AUSTRALIAN Transport Safety Bureau investigators have confirmed a water bomber that crashed fighting the Wirritin fire in the Budawang Ranges west of Ulladulla last month broke up in mid-flight.
The ATSB is investigating the fatal accident involving a M18 Dromader aircraft at about 10am on October 24, in which 43-year-old pilot David Black from Trangie was killed.
For several days the accident site had been inaccessible due to rugged terrain, high winds, low cloud and nearby bushfires.
Following the efforts of the Rural Fire Service, the NSW National Parks and Wildlife Service, and the NSW Police, a team of five ATSB investigators, including a materials specialist, were able to examine the aircraft wreckage on site last Wednesday.
The examination found that the left wing had separated at the joint between the outer wing and the centre wing sections.
The ATSB team examined the joints closely to determine the failure mode that resulted in the in-flight separation of the wing.
Preliminary examination indicated that the left outer wing lower attachment lug had fractured through an area of pre-existing fatigue cracking in the lug lower ligament.
The fatigue cracking reduced the structural integrity of the fitting to the point where operational loads produced an overstress fracture of the remaining lug material.
M18 Dromader aircraft have a cantilever wing that is anchored at one end with no mid-span supports.
The examination was completed on Thursday with several components removed from the accident site for further examination at the ATSB’s Canberra facilities.
They included both sections of the separated lower main spar lug and the remainder of the lower main spar attachment point (left wing); the entire upper main spar attachment point (left wing); part of the rear spar attachment point (left wing) and the entire lower main spar attachment point (right wing).
Following M18 Dromader accidents in the United States involving in-flight wing separation in September 2000, the US Federal Aviation Administration issued Airworthiness Directive (AD) 2000‑18-12.
This AD required repetitive inspections of the centre wing to outer wing attach joints for “cracks in the lugs, corrosion in the main holes, and ovalisation of the main holes”.
On October 19 2000, the Australian Civil Aviation Safety Authority (CASA) issued the same basic inspection requirements every 500 hours in service or every 12 months, whichever came first.
The wings of Mr Black’s aircraft were last inspected on August 8, 2013 and up to October 17, the aircraft had accumulated about 110 hours in service since the last wing inspection.
A preliminary investigation report will be released to the public by November 24.
A funeral service for Mr Black has been held at Forbes in the Central West with hundreds of people attending to pay their last respects.
The Rural Fire Service has honoured Mr Black with a posthumous Commissioners Commendation For Service.