Preliminary report -
AO-2013-187
Excerpts from page 5-6 of that report:
Wing attach fitting maintenance requirements
In 2000, investigations by the US National Transportation Safety Board into a number of US M18 accidents in which the wings separated in-flight discovered severe corrosion and cracking in the wing lower attach fittings, which led to fatigue cracking and failure of the fitting.3
On 3 August 2000, the aircraft manufacturer issued service bulletin (SB) E/02.170/2000, which provided procedures for dealing with corrosion of the centre wing-to-outboard wing attach fittings. The SB included a procedure for inspection of fittings found to be affected by corrosion and stated that:
• ‘the only acceptable inspection method is magnetic cracks detection’4 (original emphasis)
• ‘the critical area of the joints include the lower surfaces of the [main holes] in the wing lower attach joints’
• the inspections did not require outboard wing removal
• any cracked fittings and worn-out expansion mandrels were subject to mandatory replacement.
On 11 September 2000, CASA approved a procedure for inspection of the fittings using eddy-current testing as an alternative to the magnetic particle inspection required by the SB. This procedure provided information on preparing the area for inspection, instrument calibration, and other matters specific to the eddy-current testing method. It did not require wing removal and did not specify any particular critical area for inspection. Being a replacement for the inspection part of the SB, it did not provide repair instructions.
On 19 October 2000, the Civil Aviation Safety Authority (CASA) issued airworthiness directive (AD) AD/PZL/5. It specified that the centre wing-to-outboard wing attach joints were to be inspected, using magnetic particle methods, for cracks in the lugs, corrosion in the main holes, and ovalisation of the main holes. The inspection was to be carried out in accordance with the manufacturer’s SB and was mandatory for all M18s5 after the fittings accumulated 2,500 hours service. Further inspections were required every 500 airframe hours or every 12 months, whichever came first.6
Aircraft maintenance information
Maintenance records indicated that the main spar attach point fittings on TZJ were installed new in 2004. The aircraft records indicated that since then it had accrued 3,980 flight hours, and 5,784 adjusted hours. A valid maintenance release was found in the wreckage.
The main spar attach fittings were last inspected on 8 August 2013, using the CASA-approved eddy-current procedure. Records indicated that since then, the aircraft had accumulated 120.1 flight hours and 154.7 adjusted hours, not including the accident flight.
Wreckage and site information
The on-site examination found that the left wing had separated at the attachment joint between the outboard wing and centre wing sections, about 6 m from the wingtip (Figure 6). Preliminary examination of the attach fittings indicated that the left outboard wing lower attachment lug had fractured through an area of pre-existing fatigue cracking in the lug lower ligament (Figure 7 and Figure 8). 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. The detached section of lug was retained by the centre wing lugs and showed a matching fracture surface (Figure 8).
A number of aircraft components were removed from the accident site for further examination at the ATSB’s Canberra facilities, including:
• both sections of the separated lower main spar lug and the remainder of the lower main spar attach fitting (left wing)
• the entire upper main spar attach fitting (left wing)
• part of the rear spar attach fitting (left wing)
• the entire lower main spar attach fitting (right wing).