# NTSB CEN11LA566: Air Tractor AT-400, N3659N, Nelson, NE, August 11, 2011 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: August 10, 2011, 19:00 local (Aug 11, 2011, 00:00 UTC) - Place: Nelson, NE - Coordinates: 40.20111, -98.066665 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=40.2011&lon=-98.0667&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering: Powerplant sys/comp malf/fail](https://openairdocs.org/ntsb-events/340) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report April 10, 2013 - Report: Approved by Office Director - NTSB office: Central Region - Parties: Honeywell, FAA/FSDO, Transportation Safety Board of Canada - NTSB case key (mKey): 81468 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN11LA566 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/81468/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=81468 - Page: https://openairdocs.org/ntsb/CEN11LA566 ## Probable cause Failure of the fuel control unit's spool cap bearing for reasons that could not be determined during extensive postaccident examination, and the pilot’s decision to shut the engine down without sufficient altitude and distance to reach the runway. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Engine fuel and control: fuel control/carburetor](https://openairdocs.org/ntsb-findings/01057322): Aircraft › Aircraft power plant · Failure (probable cause); cases with it: [Air Tractor AT-400, 1 case](https://openairdocs.org/ntsb-findings/01057322/cases.md?model=0390202), [All aircraft, 195 cases](https://openairdocs.org/ntsb-findings/01057322/cases.md) - [Not determined](https://openairdocs.org/ntsb-findings/05000000): Not determined › Not determined · Unknown/Not determined (probable cause); cases with it: [Air Tractor AT-400, 2 cases](https://openairdocs.org/ntsb-findings/05000000/cases.md?model=0390202), [All aircraft, 2,247 cases](https://openairdocs.org/ntsb-findings/05000000/cases.md) - [Info processing/decision: decision making/judgment](https://openairdocs.org/ntsb-findings/02041520): Personnel issues › Action/decision · Pilot (probable cause); cases with it: [Air Tractor AT-400, 1 case](https://openairdocs.org/ntsb-findings/02041520/cases.md?model=0390202), [All aircraft, 3,098 cases](https://openairdocs.org/ntsb-findings/02041520/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Maneuvering: Powerplant sys/comp malf/fail](https://openairdocs.org/ntsb-events/340) (defining event); cases with it: [Air Tractor AT-400, 1 case](https://openairdocs.org/ntsb-events/340/sequence.md?model=0390202), [All aircraft, 820 cases](https://openairdocs.org/ntsb-events/340/sequence.md) ## Analysis When the pilot was unable to reduce power normally for landing, he used the fuel cut-off lever to shut down the agricultural airplane's turbine engine. However, the pilot stated that he misjudged the airplane’s altitude and distance to the runway when he shut off fuel to the engine, and the airplane landed short of the runway. During postaccident examination, the fuel control unit failed the manufacturer's functional check. The unit was then disassembled, and the spool cap bearing and its associated hardware, including the cap containing the bearing outer race and the post with the bearing inner race, were found damaged. Metallurgical examination of the bearing races found embedded alumina particles. A historical review of the manufacturer's fuel control unit revealed a history of premature spool cap bearing failures in fuel control units installed on other turboprop engines, but not on the accident airplane’s make and model engine. These failures were attributed to high diamond content and other hard particle contamination and could result in fuel flow and power/torque fluctuations. However, no direct link could be established between the accident-related failure and previous failures in other turboprop engines. Although hard alumina particles were found, there was insufficient historical data to support that these particles had attributed to previous bearing failures in this engine. The cause of the spool cap bearing failure could not be determined. ## Factual narrative On August 10, 2011, about 1900 central daylight time, N3659N, an Air Tractor AT-400, N3659N, sustained substantial damage when it landed short of a private airstrip near Nelson, Nebraska. The commercial rated pilot was not injured. The airplane was registered to and operated by the pilot. Visual meteorological conditions prevailed and no flight plan was filed for the local aerial application flight conducted under 14 Code of Federal Regulations Part 137. The flight originated at the private airstrip approximately 1830. The pilot stated that when he reduced power to land, the power would not decrease. So he shut the engine down by pulling the fuel cut-off lever. The pilot misjudged the altitude he needed to glide to the runway and landed short resulting in substantial damage to the airframe. The fuel pump, fuel control unit, and the propeller governor were removed from the Pratt &Whitney Canada (PWC) PT-6 engine for further examination. The governor was examined at Woodward, Incorporated, Rockford, Illinois, under the supervision of the National Transportation Safety Board (NTSB) and no anomalies were noted that would have precluded normal operation of the unit. The fuel pump and the fuel control unit were sent to PWC and examined under the supervision of the Transportation Safety Board of Canada. No anomalies were noted with the fuel pump when tested; however, the fuel control unit did not meet test specifications during the functional check. The unit was disassembled and it was noted that the spool cap bearing and its associated hardware were damaged. The damaged parts, which included the cap containing the bearing outer race, the post with the bearing inner race, a retainer, a shield, a second retainer and six ball bearings, and six of seven ball bearings, were sent to the NTSB Materials Laboratory, Washington