What happened
While the crew was accelerating during the climb-to-cruise segment, they felt a rumble within the aircraft. Following this vibration, it was discovered that the right wing inboard wing panel had been partially torn away. The captain decided to return to the airport, performing a landing with the inboard flaps retracted. Upon inspection, it was determined that trailing edge portions of the composite right inboard wing panel had separated, causing secondary damage to the fore and mid right inboard flaps.
The investigation
An examination of maintenance records showed that cracking had been detected in the forward outboard corner of the panel during the previous C check; this crack had been repaired, inspected, and repainted. Furthermore, the previous C check had identified disbonding and delamination within the right wing panel.
Data indicated that Boeing has received 245 operator reports regarding disbonding of this specific panel, with 95 instances involving in-flight separation. While a service bulletin exists directing operators to replace the coin tap sound inspection method with an ultrasound inspection to better detect subsurface disbonding, the operator had chosen to continue using the coin tap method prior to this event.