What happened
On June 4, 2015, a de Havilland DHC-3T Otter, registration N3125N, was operating an on-demand commercial air taxi flight near Skwentna, Alaska. The aircraft, operated by Alaska Air Taxi, LLC, was en route from Anchorage to an off-airport landing strip near Big River when the pilot experienced an anomalous vibration during cruise flight. The pilot reported feeling the vibration while applying back pressure to the control yoke.
In response to the vibration, the pilot reduced engine power and performed a precautionary landing. The pilot was not injured, but the aircraft sustained substantial damage. Post-landing inspection revealed that the skin on the right elevator servo tab was fractured and partially missing, and the internal spar structures of the right elevator were buckled and fractured.
The investigation
Investigators examined the damaged components and found that the outboard 14 inches of the servo tab had separated from the elevator. Measurements of the servo tab skin revealed that the material used was approximately 25 percent thinner than the thickness specified in the original design. Additionally, the hinge used was not in accordance with the original design specifications.
Maintenance records showed that the servo tab had been fabricated and installed following a previous incident approximately one year earlier. During that repair, the aircraft's director of maintenance used CR9163 rivets, which were a modern equivalent for the specified CR163 rivets. However, following an FAA inspection, the rivets were replaced with CR3243 Cherry Max rivets. Investigators determined that the process of drilling out the original rivets to install the new ones likely enlarged the holes beyond their nominal diameter.
Examination of the wreckage also revealed dark staining, often referred to as "smoking rivets," around many of the rivets on the upper servo tab skin. This staining indicates relative movement between the rivets and the underlying structure. Furthermore, a review of the elevator control cables following the event showed they had been tightened to 80 pounds, whereas they had been at approximately 10 pounds of tension prior to that maintenance.
Findings
- The elevator auxiliary spar and rear spar exhibited fractures and buckling consistent with aerodynamic flutter.
- The servo tab skin thickness was inadequate due to improper fabrication.
- The rivets in the servo tab likely did not adequately fill the holes, allowing them to loosen over time.
- Maintenance personnel failed to act on the visual indication of dark staining around the rivets during a 100-hour inspection performed 18.6 flight hours prior to the accident.
- Inadequate stiffness in the elevator control system was evidenced by the low tension in the control cables prior to recent maintenance.