Rigby, ID, USA
N6146G
Air Tractor AT-502B
The pilot of the Air Tractor AT-502B had reported observing no abnormalities during engine start and the subsequent application of power during takeoff. Climbing through 100 feet he heard a loud "bang," observed sparks and flames exiting the engine exhaust ports and transitioned to a power off forced landing. As the aircraft was rolling out it encountered a ditch and the main landing gear separated. Post-crash examination of the engine revealed extensive damage to the blades of the compressor turbine disk and erosive burning damage to the vanes of the vane ring situated immediately upstream of the compressor turbine disk. The 22 year old Pratt & Whitney PT6A-34 turboshaft engine had a total of 7,070 hours since new and had undergone an undocumented overhaul 5,820 hours previously to the accident. The last hot section inspection referenced in the engine log occurred 2,871 hours previous to the accident and the last logged inspection was a 100-hour inspection 324 hours and eight months previous to the accident.
On August 20, 2002, approximately 0652 mountain daylight time, an Air Tractor AT-502B spray plane, N6146G, registered to/operated by Queen Bee Air Specialties, Inc., and being flown by a commercial pilot, sustained substantial damage during a forced landing following a loss of power shortly after takeoff from the Rigby-Jefferson County airport, Rigby, Idaho. The pilot suffered minor injuries. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which was departing for a spray run, was operated under 14 CFR 137, and departed the Rigby airport approximately 0650. The pilot reported observing no abnormalities during engine start and the subsequent application of power during takeoff. While climbing through approximately 100 feet above ground he heard a loud "bang" from the engine compartment and observed sparks and flames exiting the engine exhaust ports. He then selected the fuel cutoff position on the engine condition lever and extended the flaps to full down. He reported that the aircraft touched down on all three landing gear and rolled approximately 600 feet and then impacted and passed over a raised irrigation ditch collapsing both main landing gear (refer to attached NTSB Form 6120.1/2 and photographs 1 and 2). On site examination of the aircraft by an inspector assigned to the Federal Aviation Administration's (FAA) Salt Lake City Flight Standards District Office (FSDO) revealed compressor blade and other metal fragments lying inside the engine's left exhaust port (refer to photograph 3). The aircraft was recovered to the facilities of the Operator where the FAA Inspector conducted a disassembly of the engine. The examination revealed extensive damage to the blades of the compressor turbine disk (refer to photographs 4 through 6 and Diagram I). Further examination revealed degradation and erosive burning damage to the vanes of the vane ring situated immediately upstream of the compressor turbine disk (refer to photographs 7 through 9 and Diagram I). According to the records maintained (engine service cards and maintenance record) for Pratt & Whitney PT6A-34, serial number PCE-56822, the engine was manufactured in September 1980. The first entry within the engine log following the new engine acceptance test was dated 06/18/81 and the time noted was 0.0 hours. The next entry, dated 07/22/82 showed a total engine time/time since overhaul (TSO) of 1250.3 hours. An undated line entry reading "Time since overhaul by Airwork of Millville, N.J." followed this entry. However, there was no supporting documentation within the log detailing this overhaul. The next entry, dated 11/09/84 showed the TSO as 0.0 hours. The log showed the engine accruing time through 1987 up to 11/28/88 when the TSO was recorded as 2,025.0 hours and a hot section inspection was conducted. The log continued showing the engine accruing time up to 03/07/92 at which time the TSO was recorded as 2,948.15 hours and the time since new (TSN) was recorded as 4,198.6 hours. On 04/22/95 a log entry showed another hot section inspection conducted at this same TSO. The log showed the engine continued to be operated until 06/29/01 at which time a typed entry in the log read in part "Power section repaired for chip detector light illumination. The TSO at this repair was 5,175.0 hours (TSN 6,426.1). The next entry in the log, which was also the last entry, was dated 12/20/01 and showed a total time (TSN) of 6,746.1 hours (TSO 5,495.8 hours). A 100-hour inspection was carried out on this date and the operator reported that the engine had accrued an additional 323.7 hours since this inspection. There was no record of any engine overhaul other than the line entry at 1,250.3 hours referenced above (refer to Attachment EML-I). Pratt & Whitney recommended TBO for the PT6A-34 engine is 4,000 hours. Additionally, the FAA inspector conducted a review of the maintenance records for the engine and reported a number of discrepancies (refer to Attachment FAA-I).
The failure of one or more compressor turbine blades resulting in a shutdown of the engine during initial climb resulting in a forced landing. Contributing factors were the eroded condition of the compressor turbine guide vane nozzles and the ditch which the aircraft encountered during the landing rollout.
Source: NTSB Aviation Accident Database
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