N9310RWsk Pzl Mielec M-18A · 1988

On July 12, 2021, at 0805 central daylight time, a PZL Mielec M-18A “Dromader” airplane, N9310R, was destroyed when it was involved in an accident near Seiling, Oklahoma. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

Airplane position data revealed that the pilot departed Fairview Municipal Airport (6K4), Fairview, Oklahoma, at 0744. He proceeded to an agricultural field about 19 miles southwest of the departure airport. The pilot then appeared to complete 11 north-south oriented application passes over the field. The final data point was recorded at 0805:42 with a corresponding approximate altitude of 1,787 ft mean sea level; about 35 ft above ground level. The final data point was at the north end of the field and at the end of an application pass. This was aligned with the point where, after the previous passes, the pilot had initiated a turn to reverse course for another pass.

The airplane impacted a pasture about 0.34 mile northwest of the final data point.

The pilot held a Federal Aviation Administration (FAA) mechanic certificate with airframe and powerplant ratings. He did not hold an inspection authorization associated with that certificate. An inspection authorization was required to certify an annual inspection on the accident airplane.

The airframe incorporated a fuselage-mounted center wing section. The left and right outboard wing sections were each mounted to the center wing section. The center-to-outboard wing attachment consisted of upper and lower fittings on the main wing spar and one fitting on the rear spar. The mating main spar fittings were secured by an expansion mandrel assembly comprising a split mandrel, tapered plug, retention bolt, washer, and castellated nut with cotter pin.

In August 2000, the airframe manufacturer issued service bulletin (SB) E/02.170/2000 requiring recurring inspections of the wing attachment fittings due to two accidents involving in-flight wing separations. In September 2000, the FAA issued airworthiness directive (AD) 2000-18-12 requiring compliance with the manufacturer’s SB for airplanes registered in the United States. For airplanes that had accumulated 3,000 hours time-in-service (TIS), the initial inspection was required to be completed within 30 days of the effective date of the AD; otherwise, the inspection was required upon reaching 3,000 hours TIS. Recurring inspections were required every 500 hours TIS or 12 calendar months.

The AD/SB required inspection of the center wing to outboard wing attachment joints for corrosion, cracks, and ovalization of the through-holes in the fitting lugs. The presence of any corrosion required a subsequent magnetic particle inspection to identify any cracks. Any corrosion was required to be removed before the airplane was returned to service, and any cracks required replacement of the fitting. Ovalization, within specific limits, could be repaired by reaming the fitting through-holes and installing a repair mandrel. Otherwise, fitting replacement was required. The SB specifically noted that there, “was no need for outboard wing removal upon expansion mandrels removal.” In accordance with the SB, the wing fittings and associated split mandrel hardware were to be coated as specified for corrosion protection after inspection and other maintenance.

Maintenance records revealed that the pilot performed the most recent annual inspection which included compliance with the previously noted AD. An FAA-certificated mechanic with airframe and powerplant ratings and an inspection authorization (A&P/IA) subsequently certified compliance with the annual inspection. The records noted compliance with the AD corresponding to each annual inspection from 2003 until the most recent annual inspection in 2021. With exception of the inspections in 2003, 2006, and 2016, each logbook endorsement related to AD/SB compliance was completed by the pilot/mechanic or the A&P/IA. Only the inspection in 2006 specifically noted the use of magnetic particle inspection. There was no indication in the maintenance records that the failed fitting had been replaced or the lug bore diameter had been machined at any time during the airplane’s service life.

The A&P/IA stated that the pilot/mechanic completed the required inspections and noted compliance with the AD. Then he, as an A&P/IA, verified that all inspections were completed and entered the annual inspection into the maintenance records under his inspection authorization. The A&P/IA confirmed that the pilot/mechanic removed the wing attachment bolts and mandrels to check for corrosion and dimensionally inspect the lugs; however, he was not certain if any non-destructive testing such as magnetic particle inspection was completed. To his knowledge, the AD was complied with by inspection; the wing lug fittings were not replaced at the last annual. He added that the pilot normally flew at reduced weights to minimize stress on the airframe.

Installation of a Honeywell TPE331 turboprop engine was completed in 2016.

The airframe incorporated a fuselage-mounted center wing section. The left and right outboard wing sections were each mounted to the center wing section. The center-to-outboard wing attachment consisted of upper and lower fittings on the main wing spar and one fitting on the rear spar. The mating main spar fittings were secured by an expansion mandrel assembly comprising a split mandrel, tapered plug, retention bolt, washer, and castellated nut with cotter pin.

In August 2000, the airframe manufacturer issued service bulletin (SB) E/02.170/2000 requiring recurring inspections of the wing attachment fittings due to two accidents involving in-flight wing separations. In September 2000, the FAA issued airworthiness directive (AD) 2000-18-12 requiring compliance with the manufacturer’s SB for airplanes registered in the United States. For airplanes that had accumulated 3,000 hours time-in-service (TIS), the initial inspection was required to be completed within 30 days of the effective date of the AD; otherwise, the inspection was required upon reaching 3,000 hours TIS. Recurring inspections were required every 500 hours TIS or 12 calendar months.

