QUEEN CREEK, AZ, USA
N15466
Air Tractor 502A
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 very thorough inspection of various areas could be performed, including the main wing spar center splice connection. The letter reiterated the importance of the 2,000-hour wing spar center splice inspection for cracks and other defects in the spar. It also recommended an additional calendar time limit for the inspection of 4 years for new aircraft involved in row crop applications, and 3 years for new aircraft in rice crop applications. The service letter recommended that the inspection calendar time limit interval be shortened by 1 year for older aircraft. METEOROLOGICAL INFORMATION The closest official aviation weather observation station is the Williams Gateway International Airport, which is located 5 miles northwest of the accident site. At 1059, the station was reporting clear skies with surface winds from the west-southwest at 4 knots. The hourly observations before and after the accident were similarly reporting clear conditions with winds less than 6 knots. No unusual meteorological phenomena were reported by the station or observed by witnesses to the accident. WRECKAGE AND IMPACT The accident site is in an area of level agricultural fields near the town of Queen Creek. The first identifiable ground scar was found 0.75 miles east of Elsworth Road and 0.25 miles south Ocotillo Road in a cultivated field and consisted of an oval crater-like disturbance. The left wing was found 12 feet southwest of this ground scar with the longitudinal axis oriented toward the southwest and the main wreckage mass. A large crater-like ground disturbance was found 228 feet southwest of the left wing. The fuselage and right wing were found 72 feet beyond the second crater, with the fuselage lying on it's right side and oriented back toward the northeast. The engine and propeller were separated from the fuselage and located adjacent to the second crater. The right wing had separated from the fuselage at the center section attach point and was lying parallel to the fuselage. The left wing's lower spar cap was observed to have fractured at a point that transected the inboard attach block bolt hole. Beech marks were visually apparent emanating from a bolt hole in the middle of the spar cap cross-section outward. Some corrosion was also observed in the area of the fracture. TESTS AND RESEARCH The inboard left wing spar caps (containing the fractures) and attached web plates were sectioned from the rest of the wing and forwarded to the Safety Board's Materials Laboratory for metallurgical analysis. The wing center splice connection consisting of the remaining most inboard left wing spar cap sections, the attach blocks for both wings, and the web join plates, along with a portion of the right wing main spar corresponding to the sectioned left wing components, were also forwarded for examination. The complete laboratory report is appended to this report. Examination disclosed that the spar caps met the hardness, chemistry, and microstructure of the production drawing specified material. The left upper spar cap fracture exhibited features characteristic of overstress separation. The fracture surface for the lower left spar cap intersected the lower cap surface at an angle perpendicular to the cap centerline, and also intersected the center of the most inboard attach block bolt hole. The fracture face exhibited smooth curving boundaries and visually discernable crack arrest lines, all features consistent with fatigue. Using various metallurgical examination techniques, multiple origin sites were identified within the bolt hole bore, with corrosion pitting evident at the origin points. Detailed examination of the fracture face disclosed that the fatigue region forward of the bolt hole was completely through the lower surface and had partially propagated up the forward edge of the spar cap, for a total distance of 1.063 inches away from the hole. At the upper surface the forward fatigue region had extended 0.25 inches from the bolt hole. The aft fatigue region extended a distance of 0.656 inches from the bolt hole at the lower edge and 0.031 inches from the hole at the upper edge. Using a standard sampling technique (fully described in the appended Materials Laboratory report) the number of crack arrest striations in the fatigue region was determined for two conditions. The first condition counted about 50,900 (plus or minus 10,200) striations from an initial crack size at the bolt hole of 0.002 inches to the overload portion of the fracture. The second condition counted about 9,900 (plus or minus 2,000) striations from a crack size of 0.25 inches (the first point the crack would be visible from beneath the attach block). According to an Air Tractor engineering report, the average number of wing loading cycles per flight hour is 94. The metallurgist reported that the mean time to failure from an initial 0.002-inch crack was 541 (plus or minus 108) hours and 105 (plus or minus 21) hours from a 0.25-inch crack. The attach blocks were removed and the right wing lower spar cap examined. In the outboard bolt hole bore, multiple fatigue cracks were found emanating from corrosion pits within the bolt hole bore. The largest fatigue crack was measured at 0.004-inch. The average dimension of the corrosion pits was found to be 0.0037 inches in length by 0.0010 inches in depth. MEDICAL AND PATHOLOGICAL INFORMATION The Maricopa County Medical Examiner's office conducted an autopsy, with specimens retained for toxicological examination by the FAA Civil Aeromedical Institute in Oklahoma City, Oklahoma. The results of the toxicological tests were negative for ethanol, carbon monoxide, and cyanide. Positive results were obtained for the following substances: 1) 0.058 (ug/ml, ug/g) DIPHENHYDRAMINE detected in blood; 2) 14.991 (ug/ml, ug/g) ACETAMINOPHEN detected in urine; and 3) 56.969 (ug/ml, ug/g) SALICYLATE detected in urine. According to pharmacological literature, Diphenhydramine (commonly known by the trade name Benadryl) is an over-the-counter antihistamine with sedative effects, found in several sleep aids and many multi-symptom allergy medications. Acetaminophen (commonly known by the trade name Tylenol) is an over-the-counter painkiller/fever reducer, found in many multi-symptom cold relievers. Salicylate is the active metabolite of aspirin. ADDITIONAL INFORMATION With the exception of the components retained for metallurgical examination identified in the TESTS AND RESEARCH section of this narrative, the balance of the wreckage was released to the insurance company, representing the registered owner, on April 25, 2000, at the conclusion of the on-scene investigation. The components retained for metallurgical examination were returned to the representative of the registered owner at the conclusion of those examinations.
The fatigue failure and in-flight separation of the left wing due to the manufacturer's underestimation during the aircraft design process of the time interval from fatigue crack initiation to failure, and, the manufacturer's subsequent specification of an inadequate inspection interval.
Source: NTSB Aviation Accident Database
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