Incident Details
The airplane was loaded with 700 gallons of fire retardant for fire suppression. While the pilot was maneuvering to position for a drop at 1,100 feet above ground level, the engine chip light illuminated. About 20 to 30 seconds later, the pilot heard a loud bang and the engine lost total power.
Forced Landing
The pilot intentionally ground-looped the airplane during a forced landing in an open area to avoid a brick wall, fence, and a parking lot containing vehicles and people directly ahead.
Engine Examination
Examination of the engine revealed that the loss of total power was due to the failure of the No. 1 bearing. This failure resulted in axial and radial movement of the compressor rotor and severe over temperature damage to gas path components. The bearing failure was caused by electrical arcing between the bearing races and rolling elements. Electrical arc pitting found on the starter-generator gear shaft indicated a direct electrical arcing path from the starter-generator to the bearing. However, the exact source of the electrical arcing within the starter-generator was not determined.
Maintenance History and Advisories
According to maintenance records, the starter-generator had been replaced 172 hours prior to the accident due to bearing distress. A Transport Canada Service Difficulty Advisory (No. AV-2007-05, dated October 29, 2007) for PT6A engines noted that electrical arcing occurrences have caused pitting and subsequent failure of the No. 1 bearing. The advisory states that electrical current travels from a defective starter-generator spline shaft through the engine accessory drive train to the No. 1 bearing, causing mild to severe damage such as pitting, grooves, or craters. The extent of damage depends on factors such as electrical current, exposure time, bearing load, and rotational speed.
The advisory identifies the most common and preventable cause of electrical arcing as armature leakage from an accumulation of brush dust, which can provide an electrical discharge path between the commutator and shaft. Additionally, a breakdown of commutator insulation or lamination slots can cause an electrical short. Pratt & Whitney's maintenance manual addresses oil contamination monitoring procedures for detecting pending No. 1 bearing failure after starter-generator replacement for electrical reasons, requiring oil filter inspections at 100-hour intervals for 700 hours. However, there was no record of any subsequent oil filter patch checks or other related inspections after the starter-generator change.