific daylight time, a Rockwell S2-R, N4230X, collided with the ground following an in-flight fire in the chemical spreader and a loss of control near Firebaugh, California. The aircraft was owned and operated by Tri Air, Inc., of Firebaugh, and was engaged in local area aerial application operations under 14 CFR 137 of the Federal Aviation Regulations. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft was destroyed in the collision sequence and postcrash fire. The certificated commercial pilot, the sole occupant, sustained fatal injuries. The flight originated at the Firebaugh airport on the day of the mishap at 1930.

At departure, the aircraft was loaded with 2,000 pounds of a dusting sulfur compound in powder form (containing 98% sulfur) for application to the fields. An estimated 1,500 pounds remained on the aircraft at the accident.

Three witnesses reported observing all or parts of the accident sequence.

A helicopter pilot was airborne en route to an aerial application job in a neighboring field and first noticed the aircraft about 10 miles away working a field. He stated that the outside air temperature was 96 degrees. As the pilot watched the aircraft, he observed what he believed to be an explosion and immediately radioed his company to have them notify the authorities. After arrival over the accident site, the pilot observed the aircraft on the ground burning.

A second witness lives about 3/8ths of a mile from the crash site and was mowing his lawn. He reported hearing a pop or explosion sound and turned around to see the aircraft on the ground and burning.

The third witness lives next door to the second witness and reported that she was watching the aircraft applying chemicals to the field. The witness stated that the aircraft was on a northerly heading and had just completed a spray run. The aircraft began a left turn to make what the witness assumed was a southbound run on the field. The witness then observed flames on the bottom of the aircraft in the vicinity of the dispenser apparatus. The aircraft descended a little, started to climb again, then suddenly nosed over and dove vertically into the ground.

Federal Aviation Administration (FAA) airworthiness inspectors from the Fresno, California, Flight Standards District Office responded to the site the day following the accident. They reported that, with the exception of the outboard portions of the wings, fire had totally consumed all of the nonsteel aircraft components. The inspectors noted that the fire seemed to have been more intense in the area of the chemical hopper and spray apparatus. The engine was buried in the ground; however, propeller slashes were observed in the ground and the propeller blades exhibited torsional twisting and leading edge damage.

The chemical manufacturer was contacted. A representative stated that the powdered sulfur dust compound used in aerial application operations is considered a moderate fire hazard according to National Fire Protection Association criteria. While the compound has a flash point of 374 degrees Fahrenheit, published studies (page 48, INVESTIGATION OF FIRE AND EXPLOSION HAZARDS IN CHEMICAL, MINING AND FUEL RELATED INDUSTRIES, Bureau of Mines, U.S. Department of Interior) demonstrate that atomized layers or clouds of dust may ignite at ignition temperatures of 190 degrees in air to substance density limits of less than 0.1 ounces per cubic foot. The test was conducted in a Hartman apparatus with a high voltage 24-watt spark source for ignition.

The manufacturer's representative noted that sulfur is processed in an inert atmosphere during manufacture into the powdered form to prevent potential explosions or fires.

NTSB Probable Cause

THE PILOT'S LOSS OF AIRCRAFT CONTROL AS A RESULT OF INCAPACITATION FROM AN IN-FLIGHT SULFUR FIRE.