Accident Overview
A ground-based witness observed the airplane's wings rolling rapidly to nearly vertical banks before it entered a spin and struck the ground in a nose-low attitude. The impact occurred 0.6 miles from the runway threshold. Multiple pilots in the traffic pattern reported moderate to extreme turbulence. Surface winds were 14 knots gusting to 22 knots. The airplane was calculated to be 94 to 127 pounds over its maximum certificated gross weight of 1,400 pounds.
Examination Findings
During the wreckage examination, the left wing's rear spar attachment fitting was found severely corroded, with missing rivet heads, and had separated. Metallurgical analysis revealed that rivet heads had failed and popped off in flight, and a corrosion-related crack had compromised the structural integrity of the fitting assembly. The fitting could not sustain the imposed wing loading and failed in flight. The failed rivet heads and corrosion were located on the backside of the spar, an area not visible during inspections.
Regulatory History and Inspection
In 1995, the FAA issued Airworthiness Directive (AD) 94-18-04, mandating a one-time visual inspection of the wings' internal structure for corrosion. A logbook entry dated 1997 indicated compliance, and the wings were subsequently recovered with fabric. The pilot purchased the airplane in May 1998; the airplane was manufactured in 1948. A September 1998 annual inspection did not include a logbook entry indicating reexamination of the wing's internal structure for corrosion.
Corrosion Development
The corrosion developed due to moisture and the contact between dissimilar metals: a steel bracket riveted to an aluminum spar. The severity of the ongoing corrosion would not have been evident during an external visual inspection, despite the fact that a required access panel had not been installed per the AD. The AD was noted to inadequately address the aging aircraft issue by indicating compliance through a one-time visual inspection to detect non-viewable internal corrosion.