CEN15LA399

The in-flight failure of the left wing lower forward attach point under normal loading conditions due to corrosion and fatigue cracking in the center section hinge brackets. Contributing to the accident was the operator's inadequate maintenance and overweight operation of the airplane, and the manufacturer's inadequate guidance to detect and prevent corrosion and fatigue cracking.

The commercial pilot was performing an agricultural application flight. As the airplane neared trees at the edge of the field, the pilot performed a pull-up maneuver to initiate a turn and to align for the next spray pass; during this maneuver, portions of the left wing separated from the airplane. The airplane then continued until it impacted trees and the ground beyond the edge of the field.   Postaccident examination revealed that the two left center section forward spar lower hinge brackets were fractured vertically through the pin holes. Examination of the failed brackets revealed the presence of fatigue cracking on all four of the fracture faces, which initiated at corrosion pits along the pin hole bores. An engineering study showed that the stress intensity caused by the fatigue cracks on the accident airplane would allow for failure of the center section hinge bracket at a load factor of about 2.7 g, well below the design limit load factor of 3.8 g. The operator reported that, during application flights, they typically experience load factors about 2 g during normal maneuvers, and up to 3 g for more severe maneuvers. The left wing failure initiated with the failure of the left lower forward center wing hinge bracket. After the failure of the forward bracket, the geometry of the bracket assembly allowed for eccentric loading of the remaining (aft) bracket. The bending stress and eccentric loading induced in the aft bracket, when added to the normal tension stress, quickly exceeded the capability of the bracket and resulted in its failure. The catastrophic failure of both left center section lower hinge brackets allowed the left wing to rotate upward under normal aerodynamic loads, rendering the airplane uncontrollable. Information provided by the operator indicated that the airplane departed on the accident flight at a weight of about 5,084 lbs; its weight at the time of the accident could not be determined and would have varied significantly depending on how much of the field had been sprayed before the accident occurred. The airplane's certified type design maximum gross weight was 4,000 lbs, which allowed for a useful load of about 950 lbs, to include the pilot, usable fuel, hopper load, and spray system; this load is impractical for agricultural operations. While certification in the restricted category allows for operation above the airplane's maximum gross weight, it requires that the operator verify the airplane's handling characteristics at that weight and document the increase in the aircraft logbooks. Such an entry was not found in the maintenance logs for the accident airplane. Additionally, operating an airplane at higher weights will impose higher loads on the aircraft structure and reduce the structure's static strength capabilities and the limit load factor. Further, at higher weights, the aircraft structure will accumulate fatigue damage at a faster rate. There was no evidence that the aircraft manufacturer had performed any analysis to substantiate operation at higher gross weights for the accident airplane make and model. Moderate to severe corrosion was noted on all of the forward spar hinge brackets and pins. This corrosion was present during the most recent inspection and should have resulted in more appropriate maintenance actions. The published service information stated that the wing hinge brackets should be inspected; however, instructions did not specify the means of the inspection. It is very likely that the largest fatigue crack in the left center section lower forward hinge bracket was present during the most recent inspection and would have been detectable using non-destructive inspection methods had the procedures for such an inspection been provided in published service information. Although review of maintenance records revealed that the airplane met regulatory maintenance and inspection requirements; the operator's maintenance was inadequate to detect the pending failure, and the aircraft manufacturer's published service information did not provide adequate instructions for inspection and maintenance of the wing hinge brackets.

