Incident Overview
On August 2, 2016, at approximately 0935 mountain daylight time, an Air Tractor AT 301 (registration N23923) experienced a partial loss of engine power while conducting agricultural application operations. The pilot, an airline transport pilot, executed an off-airport landing in a field near Weiser, Idaho. The pilot was not injured, but the airplane was substantially damaged during the landing roll when it collided with a sprinkler system.
Flight Details
The flight was operated by Beck's Flying Service under Title 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed. The local agricultural application flight had departed Weiser, Idaho, at about 0920.
Pilot Report
According to a report submitted to the National Transportation Safety Board investigator-in-charge, after departing for the fifth aerial application load of the morning and while approaching the target field, the cylinder head temperatures (CHTs) rose to over 400°F. The airplane had just enough power at 30 inches of manifold pressure (MP) to maintain altitude and airspeed above stall speed. After making one pass on the field, the pilot observed CHTs had risen to 414°F. To stay above gradually rising terrain, the pilot increased engine power to 32 inches MP and selected half flaps, at which point CHTs rose to 454°F with a full rich mixture. With wings level and about 40 feet above the ground and descending, the pilot dumped part of the load and set power to 30 inches MP; CHTs remained around 440°F. The pilot determined the airplane could only continue flying in a slight descent, prompting the dumping of the remainder of the load. With no improved performance, the pilot abandoned the idea of returning to the departure airport. The pilot maneuvered slightly to reach an alfalfa field, avoiding pear trees before touching down with wings in buffet. During the landing roll, the airplane collided with a wheel line sprinkler system and a tree bordering the field, resulting in substantial damage.
Postaccident Engine Examination
A postaccident examination of the engine was performed at Anderson Aeromotive, Inc., in Grangeville, Idaho. All nine cylinders remained attached to the engine crankcase. Visual examination revealed that the cylinder head temperature (CHT) and exhaust gas temperature (EGT) probes were connected to the No. 1 cylinder. The front spark plugs were removed from all cylinders with no anomalies noted. The crankshaft was manually rotated, and internal mechanical continuity was established throughout. The cylinders developed compression when the crankshaft was manually rotated.
The No. 1 cylinder was removed, and when the piston was removed, the lower piston skirt and the oil scraper ring were observed damaged. The inside of the piston had multiple indentations consistent with continued battering. The remaining eight cylinders and pistons were removed, with similar results noted. Multiple metallic fragments resembling bearing material were found in the crankcase throughout the process. The chip detector was covered with metallic debris consistent with debris found throughout the internal portion of the engine. The front case, cam ring assembly, and front crankcase assembly were removed with no anomalies noted. The master rod counterweight revealed multiple battering-like indentations consistent with the damage observed to the inside of the pistons. Multiple metallic fragments were found in the oil galley. The master rod and link rod assembly was removed from the crankshaft, and the front side of the master rod exhibited two perpendicular dents. The master rod crankpin bearing revealed substantial wear and gouging-type damage. The master rod crankpin bearing was thinned or gouged out consistent with the metallic shavings found in the crankcase and oil galley. Historical records for the engine were not made available during the investigation.