This preliminary report details factual information established in the investigation’s early evidence collection phase, and has been prepared to provide timely information to the industry and public. Preliminary reports contain no analysis or findings, which will be detailed in the investigation’s final report. The information contained in this preliminary report is released in accordance with section 25 of the Transport Safety Investigation Act 2003.
On 18 August 2021, at 0852 Eastern Standard Time,[1] an amateur built Stolp Acroduster II SA-750 registered VH-YEL, departed from Caboolture Airfield, Queensland, for a local aerobatic flight. The pilot was the sole occupant.
A short time later, the crew of a helicopter operating in the same local area, at a height of about 900 ft, witnessed red and white debris falling from the sky. After searching the immediate area, the helicopter crew identified the inverted main wreckage of VH-YEL.
The wreckage was in tidal wetlands, about 30 m from the mainland shoreline of Pumicestone Passage, adjacent to Bribie Island (Figure 1). The aircraft was destroyed and the pilot was fatally injured.
The ATSB’s preliminary assessment of the aircraft, in combination with the distribution of wreckage at the accident site, indicated that the aircraft sustained an in-flight break-up. The aircraft was recovered from the accident site and taken to a secure facility for a detailed examination.
Figure 1: Image of the aircraft departure point and accident location
Source: Google Earth, annotated by the ATSB
The pilot held a valid Private Pilot Licence (Aeroplane) with an aerobatic endorsement. The pilot had accumulated about 670 flight hours, including 108 flight hours on VH-YEL.
The Stolp Acroduster II SA-750 is a two-place homebuilt aerobatic bi-plane (Figure 2), rated to +/- 9 g.[2] The aircraft was first introduced in 1971 as a plans-built aircraft, available from a company in the United States. The aircraft fuselage is constructed of welded steel tube and the wings incorporate wooden structure with steel tube internal supports. The majority of the aircraft is covered in fabric. It is powered with a four-cylinder piston engine.
There are several other plans-built Starduster Corporation aircraft variants developed by the same designer that share similar design features, such as the Starduster and single seat Acroduster (SA-700). However, only the Acroduster series aircraft share the same upper-wing attachment points. There were about 130 Acroduster SA-700/750 aircraft that were completed. Some components for the aircraft were able to be purchased as kits or raw materials.
Figure 2: VH-YEL Stolp Acroduster II SA-750
The aircraft’s plans were supplied from the Stolp Starduster Corporation in 1976 and the aircraft was constructed in the United States with a serial number of T-02. The aircraft was first registered as N97177 and first flew in 1981. It was disassembled and shipped to Australia in 2003. At that time the aircraft had accumulated about 430 flight hours. The aircraft was re-registered in Australia in 2007 as VH-YEL and it was operated under the experimental category [3] (Figure 2). It was maintained in accordance with the Civil Aviation Safety Authority (CASA) Maintenance Schedule 5[4] and, at the time of the accident, had accumulated about 717 flight hours.
The lower wings of the Acroduster SA-700/750 were directly attached to the lower fuselage at two wing root attachment points on each side. The upper wings incorporated the left, right and centre sections. The centre section was attached to the fuselage by eye bolts that then connected to the cabane struts and roll braces. The wings were externally braced against each other using flying wires, landing wires, and interplane struts (Figure 3). In flight, the lower wing outboard sections were supported by the upper-wing outboard sections through the interplane struts and flying wires.
Figure 3: Wing bracing and attachment
Source: Starduster, annotated by the ATSB
Cabane struts and roll braces were provided to support and attach the centre-section of the upper wing to the fuselage. Eye bolts had been manufactured to thread into the upper portion of each cabane strut and completed the attachment between the upper-wing centre section and the cabane struts. The eye bolts were adjustable in length through their threaded section (Figure 4).
Source: Supplied, annotated by the ATSB
The wreckage of VH-YEL was distributed over a distance of about 2.4 km, oriented in a north-west to south-east direction. The main wreckage consisted of the fuselage, tail section, and outer-wing sections, attached to the fuselage by the flying wires. The majority of the scattered debris consisted of wooden sections of wing structures, fabric from the wing skins, panels and perspex from the windshield. The largest sections that had separated from the fuselage were the left lower wing outboard section and the upper-wing centre section (Figure 5).
Figure 5: Wreckage trail spread over mud flats and farmland
Source: Google Earth, annotated by the ATSB
The main wreckage was recovered by barge at the changing of the tide and was transported to a secure storage facility for examination. The aircraft parts were reconstructed for the examination (Figure 6). The examination revealed:
Figure 6: VH-YEL as recovered and partially reassembled
All of the cabane struts, roll braces and attaching hardware were retained for more detailed examination. Figure 7 shows the right-side cabane struts and roll brace, and the area notated as Detail A (also see Figure 8) shows the fractured upper-wing attachment point.
Figure 7: Right side cabane struts, roll brace and upper-wing attachment points
Technical examination of the cabane struts from the centre-wing structure of the aircraft confirmed that there was fatigue cracking on the fracture surfaces of the eye bolts that had been fitted in the upper-wing forward position on the left and right cabane struts. The fatigue cracking had initiated in the thread root of each eye bolt at its termination into the cabane strut.