The Fond du Lac County Medical Examiner, Fond du Lac, Wisconsin conducted an autopsy of the pilot. The autopsy stated that the cause of death was multiple blunt force injuries. The FAA Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma, performed toxicological testing on specimens from the pilot. Testing was negative for ethanol, carbon monoxide, and all tested-for drugs. The accident pilot's logbook was examined and, as of January 2, 2015, the logbook indicated 584.1 hours of total flight time in single-engine airplanes. Most of the flights annotated in the logbook were conducted in light general aviation airplanes. In 2011, the pilot only recorded 2 flights for a total of 4.9 hours, in 2013 the pilot only recorded 3 flights for a total of 2.9 hours, and in 2014 the pilot only recorded 6 flights for a total of 6.1 hours. The pilot began agricultural flight training in April 2015 at Battlefords Airspray, North Battleford, Saskatchewan, Canada. Much of the training was performed in a Gippsland GA200 two-seat agricultural training airplane. The pilot's logbook entries for the GA200 airplane did not indicate dual received flight time for these flights. Seven of the training flights were solo flights in a Piper PA-25-180 or -235 airplanes. On April 28, 2015, an entry in the logbook indicated the pilot had completed agricultural pilot training after completing 26 flights for 41.2 hours. No other flight times were contained in the pilot's logbook. The pilot began working for Agricair Flying Service in May 2015. The operator provided a desk calendar for May-August 2015 where the pilot annotated his flight time, the number of loads, and the number of acres. A Letter of Competency from the company was contained in the pilot records indicating the accident pilot had satisfactorily completed the knowledge and skills tests for an agricultural pilot under 14 CFR Part 137.19 on May 24, 2015 and was qualified to serve as pilot-in-command under the operator's certificate. The last entry on the calendar was on August 23, 2015. As of that date, the pilot had accrued 461.1 hours of agricultural operations, including his agricultural flight training. A majority of this time, 419.9 hours, was accrued in the accident airplane while working for the operator. His minimum recorded flight time on a single day was 0.4 hours and his maximum flight time was 13.6 hours with an average flight time of 5.9 hours. Fourteen days had recorded flight times that exceeded 10 hours. No information was available on the pilot's flight time on the day of the accident or the two preceding days. National Transportation Safety Board (NTSB) Special Investigation Report on the Safety of Agricultural Aircraft Operations (NTSB/SIR-14/01 PB2014-105983) stated in part, "Compared to the pilots in other [general aviation] sectors, ag operations pilots who were involved in accidents tended to be highly experienced. For 2010, the average total flight time for an ag pilot involved in an accident was about 10,400 hours with about 2,900 hours in aircraft type (NTSB 2012, 54)." The main wreckage was located in a treed area beyond the southern edge of a potato field; portions of the wreckage were found in the southwest corner of the field. The airplane impacted the tops of several large trees before impacting the ground. All the components of the left wing leading edge assembly were located in the field north of the main wreckage site. The left outboard wing fixed trailing edge section and left wingtip assembly were located in the grass between the planted area of the field and the treed area. Examination revealed that the left center section forward spar lower hinge brackets were fractured vertically through the pin holes (Figure 1). The pin remained installed through the left wing forward spar lower hinge brackets with the fractured ends of the center section brackets captured by the pin. All the remaining spar attach points on the left and right wings were intact. The left wing spars, interspar structure, and skins were recovered at the main wreckage site with varying amounts of damage. The left wing leading edge, left wing tip and left fixed trailing edge section separated during the accident sequence and were recovered in the field. There was mechanical damage and deformation noted on the left wing and center section upper spar caps adjacent to the forward spar attach point. The damage was consistent with the outboard wing having rotated up more than 120º with respect to the center wing. The left wing rear spar was fractured through the outboard wing hinge bracket holes even though the brackets remained intact. The left wing structure was crushed downward between the spars and curled upward with yellow paint transfer noted on the internal wing ribs. The left aileron control tubes and left wing spray bar displayed significant upward deformation. Figure: The sections of the left and right forward wing spar assembly sections, viewed aft looking forward angled from below, as received. The figure shows the separated pieces of the left center section forward spar lower hinge brackets, which are placed at the bottom of the left wing spar and left center section spar for the photo. The accident airplane, serial number (S/N) 1557, was manufactured in 1993 and registered to the operator in June 2014. It was a single-seat, single-engine, low-wing, all-metal airplane with conventional landing gear and was designed for agricultural spraying operations. The airplane was powered by a Pratt & Whitney R985 radial engine which drove a 3-blade tractor propeller. The Weatherly 620B was certified under FAR 21.25(a) effective February 1, 1965, with policies contained in Civil Aeronautics Manual 8 (CAM 8) Appendix B. The airplane type certificate (TC) ownership was transferred to Weatherly Aircraft Company, Chicago, Illinois, on November 6, 2000. The FAA responsible office for the TC was the Los Angeles Aircraft Certification Office, ANM-100L. Aircraft Maintenance Information Maintenance records indicated that the accident airplane was not flown between May 1, 2000, and March 21, 2003. Annual inspections were performed on the airplane each year since manufacture except in 2000, 2001, and 2002. The entries for the annual inspections performed in 2004-2008 and 2010-2012 specifically noted that the wing attach bolts were torqued. The wing hinge pins were inspected in accordance with Weatherly Service Note No. 15 on July 23, 1996, at a total time of 1,179 hours. A logbook entry on June 19, 2001 indicated that the wing leading edges were removed, stripped, primed, painted, and reinstalled. Two wingtip skins and two wingtip ribs were also replaced at this time. All applicable airworthiness directives were tracked in the maintenance records and had been complied with. The manufacturer's maintenance manual contained limited inspection information in Section XV, Periodic Aircraft Inspections. Step (n) in this section stated, "Remove wing bands and inspect wing hinge fittings." No other information in the manual addressed inspection of the wing hinge brackets. The most recent inspection was an annual inspection completed on April 15, 2015, at a total airplane and tachometer time of 4,468.9 hours. The logbook entry indicated that the inspection was completed in accordance with 14 CFR Part 43, Appendix D, and stated, "Removed all inspection plates & checked structure." The tachometer time at the time of the accident was unknown due to impact damage. The last recorded maintenance logbook entry before the accident was for an oil change on August 22, 2015, at a tachometer time of 4,811.83 hours. On August 26, 2015, about 1130 central daylight time, a Weatherly Aircraft Company 620B, N2005C, impacted terrain after the left wing partially separated during an aerial application flight near Hancock, Wisconsin. The pilot was fatally injured, and the airplane was destroyed. The airplane