ANC15LA037

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.

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.

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 th