What happened
On May 30, 2014, at approximately 0930 Alaska daylight time, a de Havilland DHC-3T Otter, registration N3125N, was performing a commercial taxi flight near Homer, Alaska. The aircraft, which was equipped with a Honeywell TPE331-12JR turboprop engine, was en route from Seldovia to Anchorage under visual meteorological conditions.
During level cruise flight at approximately 5,000 feet above mean sea level, the pilot experienced an anomalous vibration and an uncommanded nose-down pitch. The pilot noted that the vibration initially felt similar to engine or propeller issues, but flight instruments showed no abnormalities. As the vibration intensified, the pilot applied gradual pitch corrections and reduced airspeed to avoid imposing excessive aerodynamic loads on the tail. Once the aircraft was stabilized, the vibration ceased, and the flight continued to the destination without further incident. The pilot and two passengers were not injured.
The investigation
Post-flight inspections by the pilot and a mechanic revealed that the skin on the right elevator servo tab had separated from the hinge, and the spar structures inside the right elevator were fractured.
NTSB examination of the components showed that the tab lower skin had separated from the hinge, causing the structure to splayed open. While the upper skin remained mostly attached, dark staining—often referred to as "smoking rivets"—was present around every rivet on the upper skin. This staining typically indicates relative movement between the rivets and the underlying structure.
Investigation of the servo tab revealed that the material used for the tab skin was approximately 25 percent thinner than the original design specifications. Additionally, the hinge used was not in accordance with the original design. The investigation also found that the elevator auxiliary spar and rear spar exhibited fractures and buckling consistent with an overload event.
Findings
- The vibration and structural damage were consistent with aerodynamic flutter of the right elevator servo tab.
- The servo tab lacked adequate structural stiffness due to the use of thinner skin material and loose rivets.
- The rivet holes in the C-channel were likely enlarged during a previous hinge replacement, preventing the rivets from adequately filling the holes.
- Although a recent inspection had been performed, the dark staining on the rivets should have served as a visual indication of movement within the structure, but no maintenance action was taken.