Aviation Accident Final Report - National Transportation Safety Board: https://app.ntsb.gov/pdf
Docket And Docket Items - National Transportation Safety Board: https://dms.ntsb.gov/pubdms
Aviation Accident Data Summary - National Transportation Safety Board: https://app.ntsb.gov/pdf
NTSB Identification: ANC15LA037
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Thursday, June 04, 2015 in Skwentna, AK
Probable Cause Approval Date: 01/18/2017
Aircraft: DEHAVILLAND DHC-3T, registration: N3125N
Injuries: 2 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
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.
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
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.
HISTORY OF THE 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.
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.
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.
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.
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.