Accident Overview
An aerobatic airplane was destroyed when it impacted terrain after the in-flight separation of its left wing. The airplane was observed traveling southeast at the bottom of an established aerobatic box (1,500 feet msl) when the left wing separated.
Wing Spar Failure Analysis
Examination of the fracture surfaces indicated that the wing spar failed in upward bending. Flat compressive failure zones near the upper surfaces of the spar caps and fibrous tensile fractures near the bottom surfaces were consistent with this failure mode. Both spar caps exhibited the same type of bending failure, suggesting that the shear web interconnection between the spar caps failed before the final failure of the spar caps themselves.
The failure was almost certainly initiated by a progressive compressive failure zone at the top surface of the upper spar cap. This zone was visible as a band of lighter color on the fracture surface, extending approximately 12 mm to 15 mm from the top surface through the top layer and part of the second layer of spruce in the spar cap.
Discoloration and material loss at the upper forward corner of the fuselage piece of the spar were noted, but these were more likely a result of impact with the ground rather than contributing to the failure. The materials and adhesive bonds used in construction and previous repair were sound.
Maintenance and Inspection History
There is a long history of airworthiness directives (ADs) and service bulletins (SBs) requiring inspections for compressive crack-like features on the top surface of the spar. Representatives from the DGAC and BEA indicated that the first accident from this type of progressive compressive damage occurred in 1968, with approximately 10 similar failures total.
A significant portion of the upper spar cap of the accident aircraft was replaced in 1980 after an inspection detected such damage. Maintenance records indicated that the appropriate ADs and SBs were signed off as complied with since 1980, but the compressive damage leading to this accident was not detected.
The position of the inspection opening on the left wing was not strictly in keeping with the instructions of SB 16. Additionally, other problems with the structure and inspection procedures were identified.
Issues Identified
The highest compressive stresses would occur at the top of the spar, just at the outboard edge of the fuselage attachment reinforcement block, which coincided with the fracture location. This location also closely matched the edge of the wing-walk support, further increasing stress concentrations.
During preparation of the inspection openings, the upper spar caps were damaged, leaving small steps in the spar cap material at the edges of the wing-walk supports. These steps would cause additional stress concentrations, making progressive damage more likely, particularly as the step sat almost directly above the outboard edge of the fuselage attachment reinforcement block. The steps probably also obscured the appearance of crack-like features forming at their base.
The step on the right wing was much smaller than on the left wing, and the right wing had only a small area of progressive compressive damage compared to the left wing.
Based on the circumstances, several problems with the service bulletins intended to prevent such failures were identified. The position of the inspection opening is described in relation to rib 1, but this position is unreliable. The top surface of the spar above the edge of the fuselage attachment reinforcement block is of interest, but once the box beam is assembled, the block is no longer visible, and the position of rib 1 may not correlate with the block's edge, particularly after modifications or repairs involving rib 1. Additionally, the SBs do not address the location of the wing-walk support. In this case, the wing-walk support possibly limited the size of the inspection opening, created an additional stress concentration in precisely the wrong place, and obscured the progressive damage.