The July 10, 2021, fatal crash of a Beechcraft King Air C90 was caused by “separation of the left wing’s outboard section due to a fatigue crack in the lower spar cap,” according to the NTSB final report published late last week. “Contributing to the accident was the failure of the non-destructive-testing (NDT) inspector to detect the crack” and the “operator’s decision to repair the wing spar instead of replacing it as recommended by the aircraft manufacturer,” the safety agency said.
The turboprop twin was being operated as a public-use firefighting aircraft conducting reconnaissance and supervision. It was about 2,300 feet agl over an area of fire near Wikieup, Arizona, when the left wing separated. Killed were the pilot and an aerial tactical supervisor. Missions are typically flown at speeds from 120 to 150 knots and require steep bank angles.
During an inspection two months before the accident, eddy-current NDT of the left wing’s lower forward spar cap detected a crack in a fastener hole. Though Textron Aviation advised the operator to replace the spars, a repair was made instead.
The repair, which involved enlarging the hole and installing a doubler around it, was approved by an FAA-designated engineering representative (DER). The DER claimed no knowledge of the communication between Textron Aviation and the maintenance provider about the crack, though the maintenance provider alleged otherwise. A follow-up inspection by a third-party NDT inspector showed the wing spar repair to be successful and “there were no other cracks observed.”
However, post-accident examination revealed that the left wing’s lower spar cap fractured from a 2.5-inch-long fatigue crack that initiated at the aft inboard fastener hole, “large enough to have been seen visually at the last inspection,” said the NTSB. “Therefore, it is likely that the NDT inspector omitted the [eddy-current] inspection or missed the fatigue crack indication.”
Although the repaired section was inboard of where the wing separated, the report said the repaired crack and the undetected crack that caused the wing failure would have been removed if the operator had followed Textron’s recommended procedure of replacing the spars.