# NTSB WPR11LA282: Hiller UH12E, N99579, Hanford, CA, June 23, 2011 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: June 23, 2011, 04:00 local (11:00 UTC) - Place: Hanford, CA - Coordinates: 36.316665, -119.627502 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=36.3167&lon=-119.6275&mi=10 - Phase of flight: [Takeoff](https://openairdocs.org/ntsb-phases/300) (OpenAirDocs' grouping) - Defining event: [Takeoff: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report April 24, 2012 - Report: Approved by Office Director - NTSB office: Western Pacific Region - Parties: Hiller Aircraft Corporation, Federal Aviation Administration - NTSB case key (mKey): 80877 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR11LA282 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/80877/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=80877 - Page: https://openairdocs.org/ntsb/WPR11LA282 ## Probable cause The separation of the tail rotor drive gear shaft due to torsional fatigue of its threaded portion, which resulted in a loss of tail rotor control. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Tail rotor drive system: tail rotor drive shaft](https://openairdocs.org/ntsb-findings/01046510): Aircraft › Aircraft propeller/rotor · Fatigue/wear/corrosion (probable cause); cases with it: [All aircraft, 25 cases](https://openairdocs.org/ntsb-findings/01046510/cases.md) - [Performance/control parameters: directional control](https://openairdocs.org/ntsb-findings/01062020): Aircraft › Aircraft oper/perf/capability · Attain/maintain not possible (probable cause); cases with it: [All aircraft, 3,958 cases](https://openairdocs.org/ntsb-findings/01062020/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Takeoff: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) (defining event); cases with it: [All aircraft, 994 cases](https://openairdocs.org/ntsb-events/330/sequence.md) - [Takeoff: Loss of control in flight](https://openairdocs.org/ntsb-events/240); cases with it: [All aircraft, 5,002 cases](https://openairdocs.org/ntsb-events/240/sequence.md) - [Uncontrolled descent: Collision with terr/obj (non-CFIT) ](https://openairdocs.org/ntsb-events/470); cases with it: [All aircraft, 10,189 cases](https://openairdocs.org/ntsb-events/470/sequence.md) ## Analysis The pilot reported that during a preflight inspection he observed a small oil leak originating where the tail rotor drive shaft attaches to the transmission. He said that, following a discussion with his boss, they decided to reposition the helicopter from a vehicle-mounted platform onto the ground so they could perform a more detailed examination of the oil leak. The pilot stated that following a normal engine start, he began to lift the helicopter off the platform and heard a "growling" sound originating from the transmission followed by an immediate loss of tail rotor control. The pilot said that the helicopter immediately began to rotate to the right as he rolled the throttle off and lowered the collective. Subsequently, the helicopter landed hard onto the ground adjacent to the truck and platform. Postaccident examination of the helicopter revealed that the tail rotor gear shaft was separated from the tail rotor drive shaft coupling. Metallurgical analysis of the fracture surface of the tail rotor gear shaft revealed that the fracture was consistent with fatigue cracking that emanated from multiple origins at the root portion of a thread on the threaded portion of the shaft. The fatigue crack origin exhibited signatures consistent with being open for an extended period of time compared to the other regions of the fracture face; the amount of time could not be determined. The fatigue crack eventually propagated around the gear shaft as torsional fatigue. No maintenance records for the helicopter were recovered during the course of the investigation, and therefore the amount of flight time on the tail rotor gear shaft was undetermined. ## Factual narrative On June 23, 2011, about 0400 Pacific daylight time, a Hiller UH-12E, N99579, sustained substantial damage during a hard landing following a loss of tail rotor control near Hanford, California. The helicopter was registered to and operated by Central Valley Aviation Inc., Selma, California, under the provisions of 14 Code of Federal Regulations Part 91. The commercial pilot, sole occupant of the helicopter, was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the positioning flight that was originating at the time of the accident. During a telephone conversation, the pilot reported that upon completing a night time aerial application on a nearby field, he landed the helicopter on a platform that was mounted on a truck to refuel the helicopter and refill the chemical hopper. Following refueling operations, the pilot conducted a quick pre-flight inspection to look over the helicopter. Upon inspection of the transmission, the pilot observed a small oil leak originating where the tail rotor drive shaft attaches to the transmission. The pilot said that following a discussion with his boss, they decided to reposition the helicopter from the platform onto the ground so they could perform a more detailed examination of the oil leak. The pilot stated that following a normal engine start, he began to lift the helicopter off the platform and heard a "growling" sound originating from the transmission, followed