# NTSB ANC99LA042: Cessna 180, N420SB, Iliamna, AK, April 4, 1999 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: April 4, 1999, 14:30 local (22:30 UTC) - Place: Iliamna, AK - Coordinates: 59.850475, -154.55928 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=59.8505&lon=-154.5593&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report June 22, 2000 - Report: Approved by Office Director - NTSB office: Alaska Regional Office - NTSB case key (mKey): 46096 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ANC99LA042 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/46096/pdf - Page: https://openairdocs.org/ntsb/ANC99LA042 ## Probable cause The improper installation of the main landing gear attaching bolt by maintenance personnel, which resulted in total failure of the bolt and collapse of the landing gear. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Gear collapsed, landing - roll - landing gear,main gear attachment - collapsed (cause) - miscellaneous,bolt/nut/fastener/clamp/spring - failure,total (cause) - maintenance,installation - improper, other maintenance personnel (cause) ## Analysis The pilot was landing the wheel/ski equipped airplane on a frozen lake to wait for improving weather. During the landing roll, the pilot heard a 'whump' and the airplane settled onto the left wing. Inspection revealed the main landing gear attaching bolt had fractured in overload across a threaded area. The installed NAS-147R bolt was not the one specified for installation, and had a shorter smooth shank section (1.3 inches) than the specified AN bolt (1.43 inches). When an undamaged NAS-147R bolt is installed in the landing gear, a threaded area remains within the bolt hole. The bolt had been installed three weeks prior to the accident during the last annual inspection. ## Factual narrative On April 4, 1999, about 1430 Alaska daylight time, a wheel/ski equipped, Cessna 180 airplane, N420SB, sustained substantial damage during landing on a frozen lake, about 27 miles south-southwest of Iliamna, Alaska. The pilot and sole passenger were not injured. The airplane was being operated under 14 CFR Part 91 as a personal flight. The flight had departed from a frozen lake about 10 miles southwest of the accident site about 1400. Visual meteorological condition prevailed at the time of the accident, and no flight plan was filed. The pilot told the NTSB investigator-in-charge during a telephone interview on April 9 that he was returning to Palmer, Alaska, at the completion of a hunting trip. He indicated that after takeoff, the ceilings and visibility deteriorated, with low ceilings and visibility in snow showers. He decided to land on another frozen lake and wait for the weather to improve. During the landing roll, the left main gear leg separated from its mount, damaging the main gear supporting structure. The pilot and passenger were picked up by a passing helicopter. Research revealed that the main landing gear legs had been installed three weeks prior to the accident, during the last annual inspection. Inspection revealed a fracture of the main landing gear attaching bolt. The bolt which fractured was identified as a NAS-147R bolt, with a smooth shank length of 1.3 inches. Examination at the NTSB metallurgical laboratory revealed an overstress failure through a threaded area of the bolt. The bolt which is specified in the Cessna 180 parts manual is an AN bolt, which has a smooth shank length of 1.43 inches. When an undamaged NAS-147R bolt is installed in the landing gear, a threaded area remains within the bolt hole. ## Aircraft - Registration: [N420SB](https://openairdocs.org/aircraft/N420SB), registered now, S/N 18052313 (OpenAirDocs) - Reported make and model: Cessna 180 - Registry model: [Cessna 180J](https://openairdocs.org/model/cessna-180j) (registry code 2072622) (OpenAirDocs) - Registry engine: [Continental O-470 series](https://openairdocs.org/engine/continental-o-470-series) (registry code 17026, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: 18052313 - Airframe hours: 3990 - Engines: Continental O-470-R 230 horsepower reciprocating - Damage: Substantial damage - Flight: Personal, Part 91 - Registered owner: Bobby Graham - Airworthiness certificate: Standard, Normal - Last inspection: Annual, March 5, 1999, 10 hours before - Departed: AK - Destination: PAQ, PALMER, AK ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 58; commercial; airplane ratings: multi-engine land, single-engine sea, single-engine land; instrument ratings: airplane; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 4,185 total, 980 in this make and model, 4,177 as pilot in command, 18 in the last 90 days, 18 in the last 30 days, 4 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 90° at 20 knots, gusting 30 - Visibility: 4 statute miles - Lowest ceiling: 1,000 ft overcast - Temperature: 28 °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 19990404002979A](https://openairdocs.org/incident/19990404002979A), Apr 4, 1999: Cessna 180J, Iliamna, AK, Substantial damage. Acft crashed on landing under unknown circumstances and sustained substantial damage. (-23) gear collapsed due to wrong gear bolt installed on the a/c. (.19)on… ## Other investigations on N420SB (4) 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. - [NTSB ANC91LA073](https://openairdocs.org/ntsb/ANC91LA073), Jun 13, 1991: Cessna 180 (N420SB), Pilot Point, AK, substantial damage. Probable cause: FUEL EXHAUSTION, DUE TO THE PILOT'S INADEQUATE PREFLIGHT. CONTRIBUTING TO THE ACCIDENT WAS THE SOFT AND ROUGH TERRAIN. - [NTSB ANC89LA016](https://openairdocs.org/ntsb/ANC89LA016), Nov 11, 1988: Cessna 180 (N420SB), Meshik Lake, AK, substantial damage - [NTSB ANC89LA001](https://openairdocs.org/ntsb/ANC89LA001), Oct 2, 1988: Cessna 180 (N420SB), Wasilla, AK, substantial damage - [NTSB ANC85LA180](https://openairdocs.org/ntsb/ANC85LA180), Sep 7, 1985: Cessna 180-J (N420SB), Egegik, AK, substantial damage 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.