# NTSB ANC05LA051: Arctic S-1B2, N67AT, Anchorage, AK, March 19, 2005 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: March 19, 2005, 16:15 local (20:15 UTC) - Place: Anchorage, AK - Coordinates: 61.308334, -150.455276 - Nearby events: https://openairdocs.org/map.md?lat=61.3083&lon=-150.4553&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report December 20, 2005 - Report: Approved by Office Director - NTSB office: Alaska Regional Office - Parties: FAA-AL-ANC FSDO 03 - NTSB case key (mKey): 61183 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ANC05LA051 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/61183/pdf - Page: https://openairdocs.org/ntsb/ANC05LA051 ## Probable cause A main landing gear collapse during the landing touch down due to a non-secured clevis bolt at the common attach point on the landing gear oleo shock. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Airframe/component/system failure/malfunction, landing - flare/touchdown - miscellaneous,bolt/nut/fastener/clamp/spring - not secured (cause) - Main gear collapsed, landing - roll ## Analysis The private certificated pilot was conducting short-field landings with the wheels extended on a frozen lake in a wheel/ski-equipped airplane. On the ninth landing, the main landing gear collapsed, and the airplane settled onto its belly. The pilot said both landing gear A-frame type gear assemblies rotated outward and upward, as the airplane settled onto the ice. He said that the touchdown was not hard, and the common point of failure was at the center, fuselage-mounted spring shock assembly. Each landing gear assembly has an extended arm that attaches in the center of the fuselage to the bottom end of a vertically mounted oleo shock. The left gear arm attaches to the oleo lug by a clevis bolt that is installed front-to-back. The bolt also retains the top end of two oval-shaped metal link plates that are positioned vertically at the front and aft side of the oleo attach lug. A clevis bolt, through the bottom hole of each link, retains a spacer and the right landing gear extended attaching arm. Each clevis bolt uses a tension castle nut, with a cotter pin for security. The aft link plate had extensive elongation of the upper bolt hole radius. The forward link plate upper bolt hole was fractured, and had opening and elongation of a segment of the bolt hole radius. The upper clevis bolt was found at the scene, but its castle nut and cotter pin were not located. The clevis bolt was bent along its span. The threaded portion of the bolt had flattening of the threads about 1/3 of its circumference, and partial threads on the remaining 2/3 of its circumference, with slight necking of the bolt end. An FAA aerospace engineer examined the recovered parts and reported, in part, that: "...the cotter pin failed in service, or was never installed, and the shear nut loosened and fell off the aft end of the upper bolt, or was never installed." The airplane had accrued 11 hours since the most recent annual inspection. ## Factual narrative On March 19, 2005, about 1615 Alaska standard time, a wheel/ski-equipped Arctic Aircraft Company, S-1B2 airplane, N67AT, sustained substantial damage when the main landing gear collapsed during touchdown on a frozen lake, about 18 miles west of Anchorage, Alaska. The airplane was being operated as a visual flight rules (VFR) local area personal flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by the pilot. The private certificated pilot, and the sole passenger, were not injured. Visual meteorological conditions prevailed. The flight originated at Merrill Field, Anchorage, Alaska, about 1555, and no flight plan was filed, nor was one required. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on March 22, the pilot reported that he was conducting touch and go landings toward the north on Figure-Eight Lake. He said the weather conditions were clear and calm. He decided to do short-field landings with the wheels extended. On the ninth landing, as the airplane touched down, the main landing gear collapsed and the airplane settled onto its belly. The pilot said both landing gear A-frame type gear assemblies rotated outward and upward, as the airplane settled onto the ice. The pilot also stated that the touchdown was not hard, and the common point of failure appeared at the center, fuselage-mounted spring/oleo shock assembly. Inspection of the airplane revealed the left front landing gear attach point was torn from its fuselage mounting lug. The main landing gear assembly (left and right) is an A-frame design with two attaching/pivot mounts on the fuselage tubing. The aft side of each A-frame assembly has an extended arm that attaches in the center of the fuselage to the bottom end of a vertically mounted oleo shock. The left gear arm attaches to the oleo lug by a clevis bolt that is installed front-to-back. The bolt also retains the top end of two oval-shaped metal link plates that are positioned vertically at the front and aft side of the oleo attach lug. A clevis bolt, through the bottom hole of each link, retains a spacer and the right landing gear extended attaching arm. Each clevis bolt uses a tension castle nut, and a cotter pin for security. After the airplane was recovered, a Federal Aviation Administration (FAA) airworthiness inspector retrieved the landing gear link plates, which were retained by the lower clevis bolt attached to the right landing gear arm. The link plates and clevis bolts were examined on July 26, 2005, by a aerospace engineer with the FAA's Alaska Region Aircraft Certification Office, Anchorage, Alaska, which has engineering certification responsibility for the accident airplane. The engineer reported that each link plate was bent, and each lower bolt hole in the links were elongated. The aft link plate had extensive elongation of the upper bolt hole radius. The forward link plate upper bolt hole was fractured, and had opening and elongation of a segment of the bolt hole radius. The upper clevis bolt was found at the scene, but its castle nut and cotter pin were not located. The clevis bolt was bent along its span. The threaded portion of the bolt had flattening of the threads about 1/3 of its circumference, and partial threads on the remaining 2/3 of its circumference, with slight necking of the bolt end. On July 29, the FAA engineer reported, in part, that: "...the cotter pin failed in service, or was never installed, and the shear nut loosened and fell off the aft end of the upper bolt, or was never installed." The airplane had an annual inspection on August 20, 2004, and had accrued about 11 hours since the inspection. ## Aircraft - Registration: [N67AT](https://openairdocs.org/aircraft/N67AT), registered now, S/N 1018, built 1981 (OpenAirDocs) - Reported make and model: Arctic Aircraft Company S-1B2 - Registry model: [Arctic S-1B2](https://openairdocs.org/model/arctic-s-1b2) (registry code 0790208) (OpenAirDocs) - Registry engine: [Lycoming 0-320 series](https://openairdocs.org/engine/lycoming-0-320-series) (registry code 41508, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: 1018 - Airframe hours: 1131 - Engines: Lycoming O-320-B2B 160 horsepower reciprocating - Damage: Substantial damage - Flight: Personal, Part 91 - Registered owner: Bill L. Brown - Airworthiness certificate: Standard, Normal - Last inspection: Annual, August 1, 2004, 11 hours before - Departed: PAMR, Anchorage, AK ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 46; injuries: none; private; airplane ratings: single-engine land; medical: class 3, with waivers/limitations; last flight review January 1, 2005; seat: front; flight hours: 383 total, 212 in this make and model, 261 as pilot in command, 12 in the last 90 days, 3 in the last 30 days ## Weather - Conditions: Visual (VMC), Day - Wind: calm - Visibility: 20 statute miles - Lowest ceiling: none - Temperature: 37 °F ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20050319006509A](https://openairdocs.org/incident/20050319006509A), Mar 19, 2005: Arctic S1B2, Anchorage, AK, Substantial damage. (-23) The pilot was attempting to land the aircraft when it stalled about 5 to 10 feet above the ground which resulted in a hard landing. From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.