In the preliminary report of July 2021 the National Transportation Safety Board said that one of the wings of the air attack aircraft that crashed northeast of Wikieup, Arizona on July 10, 2021 was found 0.79 miles from the main wreckage.
The Beechcraft King Air C90 had been on station for 45 minutes orbiting the fire at about 2,500′ AGL. The last radar data point showed airspeed about 151 knots, altitude about 2,300′ AGL, and that the aircraft was descending, about 805 ft ESE of the accident site. The main wreckage, but not the separated wing, was consumed in a post-crash fire.
An eyewitness said she saw the aircraft coming down at a “steep angle” and then “slam into the ground.” An hour later she and her husband drove to an area near the crash. They said BLM employees told them they they had witnessed a wing fall off the plane in the air before it crashed.
At the time of the crash Pilot Matthew Miller, 48, and Air Tactical Group Supervisor (ATGS) Jeff Piechura, 62, were on board conducting visual reconnaissance and aviation command and control over the fire. Both of them were killed.
Bill Gabbert wrote back in July 2021 that the investigation of the crash would consider that the C90 was one of the models mentioned in an FAA Airworthiness Directive AD #85 FR 78699, which required inspection of certain Beechcraft King Air and Queen Air models due to a danger of wing separation.
The directive was issued after Textron Aviation, who then held the type certificates, received reports of fatigue cracks in the lower forward wing fitting on two airplanes. An investigation revealed that installing washer P/N 90-380058-1 on the wing bolt would cause a premature torque indication. The fix was to inspect the washer and replace it, if applicable, with a different washer.
The final report issued August 23, 2023 notes that the plane descended in a steep dive and impacted the side of a ridgeline in mountainous desert terrain. No distress call from the airplane was overheard on the radio. Another witness observed the outboard left wing falling to the ground after the aircraft had impacted terrain. The outboard left wing, which had separated outboard of the engine nacelle, was located about 0.79 miles northeast of the main wreckage and did not sustain thermal damage.
During a scheduled maintenance inspection several months before the accident, eddy current (EC) non-destructive testing (NDT) of a left wing’s lower forward spar cap detected a crack in a fastener hole. The hole was then oversized/reamed to a larger size, as per instructions, but the EC reinspection still produced a crack indication. The operator then submitted a structural damage report and service request detailing the crack indication to the aircraft manufacturer, who responded that the crack indication required replacement of “the center section forward spar cap, center section forward lower fittings, and both outboard main spar assemblies.”
However, the operator and their maintenance provider chose to repair the wing spar instead of replacing the spars as instructed by the manufacturer. The maintenance facility owner contacted an FAA-Designated Engineering Representative (DER) for the design of the repair, which was installed and signed off on several months before the accident. The DER later claimed no knowledge of the communication between the aircraft manufacturer and the maintenance provider about the crack indication, though the maintenance provider claimed otherwise.
After the repair, an eddy current inspection conducted by a commercial NDT inspector showed the wing spar repair to be successful and did not reveal a crack indication. Further, there were no other crack indications on the airplane.
What is non-destructive testing?
A postaccident examination of the spar fracture surface revealed that the left wing’s lower spar cap had fractured 8from a fatigue crack that began at the aft inboard fastener hole. The fatigue crack measured 2.484 inches in length and exhibited striations consistent with crack propagation. A study comparing the crack length, striations, flight hours, and number of cycles suggests the crack was probably large enough to have been seen visually at the last inspection; therefore, it is likely that the NDT inspector omitted the EC inspection of the fastener hole — or just missed the fatigue crack indication.
The fatigue separation of the lower spar cap was not in the same area where the repair had been accomplished. The repaired area was inboard of where the left wing separated. The DER-approved repaired area was not identified in the recovered wreckage and therefore could not be examined.
The NTSB determines the probable cause of this accident to be:
The failure and separation of the left wing’s outboard section due to a fatigue crack in the lower spar cap. Contributing to the accident was the operator’s decision to repair the wing spar instead of replacing it as recommended by the aircraft manufacturer. Also contributing to the accident was the failure of the Non-Destructive Testing inspector to detect the fatigue crack during inspection.