Incident Overview
The crew of a commercial aircraft heard a rumbling noise while accelerating in the climb-to-cruise segment. Subsequently, they discovered that the right wing inboard wing panel had been partially torn away. The captain decided to return to the departure airport, executing a landing with the inboard flaps retracted, as they had sustained secondary damage.
Inspection Findings
Post-flight inspection revealed that trailing edge portions of the composite right inboard wing panel had separated. This separation caused damage to the forward and mid sections of the right inboard flaps. The inboard flaps exhibited secondary damage consistent with impact from the detached panel pieces.
Maintenance History
Records showed that during the most recent C check, cracking had been detected at the forward outboard corner of the right inboard wing panel. The crack was repaired, inspected, and repainted. The previous C check had also identified disbonding and delamination in the same right wing panel.
Industry Context
Boeing has received a total of 245 operator reports concerning disbonding of this wing panel. Among those reports, 95 instances involved the panel separating from the aircraft in flight. The wing flaps are noted to be susceptible to secondary damage resulting from such panel separations.
Service Actions
A service bulletin addressing the disbonding issue has been issued and revised five times. Operators have been directed to replace the coin tap sound inspection method with an ultrasonic inspection of the panels, as ultrasonic inspection improves the detection of disbonding occurring below the panel surface. Prior to this incident, the operator had chosen to retain the coin tap sound inspection method instead of adopting the ultrasonic inspection.