LAX07TA277 · Air Tractor AT-802A

The airplane was loaded with 700 gallons of fire retardant to support fire suppression efforts and the engine chip light illuminated while the pilot was maneuvering to position for a drop at 1,100 feet above ground level. About 20 to 30 seconds later, the pilot heard a loud bang and the engine lost total power. The pilot intentionally ground looped the airplane during a forced landing in an open area in order to avoid a brick wall, fence, and a parking lot containing vehicles and people directly ahead. Examination of the engine revealed the engine lost total power due to the failure of the No. 1 bearing, which resulted in axial and radial movement of the compressor rotor and severe over temperature damage to the gas path components. The bearing failure was due to electrical arcing between the bearing races and rolling elements. The electrical arc pitting of the starter-generator gear shaft indicated a direct electrical arcing path from the starter-generator to the bearing. The exact source of the electrical arcing within the starter-generator was not determined. According to the maintenance records, the starter-generator was replaced 172 hours prior to the accident due to bearing distress. According to a Transport Canada Service Difficulty Advisory, No. AV-2007-05, dated October 29, 2007, in the PT6A engine, electrical arcing occurrences have caused pitting and subsequent failure of the No. 1 bearing. Electrical current travels from the defective starter-generator (SG) spline shaft, through the engine accessory drive train to the engine No. 1 bearing causing mild to severe bearing damage such as pitting, grooves, or craters. The extent of bearing damage, and therefore, the time before failure, is dependent on various factors such as the electrical current, exposure time, bearing load, and rotational speed. The most common and preventable cause of electrical arcing, is from armature leakage occurring as a result of an accumulation of brush dust. This dust can provide an electrical discharge path between the commutator and the shaft. Secondly, a breakdown of the commutator insulation and/or the lamination slots can cause an electrical short. In order to detect pending failure of the No. 1 bearing, the Pratt and Whitney maintenance manual addresses oil contamination monitoring procedures in the event of starter-generator replacement for electrical reasons. The oil filter is to be inspected at 100-hour intervals for engine bearing material for 700 hours following starter-generator installation. There was no record of any subsequent oil filter patch checks or other related inspections after the starter-generator change.

Verbatim NTSB analysis and probable cause from the NTSB dataset