Air Tractor AT-802A accident near Big Bear City, California, September 17, 2007
On September 17, 2007 at about 8:10 pm local time, a Air Tractor AT-802A, registered N91357, was substantially damaged in an accident near Big Bear City, California. It was a public-use flight (federal) under general aviation rules (Part 91). No one was hurt; 1 person was on board or involved. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
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, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- September 17, 2007 · about 8:10 pm local time
- Place
- Big Bear City, California
- Type
- Accident
- Injuries
- No one was hurt; 1 person was on board or involved.
- Weather
- visual conditions (good weather)
- Aircraft
- Air Tractor AT-802A
- Registration
- N91357 · registry record · serial 802A-0107
- Damage
- Substantial damage
- Flight
- Public-use flight (federal) · general aviation rules (Part 91)
The NTSB's narrative final · quoted from the NTSB record
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.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
Pilot
- Certificate: airline transport pilot, flight instructor
- Ratings: multi-engine land; single-engine land; instructor: airplane multi-engine; instructor: airplane single-engine; instructor: instrument airplane; instrument: airplane; instrument: helicopter; rotorcraft: helicopter
- Flight time: 15,000 hours in all; 300 in this make and model; 200 in the last 90 days; 100 in the last 30 days; 14,000 as pilot in command
- Last flight review: September 1, 2006
- Medical certificate: Class 2 (with waivers/limitations)
- Seat: sngl
- Injury: no injuries
The aircraft
- Airframe total time: 2,872 hours
- Last inspection: 100-hour inspection, September 1, 2007; 15 hours since
- Maximum gross weight: 16,000 lb
- Seats: 1
- Landing gear: fixed
- Engine: Pratt & Whitney Canada PT6A-67AG (turboprop); 0 hours total
The flight
- Departed from: SBD San Bernardino CA at 7:53 pm
Weather at the time
- Light: daylight
- Visibility: 10 statute miles
- Sky: clear
- Temperature: 0°F (-18°C), dew point 0°F (-18°C)
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Other NTSB records under N91357 the same tail number, which may have belonged to a different aircraft at the time
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.