by an immediate loss of tail rotor control. The pilot said that the helicopter immediately began to rotate to the right as he rolled the throttle off and lowered the collective. Subsequently, the helicopter landed hard onto the ground adjacent to the truck. Postaccident examination of the helicopter by a Federal Aviation Administration (FAA) inspector and a representative from Hiller Aircraft revealed that the tail boom exhibited structural damage. One of the two tail rotor blades was separated at the blade root. The tail rotor gear shaft was found separated from the tail rotor drive shaft coupling. The gear shaft was found separated into two pieces at the threaded portion of the shaft. The gear shaft and tail rotor drive shaft coupling were removed from the wreckage and sent to the NTSB Materials Laboratory, Washington, D.C. for further examination. An NTSB senior metallurgist reported that the external gears on the gear shaft and mating internal gears on the coupler showed no evidence of deformation damage. The gear shaft was fractured at the threaded portion. A nut and cotter pin remained attached to the fractured portion of the threaded area of the gear shaft. Bench binocular microscope examination of the thread fragment revealed the fracture face contained a twist fracture pattern consistent with torsion separation. A light brown region was found on one side of the fracture that extended between the root portion of the thread and an area located approximately 0.08 inch below the thread. The areas outside ofthe dark brown region exhibited a metallic luster. The fracture also showed ratchet marks typical of fatigue cracking that emanated from multiple origins at the root portion of the thread. The ratchet marks extended all around the thread. Scanning electron microscope examination of the thread fragment revealed the fracture face in the areas within the ratchet marks showed striation-like features and the twist fracture pattern at the center of the fracture showed striation-like features consistent with torsional-fatigue. For further details of the metallurgical examination, see the Materials Laboratory Factual Report within the public docket. The pilot reported that the last annual inspection was conducted on the helicopter on June 14, 2011, at a total airframe time of 10,485-hours. No maintenance records for the helicopter were recovered during the course of the investigation. The amount of flight time on the tail rotor gear shaft was undetermined. A representative from Hiller Aircraft reported that the tail rotor gear shaft is subject to a 1,200 hour overhaul which includes magnetic particle inspection of the part. ## Aircraft - Registration: N99579 (not matched to a registry record) - Reported make and model: HILLER UH12E - Category: Helicopter - Serial number: 1733 - Airframe hours: 10485 - Engines: Lycoming IO-540-C2A 305 horsepower reciprocating - Damage: Substantial damage - Flight: Positioning, Part 91 - Operator: Central Valley Aviation Inc. - Registered owner: Central Valley Aviation Inc. - Airworthiness certificate: Special, Restricted (Special) - Last inspection: 100 hour, June 14, 2011 - Departed: Hanford, CA - Destination: Hanford, CA ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 60; injuries: none; commercial; airplane ratings: single-engine land; medical: class 2, without waivers/limitations; last flight review November 20, 2010; seat: right; flight hours: 15,400 total, 800 in this make and model, 15,200 as pilot in command, 200 in the last 90 days, 60 in the last 30 days, 5 in the last 24 hours ## Weather - Conditions: Visual (VMC), Night - Wind: 330° at 8 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 73 °F, dew point 46 °F - Altimeter: 29.78 inHg - Observed at: HJO, 5 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 20110623012719A](https://openairdocs.org/incident/20110623012719A), Jun 23, 2011: Hiller UH-12E, Hanford, CA, Substantial damage. At 0430 the aircraft was being service and refueling when the pilot noticed excess play and oil leaking from the tail rotor drive coupling while the aircraft… ## Docket (5 files) The files NTSB released with the investigation on March 22, 2012, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=80877. 1.8 MB in all. - Materials Laboratory Factual Report 11-087 (with 3 figures) (item 1, .PDF, 650 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40364400&FileExtension=.PDF&FileName=Materials%20Laboratory%20Factual%20Report%2011-087%20(with%203%20figures)-Master.PDF - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 2, .PDF, 573 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40363991&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF - Record of Conversation - Pilot (item 3, .PDF, 52 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40363992&FileExtension=.PDF&FileName=Record%20of%20Conversation%20-%20Pilot-Master.PDF - Evidence Control Form (item 4, .PDF, 78 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40363997&FileExtension=.PDF&FileName=Evidence%20Control%20Form-Redacted.PDF - Statement of Party Representatives to NTSB Investigation - Hiller Aircraft Corporation (item 5, .PDF, 433 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40363995&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation%20-%20Hiller%20Aircraft%20Corporation-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.