Investigation update: 7 June 2019

The information contained in this investigation update is derived from the initial investigation of the occurrence. Readers are cautioned that there is the possibility that new evidence may become available that alters the circumstances as depicted in the report.

What happened

At 1407 Eastern Standard Time^[1] on 26 May 2019, a Cessna Aircraft Company 210M, registered VH-SUX and operated by Thomson Aviation, departed Mount Isa Airport for an aerial geological survey flight. There were two pilots on board, one operating the aircraft and the other observing the flight to familiarise himself with the survey area. The survey was to be conducted at a target height of about 200 ft above ground level along parallel east and west lines, spaced about 90 m apart.

The evidence indicated that about 1 hour and 40 minutes into the flight, as the aircraft tracked west along the sixth survey line, the right wing separated, resulting in a rapid loss of control and subsequent collision with terrain. The pilots were fatally injured and the aircraft was destroyed.

Wreckage and site information

The accident site was located about 25 km north-east of Mount Isa Airport in flat, arid scrub land (Figure 1). The wreckage trail was on an approximate east to west heading, in line with the expected survey flight path. The right wing was the first major component in the wreckage trail and was located about 130 m from the main wreckage. The aircraft impacted terrain about 90 m from the right wing, coming to rest a further 40 m away. All major aircraft components were accounted for at the site.

Figure 1: Accident site overview looking east

Source: ATSB

On-site examination of the wreckage indicated the right wing and part of the carry-through spar^[2] had separated from the aircraft (Figure 2). Preliminary examination of the spar identified that it had fractured through an area of pre-existing fatigue cracking. The fracture was located inboard of the wing attachment lugs.

The carry-through structure was removed for detailed examination.

Figure 2: Right wing viewed from inboard end showing section of carry-through structure

Source: ATSB

Initial findings

Technical examination of the carry-through structure was conducted at the ATSB’s Canberra facilities. This examination confirmed that the fatigue cracking reduced the structural integrity of the carry‑through to the point where operational loads produced an overstress fracture of the remaining material. The fracture location was approximately 290 mm inboard of the right wing fuselage attachment lugs and coincident with a change in section thickness of the lower flange (Figures 3, 4 and 5). Characteristic features identified across the fracture surfaces confirmed that the fatigue cracking had initiated from the underside surface, growing across the lower flange and penetrating vertically into the structure.

Figure 3: The approximate location of fracture

Source: Cessna, modified by the ATSB

Figure 4: Carry-through structure, as received at the ATSB’s facilities

Source: ATSB

Figure 5: Close view of the outboard portion of the fracture surface (fatigue cracking has initiated on the underside surface)

Source: ATSB

Aircraft details

The Cessna Aircraft Company (Cessna) 210M is a high cantilever wing, piston-engine aircraft with a three-blade variable-pitch propeller and retractable tricycle landing gear (Figure 6). The aircraft is normally fitted with six seats. The accident aircraft (serial number 21061042) was manufactured in the United States in 1976 where it operated until 2013. It was imported to Australia and registered as VH-SUX in June 2013. At that time, the aircraft was modified for geological survey work, which included the removal of the passenger seats and the installation of specialised equipment.

Figure 6: VH-SUX

Source: Operator

The aircraft also had Supplemental Type Certificates for the installation of integral wing tip fuel tanks and a non-standard engine and propeller installation.

VH-SUX accumulated about 6,000 flight hours in the 6 years it was on the Australian register. It was operated exclusively as a geological survey aircraft during that time. The aircraft had 12,174 flight hours total time in service at the time of the accident.

Operation

The flight was one of a number of flights undertaken for the purpose of a geological survey to the north and north-east of Mount Isa.

The survey was conducted in a grid pattern, with closely spaced east and west lines along with more widely spaced north and south lines flown for data verification purposes. The flight profile closely followed the topography of the survey area at a speed of about 140-150 kt with procedure turns flown at each end of a survey line.

Each flight typically lasted for about 5 hours with multiple flights required to complete each survey. Two flights were normally flown each day in accordance with allowable environmental and daylight conditions, each flight on a given day being operated by a different pilot.

The aircraft typically departed with full fuel, resulting in it operating at close to the maximum allowable take-off weight.

Safety action

On 31 May 2019, the ATSB notified the Civil Aviation Safety Authority, the US National Transportation Safety Board, the aircraft manufacturer and operator of the initial finding of fatigue cracking within the wing spar carry-through structure.

The ATSB is working closely with those organisations to ensure the continued safe operation of the aircraft type.

Further investigation

The investigation is continuing and will include:

• a metallurgical examination of the wing carry-through structure and associated parts
• further examination and assessment of the aircraft wreckage
• examination of the maintenance procedures and inspections associated with the wing spar carry-through structure
• factors that may have contributed to the development of fatigue cracking
• the operational history of the aircraft from 1976 until 2013.

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The information contained in this update is released in accordance with section 25 of the Transport Safety Investigation Act 2003 and is derived from the initial investigation of the occurrence. Readers are cautioned that new evidence will become available as the investigation progresses that will enhance the ATSB's understanding of the accident as outlined in this update. As such, no analysis or findings are included.

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1. Eastern Standard Time (EST): Universal Coordinated Time (UTC) + 10 hours.
2. A major span-wise structural member to which the wings are attached.

Initial summary: 25 May 2019

The ATSB is investigating the collision with terrain of a Cessna 210, registered VH-SUX, 25 km north-east of Mount Isa Airport, Queensland, on 26 May 2019.

The aircraft had been conducting aerial survey operations at the time of accident. The aircraft was destroyed and its two occupants were fatally injured as a result of the accident.

As part of the investigation, ATSB investigators will attend the accident site to examine the wreckage and collect necessary evidence. The ATSB will also interview relevant persons, and obtain available data, maintenance records and other reports, and review operational procedures.

A report will be released at the conclusion of the investigation. However, should a critical safety issue be identified during the course of the investigation, the ATSB will immediately notify relevant parties so appropriate action can be taken.