History of Flight
On June 10, 2016, about 0900 eastern daylight time, an experimental WSK PZL MIELEC M-18A, registration N2283M, was destroyed following an inflight separation of the right wing while maneuvering near Brunswick, North Carolina. The commercial pilot was fatally injured. Visual meteorological conditions prevailed, and no flight plan was filed for the aerial application flight, which was operated by Canam Aviators Inc. under 14 CFR Part 137. The flight originated from Bear Pen Airstrip (NC43), Supply, North Carolina, around 0845. According to ground personnel, the airplane departed with a full load of fertilizer and 45 minutes of fuel with 30 minutes of reserve fuel. After 1 hour passed and the airplane did not return, ground personnel called 911. The airplane was subsequently located in a wooded area.
Aircraft and Pilot Information
The pilot held a commercial pilot certificate with multiple ratings and a second-class medical certificate issued May 13, 2016, reporting 30,576 total flight hours. The airplane was manufactured in 1994 and imported to the US, issued a special airworthiness certificate in the restricted category. The accident operator purchased it in 2003. The most recent annual inspection was January 15, 2016, at 8,567.9 hours total time. All inspections from October 2012 onward were performed by the same company and mechanic, with entries stating compliance with the M-18 Service Manual. No specific information indicated which published inspections were performed.
The takeoff weight was estimated by an FAA inspector as about 11,787 lbs, using an empty weight of 5,975 lbs, chemical weight of 4,940 lbs (800 gallons), fuel weight of 692 lbs (1.5 hours), and pilot weight of 180 lbs.
Wing Structure and Modifications
The wing consists of three sections: center, right outboard, and left outboard. Outboard sections connect to the center wing at three points: two on the forward spar and one on the aft spar. The forward spar connections use expansion mandrels; the aft spar connection uses a bolt.
The original engine was replaced in April 1996 with a Honeywell TPE 331 turbine engine under STC SA09039SC, and geared servo tabs were installed on control surfaces under STC SA09063SC. In January 2005, a different Honeywell TPE331 engine was installed under the same STC, along with a 5-blade propeller and a modified hopper increasing capacity to 800 gallons. The maximum weight approved under the STC was 9,260 lbs with spray bar, 8,800 lbs with spreader.
Logbook entries documented service life increases and corrections. On February 20, 2014, both left lower wing attach fittings and expanding mandrels were replaced per SB E/02.170/2000. On July 2, 2015, the wing center section was replaced with a used unit.
Wreckage and Examination
The debris path was oriented south to north. The right wing and one propeller blade were located about 125 ft east of the main wreckage. The airplane impacted nose-down at about 60°. The left wing was atop the engine. The fuselage, cockpit, and hopper were 10 ft north of the initial impact point; the empennage 20 ft north. Small debris was within a 50-ft radius.
Examination revealed the right outboard wing forward spar lower fitting was fractured. The right outboard aileron attach bracket was fractured, and visible cracks were present on the right center and inboard aileron attach brackets. These parts were sent to the NTSB Materials Laboratory.
Metallurgical Findings
The right outboard wing forward spar lower fitting fractured through both arms. Each arm had six vertical bolt holes. Fractures in both arms contained fatigue cracks emanating from the bore of the holes on diametrically opposite sides. Most fatigue crack origins were associated with corrosion pitting in the bore. Fatigue propagation extended through about 90% of thickness in the lower arm and similar in the upper arm.
The fractured bolt (from the third vertical hole) contained fatigue cracks emanating from multiple origins all around the root thread, propagating through about 95% of the cross section.
The right outboard aileron attach bracket had a circumferential fracture in the main tube near welds, with multiple fatigue cracks from the outer surface at weld aft edges, propagating through about 50% thickness and around 30% circumference. Corrosion was noted.
The center and inboard aileron attach brackets were intact but had visible cracks in paint; after paint removal, corrosion and intermittent cracks were observed in welds.
Additional Information
The manufacturer's manuals emphasize cleaning and corrosion control, with particular attention to wing fittings. Non-destructive inspection procedures are detailed. The service life is 10,000 flight hours; a 1.35 factor applies for operations above 10,340 lbs.
Service Bulletin E/02.159/97 addressed corrosion protection of outboard wing forward spar fittings; the accident airplane lacked the specified inspection panel. SB E/02.170/2000 and Airworthiness Directive 2000-18-12 required inspections of wing attach joints for corrosion and cracking, but did not specifically address the vertical bolt holes where fittings attach to the spar. The AD was complied with on the accident airplane in January 2001 (with replacement of fittings) and again in January 2016 (no discrepancies).
The FAA approved Airplane Flight Manual for the accident airplane specifies a maximum takeoff and landing weight of 9,260 lbs, maximum hopper load 3,300 lbs, and maneuvering load factors of +3.4g and -1.4g. The FAA AFM does not specify a service life factor.
The manufacturer was not aware of the turbine engine modification details and was not consulted by the STC holders. The Australian Transport Safety Bureau investigated a similar 2013 accident involving a PZL M-18A with a TPE331 engine, finding fatigue cracking due to corrosion pitting in the left outboard wing forward spar lower fitting attach lug. The ATSB analysis indicated that the airplane had been operated at higher speeds and weights than certificated, leading to more severe flight loads and inadequate service life limits.