LAX00FA169

The airplane was spraying chemicals on a field of crops when the left wing separated in flight. Ground witnesses said that the airplane was making a level spray pass over a potato crop when the left wing failed. No unusual meteorological phenomena were observed in the area, and the airplane did not perform any unusual maneuvers prior to the failure. The left wing's lower spar cap was found to have fractured in fatigue at a point that transected the inboard attach block bolt hole where the spar attaches to the center section spar carry through member. Multiple origin sites were identified within the bolt hole bore, with corrosion pitting evident at the origin points. The spar caps met the hardness, chemistry, and microstructure of the production drawing specified material. Air Tractor Service Letter 118 and the maintenance manual specify an inspection procedure, which calls for the removal of the hopper for a detailed inspection of the spar caps for cracks; one area identified as critical for cracking was the inboard bolt hole bore. The inspection interval was specified as every 2,000 hours or 3 years. The number of crack arrest striations in the fatigue region was determined from the first point the crack would be visible from beneath the attach block during the specified inspection, and the mean time for the fatigue crack to progress from detectible during the inspection to failure was calculated to be about 105 hours. Examination of the right wing lower spar cap revealed the presence of multiple fatigue cracks emanating from corrosion pits within the outboard bolt hole bore. No maintenance records were located for either the airframe or engine. A mechanic was located who performed some periodic maintenance on the aircraft. He stated that during the removal of the hopper in May 1997, 2 years 11 months prior to the accident, he had accomplished the 2,000-hour check of the wing spars in the area of the attach bolts as required by Service Letter 118. Based on other records, the aircraft was believed to have flown an average of 500 hours per year and would have accumulated only 1,500 hours since the inspection.

History of flight

On April 22, 2000, at 1019 hours mountain standard time, an Air Tractor 502A, N15466, was destroyed after an in-flight separation of the left wing near Queen Creek, Arizona. The commercial pilot sustained fatal injuries. The airplane, operated by San Tan Dusters, Inc., under 14 CFR Part 137, was destroyed during the impact sequence. The airplane was spraying chemicals on a field of crops when the accident occurred. Visual meteorological conditions prevailed at the time, and a flight plan was not filed. The flight originated from a private agricultural strip about 20 minutes prior to the accident.

Ground witnesses in the vicinity of the field reported that the airplane was making a level spray pass over a potato crop when the left wing separated. No unusual meteorological phenomena were observed in the area, and the airplane was not observed by any witness to have performed any unusual maneuvers prior to the failure.

Personnel information

Review of the Federal Aviation Administration (FAA) Airman Certification Records disclosed that the pilot held a commercial pilot certificate with airplane ratings for single engine land, multiengine land, and instruments, the most recent issuance of which was dated December 27, 1988. In addition, he held a flight instructor certificate dated July 12, 1997, with airplane ratings for single and multiengine. The most recent second-class medical certificate was issued without limitations on June 29, 1999.

The pilot's personal flight records were not located and no information was available on the pilot's experience in the Air Tractor series aircraft, his last biennial flight review, or his recent flight activity. On his June 29, 1999, application for an FAA medical certificate, the pilot reported a total flight time of 26,190 hours. He owned San Tan Dusters, and FAA records disclosed that the firm's operating certificate under 14 CFR Part 137 was issued in 1966, although no determination was made regarding the length of his association with the company.

According to the FAA Airman Records database, the pilot did not hold either an Airframe or Powerplant technician certificate.

Aircraft information

The Air Tractor 502A, serial number 502A-0181, was equipped with a Pratt & Whitney of Canada PT6A-45A engine, serial number PCE84076.

No maintenance records were located for either the airframe or engine. The pilot's family members stated that they searched his residence and vehicles and could not locate any records relating to maintenance on the aircraft. San Tan Dusters company personnel and the accountant retained by the company stated they were unsuccessful in locating the records in the company files. The company accountant provided the name of a mechanic who had been routinely paid during the prior year for maintenance work.

