History of Flight
On May 30, 2014, about 0930 Alaska daylight time, a de Havilland DHC-3T Otter airplane, registration N3125N, was in cruise flight at approximately 5,000 feet above mean sea level near Homer, Alaska. The flight, operated by Alaska Air Taxi, LLC under 14 CFR Part 91, had departed Seldovia, Alaska, about 0900 en route to Anchorage. The pilot, an airline transport pilot, and two passengers were not injured. Visual meteorological conditions prevailed.
During level cruise, the pilot felt a vibration initially similar to propeller ice or an engine issue, but engine and systems instruments showed no anomalies. The vibration worsened, and the airplane pitched nose-down uncommanded. The pilot reduced airspeed and applied gradual pitch corrections to avoid excessive aerodynamic loads on the tail. After slowing, the vibration ceased, and the flight continued to the destination.
Aircraft Information
The de Havilland DHC-3 Otter is a single-engine, propeller-driven airplane originally powered by a reciprocating radial engine. N3125N was modified in November 2008 with a Honeywell TPE331-12JR turboprop engine and Hartzell HC-B4TN-5QL propeller under supplemental type certificate (STC) SA09866SC, held by Texas Turbine Conversions, Inc. The airplane was not equipped with a modified elevator servo tab and control linkage.
The original elevator servo tab design (P/N C3TE13-12) uses 0.016-inch thick 2024 Alclad aluminum, a continuous hinge (P/N NAS-30-3A-LT), and CR-163-4 blind rivets. The hinge specified for the original design is 0.625 inch wide with a flange thickness of 0.045 inch.
Maintenance History
A maintenance log entry dated December 1, 2013, documented an annual/100-hour inspection with compliance to AD 2011-18-11, which mandates elevator servo tab inspections. The inspection found the tabs within tolerance. A separate work item noted a worn right elevator trim tab hinge, which was replaced. The director of maintenance stated that the replacement was due to exceeding maximum free-play allowances. The repaired servo tab was reinstalled and found compliant.
A subsequent 100-hour inspection on May 28, 2014, included the AD 2011-18-11 inspection. No other servo tab maintenance occurred between December 2013 and the accident. Records also showed compliance with AD 2011-12-02, which imposed airspeed limitations.
Wreckage and Component Examination
After landing, examination revealed that skin on the right elevator servo tab had separated from the hinge, and spar structures inside the right elevator were fractured. The airplane was repaired and returned to service on June 3, 2014, before NTSB notification.
Photographs provided by the operator showed most of the servo tab remained attached. The upper skin was mostly attached to the hinge by rivets, but the lower skin had separated, splaying the tab structure. Sections of both skins were missing. Dark staining was present around rivets on the upper skin.
NTSB examination of the components found that the inboard portion of the upper skin was missing; remaining rivets were installed through the upper skin and hinge except the inboard-most. The lower skin and C-channel portion were separated, with fractures and elongation of rivet holes. The C-channel fractured about 3.5 inches from the outboard end. Skin thickness measured 0.012 to 0.013 inch. The hinge was marked MS 20001-2, which has a width of 0.531 inch and flange thickness of 0.044-0.056 inch, differing from the original specification.
The elevator auxiliary spar was fractured about 20.75 inches outboard of the inboard end, with rivets pulled through from the lower skin. The rear spar exhibited buckling and fractures.
Related Safety Activity
Several airworthiness directives (ADs) addressed elevator servo tab safety on DHC-3 airplanes, including AD 2011-12-02 (airspeed limitations) and AD 2011-18-11 (repetitive inspections). An earlier AD from 2004 initially mandated a modified servo tab and control linkage for turbine-powered DHC-3s but was later revised to exclude Honeywell-equipped airplanes. No previous in-flight servo tab failures had been documented on Honeywell-powered DHC-3s.
Postaccident Repair
Repairs completed June 3, 2014, included replacement of the elevator rear spar, false spar, and fabrication of a new servo tab per original drawing. The operator initially used CR9163 rivets (modern equivalent of CR163). An FAA inspector later required replacement with CR3243 Cherry Max rivets, which were installed on June 5, 2014, and the airplane was returned to service.