History of Flight
On June 4, 2015, at about 1230 Alaska daylight time, a de Havilland DHC-3T Otter airplane, registration N3125N, equipped with a Honeywell TPE331-12JR turboprop engine, experienced an anomalous in-flight vibration during cruise flight near Skwentna, Alaska. The flight was operated by Alaska Air Taxi, LLC under 14 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. The flight had departed Anchorage, Alaska, about 1200 en route to an off-airport landing strip near Big River, Alaska.
According to the pilot, while applying back pressure to the control yoke during cruise, a vibration was felt. The pilot immediately reduced power and executed a precautionary landing. Post-landing inspection revealed 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, originally designed in the early 1950s, is a single-engine, propeller-driven airplane originally powered by a reciprocating radial engine. The type certificate is currently held by Viking Air Limited. The accident airplane was modified in November 2008 with a Honeywell TPE331-12JR turboprop engine, a Hartzell HC-B4TN-5QL propeller, and other modifications per supplemental type certificate (STC) SA09866SC, held by Texas Turbine Conversions, Inc.
The airplane was not equipped, and was not required to be equipped, with a modified elevator servo tab and control linkage. The original servo tab design (P/N C3TE13-12) specifies a single piece of 0.016-inch thick 2024 Alclad aluminum formed into a triangular shape with a C-channel at the forward edge, a continuous hinge (P/N NAS-30-3A-LT) installed at the forward edge, and CR-163-4 blind rivets. The hinge is made from anodized 61S-T6 aluminum alloy.
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 tab was built per the original drawing. The DOM used CR9163 rivets, the modern equivalent of CR163 rivets. This maintenance followed a previous anomalous event involving the elevator servo tab (see NTSB accident ANC14LA035). An FAA Form 337 was submitted for the repair on June 6, 2014.
The DOM stated that an FAA inspector initially wanted to ground the airplane because the DOM did not use CR3243 Cherry Max rivets, but the DOM argued that CR9163 was correct per the drawing. A maintenance record dated June 5, 2014, shows that CR9163 rivets were removed and 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 and reference to compliance with AD 2011-18-11 (elevator servo tab inspection) at airframe total time 16,688.0 hours. The review did not locate a record for compliance with AD 2011-12-02 (airspeed limitations), but a compliance record showed it was complied with on July 15, 2011. The airspeed indicator had red radial markings at 134 mph and 144 mph.
Wreckage Examination
The airplane was repaired and returned to service on June 4, 2015, before the NTSB was notified. Examination of photographs provided by the operator showed that more than half of the servo tab remained attached to the elevator. Dark staining was observed around most rivets on the tab upper skin, extending aft in the airflow direction. Staining was also present on the forward face of the servo tab and the aft face of the auxiliary spar at the hinge.
Tests and Research
Photographic evidence showed that the outboard, aft corner of the tab with outboard filler and control horn remained attached to the control rod but was not supplied to the NTSB. The outboard 14 inches of tab separated from the elevator and was not recovered except for 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. Three additional fractures of the upper and lower tab skins were located about 7, 14.5, and 19 inches outboard of the inboard end, extending forward from the trailing edge about 0.5, 0.75, and 1.5 inches respectively. About 31 inches of the tab hinge remained installed; the remaining portion was separated and not recovered.
Skin thickness measurements of the tab skin pieces (paint not removed) were between 0.012 and 0.013 inch. The hinge was marked "MS 20001-2" along its length. Specifications for the MS 20001-2 hinge indicate it is made from anodized 2024-T3511 aluminum alloy with a width from hinge centerline to flange edge of 0.531 inch and flange thickness 0.044-0.056 inch.
The elevator auxiliary spar remained installed but was fractured in two locations about 29 and 41.5 inches outboard of the inboard edge of the tab. The upper flange was intact on each of three pieces examined. The elevator hinge half remained attached to the upper flange on the inboard piece and was marked "MS 20001-2". Most of the lower flange of the auxiliary spar was fractured between about 3.5 and 25 inches outboard of the inboard end. There was a branched fracture in a formed rib on the lower skin at about 41.5 inches outboard. The aft edge of the upper skin was fractured at the auxiliary spar through rivet holes from about 14 to 35 inches outboard.
About 59 inches of the rear spar was removed and examined. The spar was buckled and fractured through the inboard three lightening holes. There was a fracture of the upper flange about 31 inches outboard above the 6th lightening hole. A repair was installed between the 7th and 8th lightening holes (about 36 to 41.5 inches outboard), consisting of doublers on the web and flanges. The spar was fractured through the upper flange at the 8th lightening hole and through both flanges at the 9th.
After the 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 upper and lower skins, and installation of a newly built servo tab. On June 13, 2015, a logbook entry documented tightening of elevator control cables to the maximum permissible value; according to the DOM, the cables had about 10 pounds of tension before tightening to about 80 pounds.
Radar data from three previous flights (February 23, April 9, and May 24, 2015) was available from Flight Aware. Winds aloft data was used to determine airspeed. On the February 23 flight, the airplane maintained an average calibrated airspeed of about 130 knots for about 20 minutes, which is 5 knots beyond the Vne established by AD 2011-12-02. On the April 9 flight, average airspeed was between 115 and 120 knots over about 10 minutes. On the May 24 flight, the airplane remained between 110 and 140 knots over 10 minutes.
Additional Information
Flutter is an aeroelastic phenomenon where an airplane's natural structural vibration mode couples with aerodynamic forces, producing rapid periodic motion or oscillation. Flutter can be stable if damping prevents increase, or become dynamically unstable if damping is inadequate, leading to destructive forces. Aircraft speed and structural stiffness influence flutter susceptibility; higher speed or lower stiffness increases risk.
A history of safety activity related to DHC-3 elevator servo tabs exists. In June 2011, FAA AD 2011-12-02 imposed airspeed limitations for DHC-3 airplanes with Honeywell TPE331-10 or -12JR engines per STC SA09866SC: maximum operating airspeed of 144 mph for land/ski and 134 mph for seaplane, with corresponding airspeed indicator markings. In August 2011, AD 2011-18-11 required repetitive inspections of the elevator servo tab every 100 hours for all DHC-3 airplanes. In March 2004, an AD initially mandated installation of an STC for a modified elevator servo tab and redundant control linkage on all turbine-powered DHC-3 airplanes, but in April 2004, revised AD 2004-05-01R1 removed the mandate for Honeywell- and Walter-equipped airplanes after evaluation of technical information. The NTSB review found only one previous documented in-flight failure of an elevator servo tab on a Honeywell-equipped DHC-3, which occurred on the same airframe about one year earlier (see NTSB accident ANC14LA035).