Big Bear City, CA, USA
N91357
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.
On September 17, 2007, at 1310 Pacific daylight time, an Air Tractor AT-802A single-engine air tanker (SEAT), N91357, impacted terrain during a forced landing following a loss of engine power near Big Bear City, California. The commercial pilot, the sole occupant, was not injured. The airplane sustained substantial damage. The United States Department of Agriculture - US Forest Service operated the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91 as a public-use aerial application flight. Visual meteorological conditions prevailed for the firefighting mission, and a company visual flight rules (VFR) flight plan had been filed. The local area fire suppression support flight departed San Bernardino, California, at 1253. Prior to departure, the airplane was loaded with 700 gallons of fire retardant to support fire suppression efforts on the Bulter 2 fire near the Big Bear Lake area. While maneuvering to position for a drop at 1,100 feet above ground level (agl), the engine chip light illuminated. The pilot checked all the engine gauges and "they appeared to all be in the normal range." Approximately 20 to 30 seconds later, the pilot heard a loud bang and the engine lost total power. The pilot immediately jettisoned the fire retardant load, shut off the main fuel, and turned toward an open area. Due to smoke in the area, the pilot could not locate the Big Bear airport, which he knew was somewhere in the area. During touchdown to an open area, the airplane struck a dirt berm and bounced back into the air. After recovery from the bounced landing, the pilot noticed a brick wall, fence, and a parking lot containing people and vehicles. The pilot applied hard left brake to avoid the parking area and the right main landing gear collapsed. The airplane rotated 90 degrees and came to rest upright. The engine examination took place on September 25 to 27, 2007. During the visual examination investigators noted no indications of preimpact discontinuity of any of the airframe to engine connections. Investigators noted that the airframe exhaust stubs displayed external pockmarks and dimpling. The engine was removed from the nacelle along with associated airframe components and the engine was transferred to the Pratt and Whitney Engine Services facility for further inspection. An initial examination of the engine revealed that the number 1 bearing displayed operational distress that consisted of adhesion pitting and spalling of the bearing raceways and balls, and fracture of the bearing cage. The bearing components displayed high residual magnetism and evidence of an electrical arcing pit on at least one bearing ball characteristic of electrical discharge damage. The starter-generator gear shaft, the accessory drive gear shaft, the accessory gearbox coupling shaft, and the rear compressor rotor shaft displayed high residual magnetism, and electrical pit indications were evident on the gear shaft tooth faces of those components. The compressor rotor displayed circumferential rubbing and machining from contact with adjacent static components due to radial and axial movement of the compressor rotor. The downstream gas path components displayed severe over temperature damage. A materials examination was performed by Pratt and Whitney under the auspices of the Transportation Safety Board of Canada to determine the source of the No. 1 bearing distress and failure. The examination revealed that the No. 1 bearing components high level of magnetism combined with the presence of an electrical pit found on one of the balls indicated that the distress was a result of electrical discharge damage (EDD). The degradation of the bearing components is considered as secondary damage resulting from the EDD. The residual magnetism combined with the evidence of EDD pits on the mechanical components linking the engine accessory drive train to the starter-generator output shaft indicated that the arcing originated in the starter-generator. The source of the arcing produced by the starter-generator was not determined. The following is an excerpt from the Pratt and Whitney maintenance manual, 72-00-00 Engine, Turboprop - Inspection, Starter-Generator Replacement: (1) If the starter-generator is replaced in order to rectify a reported engine starting or electrical generation defect, that is suspected as an electrical fault or bearing distress of the starter-generator, inspect the main oil filter as follows: (a) Do a main oil filter patch check (Ref. 79-20-02). The results of the filter patch analysis should be reviewed within the next 25 flight hours. If non allowable debris is found, follow the recommended maintenance actions (Ref. 79-20-02). (b) Regardless of the results of the patch analysis, repeat step (a) every 100 hours, for the next 700 flight hours. (c) If bearing material (AMS 6440/6444 or AMS 6490/6491, (Ref. 79-20-02) is found, remove the engine/gas generator module and send to an approved overhaul facility for repair. According to the maintenance records, the starter-generator was replaced on May 31, 2007, at 2,714 hours, approximately 172 hours prior to the accident. The starter-generator was replaced by the operator due to bearing distress. There was no record of any subsequent oil filter patch checks or other related inspections. According to a Transport Canada Service Difficulty Advisory, No. AV-2007-05, dated October 29, 2007, in the PT6A engine, EDD occurrences have caused failure of the No. 1 bearing. Electrical current travels from the defective starter-generator (SG) spline shaft, thru 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 EDD, is from SG 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 SG commutator insulation and/or the lamination slots can cause an electrical short. Periodic field cleaning and resistance checks may provide an indication of armature insulation breakdown. Close visual inspection of the SG spline shaft for arc damage anytime the SG is removed is an excellent indicator of possible EDD.
Loss of engine power while maneuvering due to the failure of the number 1 bearing that was caused by electrical discharge damage to the bearing originating from the starter-generator. Contributing to the accident was the operator's failure to follow the engine maintenance manual procedures for oil monitoring following generator replacement and the unsuitable terrain for the forced landing.
Source: NTSB Aviation Accident Database
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