The Australian Transport Safety Bureau has made a formal recommendation to the manufacturer of the Cessna 210 aircraft to further address the risk of fatigue cracking within the aircraft’s carry-through structure.
The safety recommendation forms part of the ATSB’s final report from its investigation into the 26 May 2019 collision with terrain of a Cessna T210M which was conducting low-level geophysical survey work about 25 km north‑east of Mount Isa, Queensland.
The aircraft, with a pilot and observer on-board, was operating at a height of 193 ft above ground level and a speed of 147 kt when its right wing separated in flight, leading to a rapid loss of control. The aircraft collided with the ground, and both crewmembers were fatally injured.
Relevant components of the aircraft were subject to detailed examination at the ATSB’s technical facilities in Canberra where it was found that relatively minor corrosion near a highly-stressed location on the lower surface of the wing spar carry-through had progressed into the aluminium alloy structure, initiating a fatigue crack. The crack propagated to a critical size resulting in an overstress fracture of the remaining wing carry-through structure material and separation of the right wing.
This information was immediately communicated to the aircraft manufacturer, Textron Aviation (which now owns Cessna), and Australian and US aviation regulators, and resulted in a number of initial safety actions.
These included Textron issuing service bulletins to owners of relevant Cessna 210 and Cessna 177 aircraft on 24 June 2019 requiring a one-off inspection of the carry‑through structure and communication of inspection findings to the manufacturer, and a US Federal Aviation Administration (FAA) Airworthiness Directive issued on 21 February 2020 requiring the visual and eddy current inspections of the carry-through spar lower cap along with the application of a corrosion preventative coating of certain model Cessna 210 aircraft.
At the time of finalising the ATSB’s investigation report, the FAA and Textron Aviation had received 1,119 reports from Cessna 210 owners/operators who had undertaken the visual and eddy current inspections of the carry-through spar on their aircraft. Of these, 499 reported corrosion and 68 carry‑through structures were removed from service.
Textron has advised the ATSB that it would be undertaking a fatigue analysis of the C210 wing spar carry-through in its original configuration to determine whether a modified inspection program or life limit is necessary. Textron has also advised the ATSB that it is working on a certification program to install a new spar in the C210 with an updated configuration and material.
“The ATSB acknowledges the significant safety actions taken to date by the manufacturer and regulators as a result of this accident and the ATSB’s investigation, and notes that these measures have addressed the short-term risk of further similar failures,” said ATSB Chief Commissioner Angus Mitchell.
“Further, the ATSB welcomes Textron’s ongoing efforts to address the risk of cracking in wing spar carry-through structure of Cessna 210 aircraft used for low-level geophysical survey operations. However, the ATSB remains concerned by the indefinite nature of the manufacturer’s proposed analysis and certification program and recommends that further action be taken to address this safety issue.”
In 1992, Cessna had introduced a continued airworthiness program for the Cessna 210 which included repetitive eddy current inspection for cracking of the carry-through structure. This flight-hours based inspection was more stringent for aircraft being used for low-level surveys.
Eddy current inspection is a form of non-destructive testing which can detect flaws in the internal consistency of certain types of metals.
However, following an assessment of historical data in 2011, Cessna replaced this inspection with a visual corrosion inspection, on a three-yearly frequency for all operation types, irrespective of hours flown.
“Had the previous flight-hour based eddy current inspection schedule remained in place, it is almost certain that the fatigue crack within the wing spar carry-through would have been detected before this accident occurred,” Mr Mitchell said.
The accident aircraft had accumulated 6,241 flight hours in the six years leading up to the accident, and had been operated exclusively as a geophysical survey aircraft during that time. In total the aircraft had accumulated 12,175 flight hours.
As part of the investigation the ATSB, in cooperation with the operator, undertook data gathering using an instrumented Cessna 210N to determine in-flight loads associated with the geophysical survey flight profile. Data from 95 flights over a period of 10 weeks during autumn in 2020 was sampled and analysed.
“The ATSB’s analysis showed that cyclic loads induced by the low-level survey flight profile were significantly greater than those associated with the higher-level flight profile originally intended for the aircraft type, and this probably increased the risk of a fatigue-related structural failure,” Mr Mitchell said.
“Even when flying within operational limits, if an aircraft is operated in a flight profile for which it was not originally intended, its structure can fatigue more rapidly.
“The ATSB cautions all geophysical survey aircraft operators that the terrain following flight profile may significantly increase aircraft fatigue damage accumulation.”
The investigation also determined that the airframe and system modifications incorporated into the accident aircraft did not significantly increase the fatigue damage accumulated by the wing spar carry-through structure.Last update 23 November 2021