The identified mechanic was interviewed by telephone and subsequently in-person by FAA inspectors. In the telephone interview, the mechanic stated that he was "not the maintenance operation for San Tan Dusters" and had no knowledge of the maintenance records location. He further stated that he had no other records detailing maintenance performed on the accident aircraft. The mechanic said that the pilot performed the maintenance work on his own aircraft and would only call on him (the mechanic) to do the periodic maintenance that the pilot could not do.

During the in-person interview 2 days following the telephone interview, the mechanic reiterated that he had no knowledge of the records whereabouts, and reported that he last performed maintenance on the aircraft 2 days prior to the accident when he replaced the rudder cables. The pilot did not present the logbooks for the mechanic to enter the work at that time. The mechanic then acknowledged that in the past he had replaced skin panels, removed the hopper for an inspection of the structure, and had replaced the landing gear. He further acknowledged that he had also examined the right wing for oil canning of the skin and had found a crack in a skin panel that was within acceptable limits per the maintenance manual. During the removal of the hopper in May 1997, he had accomplished a check of the wing attach bolts required by an Air Tractor service bulletin (subsequently identified as Service Letter 118) every 2,000 hours. He then stated that he did the 100-hour inspections on the aircraft. He reported that the airframe total time was about 4,000 hours, and that the last annual inspection was performed by another mechanic/IA unknown to him "11-months before the accident." He again stated that he had no written records detailing any maintenance performed on the aircraft.

The mechanic provided a notarized written statement to the insurance company for San Tan Dusters 4 days after the interview with FAA inspectors. The statement noted that he "performed 100-hour inspections as requested" on the aircraft while the pilot's loader did the daily maintenance consisting of tire, brake, and oil checks. The statement reports the rudder cable replacement previously noted and states that the aircraft total time was "approximately 4,000 hours."

Subsequent inquiry with a turbine engine overhaul maintenance facility in Texas disclosed that they performed a hot section inspection on the engine in January 2000. At the time of that inspection, the aircraft had 3,902 hours total time. According to Air Tractor, Inc., manufacturing records, the engine was the original factory installation in the airframe.

Factory production records from Air Tractor, Inc., disclosed that the aircraft was delivered new to San Tan Dusters on October 5, 1992. Mathematical averaging computations disclose that the aircraft likely averaged about 500 hours per year in the 8 years it was owned by San Tan Dusters.

According to Air Tractor, Inc., engineering documents and maintenance manual instructions, the conventional all-metal wing is designed so that the main spar takes all of the bending loads. No span wise stringers are employed in the wing design for ease of field repair, and as a result, some skin panel canning and wrinkling will be apparent during heavy weight maneuvers. The main spar consists of an upper and lower steel cap with a metal web plate that maintains cap separation. Near the inboard end, five additional web plates are joined to the main span wise web plate. The main spar caps of each wing are attached to each other in the fuselage center splice connection section area by metal attach blocks, an upper and lower block for each cap. Five bolts secure each spar cap inboard end to it's respective side of the attach blocks. The spar webs of each wing are joined to each other by two steel plates, which are then joined to the fuselage structure.

The maintenance manual notes that to inspect the wing center splice connection properly for cracks, corrosion, or other problems, the hopper must be removed. The manual identifies two critical inspection areas, located at wing station (WS) 11.5 from the fuselage centerline. The first critical area for crack inspection of the lower spar cap is at the most inboard bolt hole attaching the cap to the attach block. The second identified critical area concerns a rivet pattern in the spar web at WS 25 to WS 30. The inspection interval called out in the manual for this inspection of the spar caps and webs for cracks is every 2,000 hours.

The inspection procedure specified in the maintenance manual for the spar cap inboard bolt hole location does not call for removal of the attach blocks to fully examine the lower spar cap area. According to the engineering drawings, any crack emanating from the bolt hole would not be visible until it extended beyond the attach block edge, a distance of 0.25 inches.

Air Tractor, Inc., issued service letter 118 on May 10, 1993. The service letter stated the importance of removing the hopper so that a ver