N3125NDehavilland DHC-3 · 1960

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.

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.

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.

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 NTSB Senior Meteorologist provided winds aloft for these three dates along the flight paths that were used to determine the airplane's airspeed for each flight.

During the cruise portion of the February 23 flight, the airplane maintained an average calibrated airspeed of about 130 knots for about 20 minutes of recorded flight. This was 5 knots beyond the Vne (never exceed speed) established by AD 2011-12-02.

During about a 10 minute period on the April 9 flight, the airplane's average airspeed was between 115 knots and 120 knots.

The May 24 flight consisted of a 10 minute period where the airplane remained between 110 knots and 140 knots.

See the Aircraft Performance Study in the public docket for more detailed 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 destructive failure of the structure in a very short period of time. Aircraft speed and structural stiffness are two inputs that govern flutter; as speed increases or structural stiffness decreases, the susceptibility to flutter will increase. (Source: NTSB Aircraft Accident Brief, NTSB/AAB-12/01, "Pilot/Race 177, The Galloping Ghost, North American P-51D, N79111, Reno, Nevada, September 16, 2011.")

Summary of Related Elevator Servo Tab Safety Activity, including ADs

The FAA, Transport Canada (TC), the Civil Aviation Authority of Australia, the Civil Aviation Authority of New Zealand, Viking Air Limited, and various STC holders have a history of activity (spanning several years) related to elevator servo tab safety for a variety of configurations of the DHC-3 airplane, including original and STC-modified (turbine) powerplant configurations. A summary of the items that applied to the accident airplane's configuration for U.S. operations at the time of the accident is provided below. A detailed history of various ADs for other DHC-3 configurations, an optional Viking Air Limited flutter-prevention modification kit STC (with its own related ADs and a service bulletin), and additional requirements from TC for DHC-3 airplanes in Canada, are contained in the Airworthiness Group Factual Report in the docket for this investigation.

In June 2011, the FAA issued AD 2011-12-02 to impose limitations on the airspeed of DHC-3 airplanes equipped with a Honeywell TPE331-10 or -12JR turboprop engine per STC SA09866SC. The AD required inserting limitations into the airplane flight manual to specify a maximum operating airspeed limitation of 144 mph for a land/ski airplane and 134 mph for a seaplane and modifying the airspeed indicator to include a red radial line at 144 mph for a land/ski airplane and/or a red radial line at 134 mph for a seaplane.

In August 2011, the FAA issued AD 2011-18-11 requiring repetitive inspections of the elevator servo tab, correcting all discrepancies, and reporting results to the FAA for all DHC-3 airplanes. (The AD superseded AD 2011-05-02 issued in February 2011, retaining all of the requirements and changing the applicability to include all DHC-3 airplanes.) The AD requires operators to inspect the elevator servo tab every 100 hours in accordance with Viking DHC-3 Otter Maintenance Manual Temporary Revisions No. 18, No. 19, and No. 20.

In March 2004, the FAA issued an AD that initially mandated the installation of an STC for a modified elevator servo tab and redundant control linkage on all turbine-powered DHC-3 airplanes (including Honeywell [STC SA09866SC], Walter M601E-11 [Canada Turbine Conversions, Inc., STC SA09857SC], and Pratt & Whitney PT6A-34 or -135 [A.M. Luton STC SA3777NM]turbine engine installations). According to Federal Register Vol. 69, No. 40, Monday, March 1, 2004, page 9523, in which the FAA announced the original AD, the FAA referenced "several reports of situations where pilots of…DHC-3 airplanes with installed turbine engines have experienced buffeting of the elevators." The FAA noted that, in all cases, the airplanes had been modified with a Pratt & Whitney engine per STC SA3777NM. In April 2004, the FAA issued a revised AD 2004-05-01R1 to remove the mandate for the Honeywell- and Walter-equipped airplanes. The FAA stated that it decided to remove the applicability to the Honeywell- and Walter-modified airplanes after it evaluated the concerns, comments, and technical information related to all three STC configurations.

