N3125N

HISTORY OF FLIGHT

On May 30, 2014, about 0930 Alaska daylight time, N3125N, a de Havilland DHC-3T Otter airplane equipped with a Honeywell TPE331-12JR turboprop engine, experienced an anomalous in-flight vibration and uncommanded nose-down pitch during cruise flight in the vicinity of Homer, Alaska. The personal flight was operated by Alaska Air Taxi, LLC under the provisions of 14 Code of Federal Regulations (CFR) Part 91 with no flight plan filed. The airline transport pilot and the two passengers were not injured, and the airplane sustained substantial damage to the right elevator. Visual meteorological conditions prevailed. The flight departed Seldovia, Alaska, about 0900, en route to Anchorage, Alaska.

According to the pilot, during level cruise flight about 5,000 feet above mean sea level, he felt a vibration in the airplane that at first felt similar to the effects of propeller ice or a problem with the engine, but the engine and systems instruments showed no indications of any problems. The pilot said that the vibration then became worse, and the airplane wanted to pitch nose-down, and he felt certain that there was a change in the elevator. The pilot reduced the airspeed and applied gradual pitch corrections, being mindful that he did not want to impose excessive aerodynamic loads on the tail. The pilot said that, after he slowed the airplane and recovered it from the pitch deviation, the vibration stopped, and he continued the flight to the destination.

After landing, the pilot and a mechanic examined the airplane and found that skin on the right elevator servo tab was separated from the hinge, and spar structures inside the right elevator were fractured.

AIRCRAFT INFORMATION

The de Havilland DHC-3 Otter, as designed in the early 1950s, is a single-engine, propeller-driven airplane originally powered by a reciprocating radial engine. The type certificate for the DHC-3 is currently held by Viking Air Limited, Sidney, British Columbia, Canada. The accident airplane was modified in November 2008 to be equipped with a Honeywell TPE331-12JR turboprop engine, a Hartzell model HC-B4TN-5QL propeller, and other modifications in in accordance with supplemental type certificate (STC) SA09866SC, held by Texas Turbine Conversions, Inc., of Denison, Texas.

The accident airplane was not equipped, and was not required to be equipped, with a modified elevator servo tab and control linkage. (See the "Additional Information" section below for more information.) The original design for DHC-3 servo tab (P/N C3TE13-12) specifies the use of a single piece of 0.016-inch thick 2024 Alclad aluminum formed into a triangular shape with a C-channel incorporated into the forward edge, a continuous hinge (P/N NAS-30-3A-LT) installed at the forward edge of the tab between the upper skin and the upper leg of the C-channel (to attach it to the elevator), and CR-163-4 blind rivets to fasten the hinge and tab skin together. According to design specifications, the NAS-40-3A-LT hinge is made from anodized 61S-T6 aluminum alloy with a width from the hinge centerline to the edge of the flange of 0.625 inch and a flange thickness of 0.045 inch.

A maintenance log entry dated December 1, 2013, documented an annual/100-hour inspection for the airplane and referenced compliance with AD 2011-18-11 (which specified an elevator servo tab inspection) at an airframe total time of 16,299.1 (Hobbs 1513.1). A two-page maintenance worksheet that noted the same Hobbs time contained 11 work items. Item 3 on the worksheet recorded compliance with AD 2011-18-11 and stated "Tabs within tolerance." Item 11 on the worksheet, which was signed off by a different mechanic, noted "R-H Elevator Trim Tab Hinge Worn" and, for work accomplished, noted "R/R Trim Tab Hinge." During an interview, the Alaska Air Taxi director of maintenance (DOM) said that the reason for the hinge replacement at that time was because the AD 2011-18-11 inspection revealed that the servo tab installation exceeded the maximum free-play/trailing edge deflection allowances specified in Viking DHC-3 Otter Maintenance Manual Temporary Revisions 18, 19, and 20. The DOM said that the skin had been assembled to the hinge in accordance with the specifications in the original DHC-3 drawing. The repaired servo tab was reinstalled on the airplane, found to be compliant with the specified allowances, and the airplane was returned to service.

The maintenance records showed no other maintenance to the servo tab between its replacement and the time of the accident. A maintenance record documented that a 100-hour inspection, including the AD 2011-18-11 elevator servo tab inspection, was performed May 28, 2014, at an airframe total time of 16,398.0 hours.