The right eye bolt had sustained fatigue cracking through about 90 per cent of the cross-section, and the left eye bolt had sustained about 40 per cent fatigue cracking through its cross-section. Final fracture of the eye bolts had occurred instantaneously by overstress due to a reduction in overall tensile strength (Figure 8 and Figure 9).
Figure 8: Right-side cabane strut forward upper-wing attachment eye bolt showing evidence of significant fatigue cracking through the threaded portion (Detail A from Figure 7)
Figure 9: Left-side cabane strut forward upper-wing attachment eye bolt showing evidence of fatigue cracking through the threaded portion
From the preliminary examination findings, it is indicative that fatigue cracking and then fracture of the eye bolts has led to structural instability of the centre-wing section and a consequential in-flight break-up of the upper-wing structure.
Information provided by industry experts familiar with the aircraft type indicates that there has been a history of cabane strut eye bolt cracking in the same area of the threaded section as occurred with VH-YEL. The ATSB was provided with a picture of an Acroduster SA-750 that had a left forward upper-wing cabane strut eye bolt fractured completely through its threaded section. The left roll brace was the only remaining structure providing support for that section of the wing (Figure 10).
Figure 10: Previous example of another Acroduster SA-750 with a wing attachment eye bolt that had completely fractured through the threaded section
Source: Supplied, annotated by the ATSB
The ATSB was advised that some aircraft owners of Acroduster aircraft have made modifications to the upper-wing attachment points. The modification removed the eye bolts and replaced them with a welded structure, which attached directly to the upper-wing attachment point (Figure 11). The ATSB notes that the modification shown is not approved by the aircraft design owner and advice should be sort from a suitably qualified aeronautical engineer on any design changes undertaken.
Figure 11: Right-side upper-wing centre-section attachment point with a standard eye-bolt attachment on the left and a modified attachment on the right
Source: Supplied, annotated by the ATSB
The location of fatigue cracking in the forward upper-wing attachment eye bolts makes identifying fatigue crack during visual inspections difficult and in some cases impossible without removing the eye bolts from the cabane strut.
Apart from a general inspection of the wing structure, the aircraft type does not have a specific detailed inspection schedule for the eye bolts to ensure their ongoing airworthiness. In addition, the aircraft type does not have a time life replacement of the eye bolts at set flight hours and/or cycles.
On 20 August 2021, the ATSB notified the Civil Aviation Safety Authority and the US National Transportation Safety Board (NTSB) of the initial finding that fatigue cracking had been identified within the upper-wing centre-section attachment eye bolts. The ATSB requested the NTSB provide details of this accident to the US Federal Aviation Administration and the kit plane design/material provider for its information. The ATSB also contacted the owner of the only other Stolp Acroduster II SA-750 on the Australian civil aircraft register and informed them of the issue.
With the release of this preliminary report, the ATSB has issued a safety advisory notice SAN in an effort to inform aircraft type owners of the circumstances of the accident and the fatigue crack issue.
Action number: AO-2021-032-SAN-001
The Australian Transport Safety Bureau advises all owners, operators and maintainers of Stolp Acroduster SA‑700/750 aircraft to consider the safety implications of the initial findings of this investigation regarding the fatigue cracking on forward cabane strut upper-wing centre-section attachment eye bolts, and take action where considered appropriate to ensure that their aircraft remain airworthy.
To date, the ATSB has:
The investigation is continuing and will include:
Should any further critical safety issues be identified during the course of the investigation, the ATSB will immediately notify relevant parties so appropriate and timely safety action can be taken.
A final report will be released at the conclusion of the investigation.
The ATSB would like to acknowledge the assistance provided by the Queensland Police Service, McLarens Aviation, and Clayton’s Towing Service for their assistance in the prompt recovery of the aircraft prior to it being submerged by the incoming tide.
The ATSB is investigating an in-flight breakup involving an amateur-built Acroduster II SA750 aircraft, registered VH-YEL, which occurred about 8 NM north east of Caboolture, Queensland, on 18 August 2021.
Earlier that morning, the aircraft, with the pilot as the sole occupant, departed Caboolture Airfield, Queensland, for a local flight over the Bribie Island area. At about 0906, the crew of a helicopter operating in the area on a training flight observed what appeared to be aircraft parts falling from the sky. They then observed the fuselage of an aircraft located in the mudflats near Long Island. A dedicated rescue helicopter winched a rescue crew to the accident site, and it was confirmed that the pilot was deceased. The aircraft was destroyed.
ATSB investigators deployed to the accident site and, with the assistance of other parties, the wreckage was recovered and taken to a secured location for subsequent examination.
The Acroduster II SA750 is an amateur-built two seat biplane, designed for the purpose of conducting aerobatics. VH-YEL was registered in the experimental category.
The evidence collection phase of the investigation will include further detailed examination of the wreckage, a review of available recorded data, a review of aircraft and pilot records, and witness interviews.
A preliminary report is expected to be released in a number of weeks, while a final report will be published at the conclusion of the investigation. Should a critical safety issues be identified during the course of the investigation, the ATSB will immediately notify relevant parties so appropriate safety action can be taken.