An NTSB review of available records found only one previous documented instance of an in-flight failure of an elevator servo tab on a DHC-3 airplane equipped with a Honeywell TPE331-10 or -12JR turboprop engine. That previous event occurred on this same airframe about 1 year prior to this accident. See NTSB accident ANC14LA035 for more information.

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.

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.

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.

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.

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 destructive failure of the structure in a very short period of time. Aircraft speed and structural stiffness are two inputs that govern flutter; as speed increases or structural stiffness decreases, the susceptibility to flutter will increase. (Source: NTSB Aircraft Accident Brief, NTSB/AAB-12/01, "Pilot/Race 177, The Galloping Ghost, North American P-51D, N79111, Reno, Nevada, September 16, 2011.")

Summary of Related Elevator Servo Tab Safety Activity, including ADs

The FAA, Transport Canada (TC), the Civil Aviation Authority of Australia, the Civil Aviation Authority of New Zealand, Viking Air Limited, and various STC holders have a history of activity (spanning several years) related to elevator servo tab safety for a variety of configurations of the DHC-3 airplane, including original and STC-modified (turbine) powerplant configurations. A summary of the items that applied to the accident airplane's configuration for U.S. operations at the time of the accident is provided below. A detailed history of various ADs for other DHC-3 configurations, an optional Viking Air Limited flutter-prevention modification kit STC (with its own related ADs and a service bulletin), and additional requirements from TC for DHC-3 airplanes in Canada, are contained in the Airworthiness Group Factual Report in the docket for this investigation.

In June 2011, the FAA issued AD 2011-12-02 to impose limitations on the airspeed of DHC-3 airplanes equipped with a Honeywell TPE331-10 or -12JR turboprop engine per STC SA09866SC. The AD required inserting limitations into the airplane flight manual to specify a maximum operating airspeed limitation of 144 mph for a land/ski airplane and 134 mph for a seaplane and modifying the airspeed indicator to include a red radial line at 144 mph for a land/ski airplane and/or a red radial line at 134 mph for a seaplane.

In August 2011, the FAA issued AD 2011-18-11 requiring repetitive inspections of the elevator servo tab, correcting all discrepancies, and reporting results to the FAA for all DHC-3 airplanes. (The AD superseded AD 2011-05-02 issued in February 2011, retaining all of the requirements and changing the applicability to include all DHC-3 airplanes.) The AD requires operators to inspect the elevator servo tab every 100 hours in accordance with Viking DHC-3 Otter Maintenance Manual Temporary Revisions No. 18, No. 19, and No. 20.

In March 2004, the FAA issued an AD that initially mandated the installation of an STC for a modified elevator servo tab and redundant control linkage on all turbine-powered DHC-3 airplanes (including Honeywell [STC SA09866SC], Walter M601E-11 [Canada Turbine Conversions, Inc., STC SA09857SC], and Pratt & Whitney PT6A-34 or -135 [A.M. Luton STC SA3777NM]turbine engine installations). According to Federal Register Vol. 69, No. 40, Monday, March 1, 2004, page 9523, in which the FAA announced the original AD, the FAA referenced "several reports of situations where pilots of…DHC-3 airplanes with installed turbine engines have experienced buffeting of the elevators." The FAA noted that, in all cases, the airplanes had been modified with a Pratt & Whitney engine per STC SA3777NM. In April 2004, the FAA issued a revised AD 2004-05-01R1 to remove the mandate for the Honeywell- and Walter-equipped airplanes. The FAA stated that it decided to remove the applicability to the Honeywell- and Walter-modified airplanes after it evaluated the concerns, comments, and technical information related to all three STC configurations.

An NTSB review of available records found no previous documented instances of an in-flight failure of an elevator servo tab on a DHC-3 airplane equipped with a Honeywell TPE331-10 or -12JR turboprop engine.

Postaccident Repair and Return to Service

Following this accident, the airplane was repaired and returned to service. 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 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.

The DOM stated that following the June 3, 2014, repair, an FAA inspector had inspected the airplane and wanted to ground the airplane because CR3243 Cherry Max rivets were not used 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. The DOM 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.

Before this accident investigation was completed (and before the results of the component examinations were known), the airplane experienced another anomalous event involving the elevator servo tab. See NTSB accident ANC15LA037 for more information about that event.