What happened
During a night departure, the flight crew of a Boeing 747 noticed unusual vibrations following the retraction of the landing gear. While the crew observed additional noise during the climb, all flight deck instruments indicated that the landing gear had functioned correctly. Because sterile cockpit procedures were in effect, the crew was not alerted to the issue until the cabin services manager contacted the flight deck to report the disturbance.
Upon identifying the vibration, the crew worked with maintenance operational control and cabin crew to systematically check various aircraft systems. Although the source of the vibration could not be immediately identified, the crew determined the vibration was constant but that the aircraft was otherwise performing normally. Lacking specific checklists for this particular phenomenon, the crew used their professional judgment to continue the flight.
The investigation
The investigation focused on determining whether the vibration was related to the landing gear mechanism. While the timing of the vibration coincided with gear retraction, investigators noted that the high levels of noise and vibration present while the gear is extended could have masked an earlier event.
Post-flight examination revealed evidence of a birdstrike and a sheared attach rod. The investigation found that the Boeing 747 landing gear system utilizes hydraulics and rods to operate both the gear and associated doors. While larger doors use proximity sensors to alert the crew, certain smaller doors are attached via rods and lack independent sensors. The sheared rod prevented the L3 landing gear door from closing properly, creating the turbulence and vibration experienced during flight.