The AD/SB required inspection of the center wing to outboard wing attachment joints for corrosion, cracks, and ovalization of the through-holes in the fitting lugs. The presence of any corrosion required a subsequent magnetic particle inspection to identify any cracks. Any corrosion was required to be removed before the airplane was returned to service, and any cracks required replacement of the fitting. Ovalization, within specific limits, could be repaired by reaming the fitting through-holes and installing a repair mandrel. Otherwise, fitting replacement was required. The SB specifically noted that there, “was no need for outboard wing removal upon expansion mandrels removal.” In accordance with the SB, the wing fittings and associated split mandrel hardware were to be coated as specified for corrosion protection after inspection and other maintenance.

Maintenance records revealed that the pilot performed the most recent annual inspection which included compliance with the previously noted AD. An FAA-certificated mechanic with airframe and powerplant ratings and an inspection authorization (A&P/IA) subsequently certified compliance with the annual inspection. The records noted compliance with the AD corresponding to each annual inspection from 2003 until the most recent annual inspection in 2021. With exception of the inspections in 2003, 2006, and 2016, each logbook endorsement related to AD/SB compliance was completed by the pilot/mechanic or the A&P/IA. Only the inspection in 2006 specifically noted the use of magnetic particle inspection. There was no indication in the maintenance records that the failed fitting had been replaced or the lug bore diameter had been machined at any time during the airplane’s service life.

The A&P/IA stated that the pilot/mechanic completed the required inspections and noted compliance with the AD. Then he, as an A&P/IA, verified that all inspections were completed and entered the annual inspection into the maintenance records under his inspection authorization. The A&P/IA confirmed that the pilot/mechanic removed the wing attachment bolts and mandrels to check for corrosion and dimensionally inspect the lugs; however, he was not certain if any non-destructive testing such as magnetic particle inspection was completed. To his knowledge, the AD was complied with by inspection; the wing lug fittings were not replaced at the last annual. He added that the pilot normally flew at reduced weights to minimize stress on the airframe.

Installation of a Honeywell TPE331 turboprop engine was completed in 2016.

The main wreckage consisted of the fuselage, engine, right wing, and empennage. A ground impact depression, oriented toward the northwest, was located immediately adjacent to the main wreckage. The left wing had separated and was located about 120 ft northeast of the main wreckage. The flight controls remained attached to the airframe with the exception of the right aileron and the outboard portion of the left elevator; both were located at the accident site. Minor debris associated with the left wing was located about 190 yards southeast of the main wreckage.

An on-scene examination indicated that the left outboard wing main and rear spars had separated from the center wing structure. Further examination revealed that the lower lug common to the left/outboard wing main spar lower lug had features consistent with fatigue fracture through approximately 90% of the lower lug cross section with the remaining lug cross section fracturing in overstress. The separated upper lug fracture surface exhibited features consistent with overstress fracture.

In accordance with 14 CFR 65.81 and 14 CFR 65.85, a certificated aircraft mechanic may perform a 100-hour inspection, as well as maintenance, preventative maintenance, and alterations of aircraft. Under 14 CFR 65.95, a certificated mechanic with an inspection authorization may perform an annual inspection.

Metallurgical examination determined that the fatigue crack originated on the lug bore surface about 0.12 inch from the forward face of the lower lug. The fatigue crack origin was associated with a corrosion pit on the lug bore surface. At its longest length, the fatigue crack propagated about 0.80 inch before the remaining ligament fractured in overstress. In addition, the fatigue crack surface deposits were consistent with corrosion products. Two other corrosion pits were present near the crack origin region. The bore surface exhibited significant corrosion pitting. Furthermore, corrosion deposits were prevalent on the portion of the expansion mandrel assembly in contact with the outboard lower main attachment fitting bore. Examination of the opposing upper lug portion of the lower fitting revealed fracture features consistent with upward bending of the inboard portion of the lug.

Surface deposit samples taken from the retention bolt, tapered plug, and split mandrel revealed spectra consistent with degraded or oxidized corrosion inhibiting compound. The samples from the lugs and bore of the main attachment fitting piece revealed spectra that was not a comparative match to the corrosion inhibiting compound spectrum.

The right outboard wing to center wing section main spar lower attachment fitting was also examined. The fitting was intact and did not exhibit any crack indications; however, it exhibited mechanical pits and, similarly to the failed left fitting, no positive indications of fluorescent deposits consistent with previous magnetic particle inspection. In addition, the expansion mandrel components had a brittle and crumbling brown coating, consistent with an aged or degraded corrosion inhibitor coating.