The review of airframe logs did not locate a record for compliance with AD 2011-12-02 (which specified revised airspeed limitations and airspeed indicator markings); however, the AD compliance record for the airframe showed it was complied with on July 15, 2011. The airplane's airspeed indicator had red radial markings at 134 mph and 144 mph, consistent with the specifications in AD 2011-12-02.

WRECKAGE EXAMINATION

The airplane had been repaired and returned to service on June 3, 2014, before the NTSB was notified of the event. (See the "Postaccident Repair and Return to Service" subsection under the "Additional Information" section below for more information.) Examination of photographs of the damaged components provided by the operator showed that a majority of the servo tab remained attached to the right elevator. Most of the servo tab upper skin was attached to the hinge by its rivets, but the lower skin was separated from the rivets such that the tab structure was splayed open. Sections of both the upper and lower skin were separated and not recovered. The upper tab skin showed dark staining around every rivet, and the staining extended aft along the airflow direction from many of the rivets.

The operator provided the NTSB IIC the damaged servo tab, hinge, and elevator auxiliary and rear spar components for further examination by an NTSB structures engineer. While meeting with the NTSB IIC, both the pilot and the DOM expressed their concerns that complying with the AD could potentially introduce a new problem. Both the pilot and the DOM noted that previously replacing the hinge required the removal of blind rivets from the servo skin. The DOM noted that when a rivet is drilled out, the holes could become "hogged out," or elongated. He noted that, in a blind rivet situation, a person cannot tell if the holes are clean or not, and a person cannot tell how much material is there for the new rivets to grip into.

TESTS AND RESEARCH

Elevator Servo Tab and Hinge

NTSB examination of the elevator servo tab and hinge components revealed that the inboard 3 to 4 inches of the tab upper skin was separated and not recovered; the remaining upper skin portion was mostly intact. All of the rivets with the exception of the inboard-most rivet remained installed through the tab upper skin and hinge; however, the tab lower skin was separated from the hinge along the rivet line of the C-channel such that the tab structure was splayed open. The inboard 9.5 inches of tab lower skin and the inboard 14 inches of the C-channel portion of the tab were separated and not recovered. Examination of the remaining C-channel portion found that the rivet holes were intact but had some elongation and deformation.

The C-channel was fractured about 3.5 inches inboard of the outboard end, and the fracture extended aft on the lower skin about 2.25 inches. The outboard filler and control horn remained attached to the tab upper skin at the outboard end. Fractures in the tab lower skin were coincident with the location of the fractures in the upper skin.

Skin thickness measurements of the servo tab skin pieces were performed at several locations (the paint was not removed before the measurements were taken). All measurement readings were between 0.012 and 0.013 inch. Examination of the hinge found it was marked with "MS 20001-2" along its length. Specifications for the MS 20001-2 hinge indicate that it is made from anodized 2024-T3511 aluminum alloy with a width from the hinge centerline to the edge of the flange of 0.531-inch and a flange thickness of 0.044-0.056 inch.

Elevator Auxiliary and Rear Spar Structures

Examination of the elevator auxiliary spar (installed between the upper and lower elevator skins in the area forward of the servo tab) found that it was fractured about 20.75 inches outboard of the inboard end. Twelve rivets installed on the lower elevator skin were found still attached to the skin but pulled through (separated) from the auxiliary spar in the area of the spar fracture and outboard. The auxiliary spar had identification and inspection stamps consistent with the original manufacture. Examination of the elevator rear spar revealed buckling damage on the inboard 25 inches with fractures through the upper flange about 18 inches from the inboard end (at the third lightening hole location) and through both the upper and lower flanges about 25 inches from the inboard end (at the fourth lightening hole location).

See the Airworthiness Group Factual Report in the public docket for more detailed information.

ADDITIONAL INFORMATION

Aerodynamic Flutter

Flutter is an aeroelastic phenomenon that can occur when an airplane's natural mode of structural vibration couples with the aerodynamic forces to produce a rapid periodic motion, oscillation, or vibration. The vibration can be somewhat stable if the natural damping of the structure prevents an increase in the vibratory forces and motions. The motions can become dynamically unstable if the damping is not adequate, resulting in increasing self-excited destructive forces being applied to the structure. Flutter can range from an annoying "buzz" of a flight control or aerodynamic surface to a violent and...