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Aviation Accident

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NTSB Identification: ANC15LA037
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Thursday, June 04, 2015 in Skwentna, AK
Probable Cause Approval Date: 01/18/2017
Aircraft: DEHAVILLAND DHC-3T, registration: N3125N
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The airline transport pilot was conducting an on-demand commercial taxi flight. The pilot reported that, during cruise flight and while applying back pressure to the control yoke, he felt a vibration. The pilot reduced engine power and then executed a precautionary landing. After landing, the pilot found damage to the elevator servo tab and spar structures inside the right elevator.
Subsequent examination of the airplane revealed that the elevator auxiliary spar and rear spar forward of the servo tab location exhibited fractures and buckling consistent with overload. Dark staining was evident around many of the rivets on the upper servo tab skin. Such staining, commonly known as “smoking rivets,” is typically caused by the loosening of one or more rivets, which allows relative movement between the rivet(s) and the underlying structure. However, if the joint is adequately riveted, there should be no movement between the rivet(s) and the underlying structure.
About 1 year (309 flight hours) before this accident, a similar anomalous event involving the accident airplane occurred. A review of maintenance records indicated that, after that accident, the elevator rear spar was repaired, the false spar was replaced, and a new elevator servo tab was fabricated. The operator’s director of maintenance reported that the servo tab was made in accordance with the drawing for the original part number tab. Postaccident examination of the servo tab skin found that the material used to construct the tab was about 25 percent thinner than the skin material specified in the original servo tab design and that the hinge used was not in accordance with the original servo tab design.
The director of maintenance also reported that, initially, the elevator servo tab was fastened together using the correct rivets. A Federal Aviation Administration inspector subsequently inspected the airplane and required the removal of those rivets and replacement with the rivets based on a service bulletin applicable to airplanes equipped with an optional flutter kit; however, the optional kit was not installed on the accident airplane, so the replacement rivets were incorrect. Replacing the rivets required drilling out each of the rivets fastening the tab skin, C-channel, and hinge together; drilling out the rivets can enlarge the holes beyond their nominal diameter. In an adequately riveted joint, the rivet expands to fill the hole and should slightly deform the hole, making each hole diameter slightly larger than the original; however, if the diameter is already larger than required, the rivet may not adequately fill the hole.
The servo tab is subject to a harsh vibratory environment due to the natural airframe vibration and the propeller wash induced on the airplane’s tail. Based on the dark staining evidence, the rivets installed on the accident servo tab likely did not adequately fill the holes in the tab structure, which allowed them to loosen over time. The holes in the servo tab upper skin and upper leg of the C-channel, where the rivets were changed, were likely slightly larger than nominal following the rivet replacement.
The airplane was inspected about 1 month (18.6 flight hours) before the accident flight in accordance with an airworthiness directive (AD) that required checks for elevator servo tab condition, fastener security, and tab free-play. Although maintenance personnel found the tab to be compliant with the AD’s maximum free-play/trailing edge deflection allowances, the dark staining should have provided maintenance personnel a visual indication that there was a problem with relative movement between the rivets and the underlying structure; however, no maintenance action was taken.
The reported vibration during the accident flight and the overload damage observed on the servo tab and elevator spar structures are consistent with aerodynamic flutter of the elevator servo tab that excited to the point of imposing damaging loads on the servo tab and the elevator spar structures. For a servo tab flutter event to develop, both airspeed (airflow) and inadequate structural stiffness are required. Inadequate stiffness of the servo tab was evident by both the inadequate servo tab skin thickness and the loose rivets. Inadequate elevator control system stiffness was evident by the low elevator control cable tensions recorded after the event.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
  • The flutter failure of the right elevator servo tab due to the improper fabrication of the servo tab and maintenance personnel’s subsequent failure to detect anomalies with the servo tab.