DC, for metallurgical examination. Examination of the ball bearings revealed they were smaller than their original dimensions and exhibited wavy lines or a frosted appearance. The #5 ball bearing was metallurgically encapsulated, ground down to its diameter, and polished for micro-hardness testing. The bearing’s hardness was within tolerances. The two retainers were thin circular ribbons made of metal. Both retainers were mechanically damaged. One retainer exhibited fractured ends with visible bulges, consistent with the original location of a ball bearing. There was also a folded area that displayed a smooth bulge, consistent with it being the location of the seventh ball. The second retainer was also fractured at each end and exhibited defined bulges, consistent with the original location of each ball bearing. The shield was intact and undamaged. A wire clip used to retain the shield was not present with the cap. Examination of the inner surface of the bearing's cap (outer race) revealed circular impressions and a rectangle-shaped smear of material. The smeared material was consistent with a rolling contact path produced by the fuelcontrol arm bearing during governor operation. The inner and outer edges of the cap’s annulus were not well defined; however, measurements of each diameter were consistent with each other. Examination of the post, with the inner race of the bearing still installed revealed two parallel impressions on the surface consistent with normal contact of the counterweights. The depth of the impressions was found to gradually taper becoming deeper at the opposing ends. Sections of the bearing's inner and outer races were examined using a scanning electron microscope (SEM). Colonies of aluminus particles were found embedded in both sections. Other sections of the races were metallurgically encapsulated and polished for hardness testing. Both sections tested within specifications. Honeywell, who manufactured the fuel control unit, had a history of premature spool cap bearing failures in fuel control units installed on their LT-101 engines. These failures were attributed to high diamond content and other hard particle contamination. Honeywell took immediate corrective action to determine the root cause of these failures and implemented new manufacturing procedures with the spool cap bearing supplier. A Federal Aviation Administration (FAA) Airworthiness Directive was also issued. The NTSB’s metallurgical findings on the spool cap bearing from the PT-6 engine were compared to examination data of the failed spool cap bearings on the LT-101 engines, and a direct link between the failures could not be established. Though hard aluminus particles were found embedded in the inner and outer race of the bearing on the PT-6 engine, there was insufficient historical data to support that these particles had attributed to any previous bearings failures. The fuel control unit was overhauled and re-installed on the airplane in November, 2010. It had accrued a total of 197.4 hours at the time the spool cap bearing failed. Failure of the spool cap bearing will result in power/torque fluctuations, either higher or lower than the set metered fuel flow conditions. ## Aircraft - Registration: [N3659N](https://openairdocs.org/aircraft/N3659N), registered now, S/N 400-0294 (OpenAirDocs) - Reported make and model: AIR TRACTOR INC AT-400 - Registry model: [Air Tractor AT-400](https://openairdocs.org/model/air-tractor-at-400) (registry code 0390202) (OpenAirDocs) - Registry engine: [Amateur-built UNKNOWN ENG](https://openairdocs.org/engine/amateur-built-unknown-eng) (registry code 99999) (OpenAirDocs) - Category: Airplane - Serial number: 400-0294 - Airframe hours: 3900 - Engines: Pratt &Whitney Canada PT6-15-AG 680 horsepower turbo prop - Damage: Substantial damage - Flight: Aerial application, Part 137 - Operator: Wehrman Jim - Registered owner: Wehrman Jim - Airworthiness certificate: Special, Restricted (Special) - Last inspection: Annual, December 5, 2010, 197 hours before - Departed: None, Private, NE - Destination: None, Private, NE - Airport: None, Private ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 57; injuries: minor; commercial; airplane ratings: single-engine land; medical: class 2, without waivers/limitations; seat: single; flight hours: 8,000 total, 400 in this make and model, 100 in the last 90 days, 7 in the last 30 days, 0 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 80° at 4 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 64 °F, dew point 61 °F - Altimeter: 29.96 inHg - Observed at: HJH, 23 nm away ## 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 20110810020579A](https://openairdocs.org/incident/20110810020579A), Aug 10, 2011: Air Tractor Inc AT-400, Nelson, NE, Substantial damage. Pilot noticed engine torque fluctuation of 10-15 lbs, yet continued to operate at 75% power. Pilot pulled fuel cutoff to shut down engine. Pilot landed 253… ## Docket (4 files) The files NTSB released with the investigation on February 27, 2013, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=81468. 2.1 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 520 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40379902&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF - Materials Laboratory 15 - Factual Report 12-030 (item 2, .PDF, 18 photos, 1.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40376641&FileExtension=.PDF&FileName=Materials%20Laboratory%2015%20-%20Factual%20Report%2012-030-Master.PDF - Statement of Party Representatives to NTSB Investigation (item 3, .PDF, 111 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40379903&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Redacted.PDF - Statement of Party Representatives to NTSB Investigation (item 4, .PDF, 109 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40379904&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Redacted.PDF 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.