N3125N

HISTORY OF FLIGHT

On June 4, 2015, about 1230 Alaska daylight time, a de Havilland DHC-3T Otter airplane, N3125N, equipped with a Honeywell TPE331-12JR turboprop engine, experienced an anomalous in-flight vibration during cruise flight in the vicinity of Skwentna, Alaska. The flight was being operated by Alaska Air Taxi, LLC under the provisions of Title 14 Code of Federal Regulations (CFR) Part 135 as an on-demand commercial air taxi. The airline transport pilot was not injured, and the airplane sustained substantial damage to the right elevator. Instrument meteorological conditions prevailed in the area. The flight departed Anchorage, Alaska, about 1200, en route to an off-airport landing strip near Big River, Alaska.

According to the pilot, during cruise flight, while applying back pressure to the control yoke, a vibration was felt. The pilot immediately reduced power and executed a precautionary landing. After landing, the pilot examined the airplane and found that the skin on the right elevator servo tab was fractured in several places, a portion of the tab was missing, and spar structures inside the right elevator were buckled and 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 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 record dated June 3, 2014, documented repairs to the elevator rear spar, replacement of the false spar, and fabrication of a new servo tab. According to the Alaska Air Taxi Director of Maintenance (DOM), the new servo tab was built in accordance with the drawing for the original P/N C3TE13-12 servo tab. He pointed out that the original drawing specified the use of CR163 blind rivets to fasten the tab and hinge together. The DOM used CR9163 rivets, the modern equivalent of CR163 rivets (which were no longer available), for the repair. This maintenance was performed on the airplane following a previous anomalous event involving the elevator servo tab. See NTSB accident ANC14LA035 for more information about that event. An FAA Form 337 dated June 6, 2014, was submitted for the repair.

The DOM stated that following the June 3, 2014, repair, an FAA inspector had inspected the airplane and wanted to ground the airplane because he did not use CR3243 Cherry Max rivets to fasten the tab together. The DOM said that the FAA inspector was incorrect and that, per the drawing, he had used the correct rivets. He noted that the Cherry Max rivets were used only for DHC-3 airplanes that were equipped with an optional flutter kit, which the accident airplane had not incorporated. The DOM provided a copy of the service bulletin that applied to modified airplanes for reference. A maintenance record for the airplane dated June 5, 2014, showed that the CR9163 rivets were removed from the servo tab hinge, CR3243 Cherry Max rivets were installed, and the airplane was returned to service.

A maintenance log entry dated May 5, 2015, documented a 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 16,688.0 (Hobbs 1902.0).

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 4, 2015, before the NTSB was notified of the event. Examination of photographs of the damaged components provided by the operator showed that more than half of the servo tab remained attached to the elevator. There was dark staining around most rivets on the servo tab upper skin portions recovered. The staining extended aft along the airflow direction from some of the rivets. There was also dark staining on the forward face of the servo tab and the aft face of the auxiliary spar that emanated from the hinge.

TESTS AND RESEARCH

Elevator Servo Tab and Hinge

Photographic evidence showed that the outboard, aft corner of the tab with outboard filler and control horn attached remained attached to the control rod after the event but was not supplied to the NTSB. NTSB examination of the elevator servo tab and hinge components revealed that the outboard 14 inches of tab separated from the elevator and was not recovered with the exception of the outboard aft corner. The inboard 31 inches of tab was mostly intact. The upper and lower skins were fractured about 26.5 inches outboard of the inboard end from the trailing edge forward to the C-channel area but the channel and hinge were not fractured.

The C-channel and hinge were cut during disassembly separating the remaining tab into 2 pieces. There were three additional fractures of the upper and lower tab skins located about 7, 14.5 and 19 inches outboard of the inboard end. They extended forward from the trailing edge about 0.5, 0.75 and 1.5 inches, respectively. About 31 inches of the tab hinge remained installed on the tab and the elevator. The remaining portion of hinge was separated and not recovered.

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

The elevator auxiliary spar (installed between the upper and lower elevator skins in the area forward of the servo tab) remained installed in the elevator but was fractured in two locations about 29 inches and 41.5 inches outboard of the inboard edge of the tab. The inboard 3.5 inches of auxiliary spar was cut off during disassembly and not retained. The upper flange of the auxiliary spar was intact on each of the three pieces examined. The elevator hinge half remained attached to the upper flange of the auxiliary spar on the inboard piece. The hinge half was marked with "MS 20001-2" along its length. Most of the lower flange of the auxiliary spar was fractured from the spar between about 3.5 and 25 inches outboard of the inboard end. Portions of the fractured flange were attached to the lower elevator skin. There was a branched fracture in one of the formed ribs on the lower skin running forward about 5 inches and located about 41.5 inches outboard of the inboard end of the tab cutout coincident with the fracture in the auxiliary spar. The aft edge of the upper skin was fractured at the auxiliary spar through the rivet holes from about 14 inches to 35 inches outboard of the inboard end.

About 59 inches of the rear spar was removed and retained for examination consisting of 4 pieces. The spar was buckled and fractured through the inboard 3 lightening holes. There was a fracture of the upper flange of the rear spar about 31 inches outboard of the inboard end above the 6th lightening hole. There was a repair installed between the 7th and 8th lightening holes, about 36 inches to 41.5 inches outboard of the inboard end. The repair consisted of a doubler on the aft face of the web, a doubler on the forward face of the web and upper flange and a doubler on the forward face of the web and lower flange. The spar was fractured through the upper flange at the 8th lightening hole and through both the upper and lower flanges at the 9th lightening hole.

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

Elevator Controls

After this event, the airplane was repaired by replacement of the right elevator auxiliary spar, hinge, a portion of the rear spar, the outboard closeout rib, and the upper and lower skins and installation of a newly built servo tab. On June 13, 2015, a logbook entry documented the tightening of the elevator control cables to the maximum permissible value. According to the DOM, the cables had about 10 pounds of tension prior to tightening them to about 80 pounds.

Radar and Airspeed Data

Radar data from three previous flights with this airplane was available from Flight Aware. The previous flights occurred on February 23, 2015, April 9, 2015 and May 24, 2015. An NT...