Saturday, April 15, 2017

Hughes 369D, Rotor Blade LLC, N920JP: Accident occurred October 30, 2015 in Marion County, South Carolina

Aviation Accident Final Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

The National Transportation Safety Board did not travel to the scene of this accident.

Additional Participating Entities:

Federal Aviation Administration / Flight Standards District Office; West Columbia, SC
MD Helicopters; Mesa, Arizona
Boeing; Mesa, Arizona 

Aviation Accident Factual Report -  National Transportation Safety Board:  https://app.ntsb.gov/pdf


Docket And Docket Items - National Transportation Safety Board: https://dms.ntsb.gov/pubdms


NTSB Identification: ERA16LA030
14 CFR Part 133: Rotorcraft Ext. Load
Accident occurred Friday, October 30, 2015 in Marion, SC
Probable Cause Approval Date: 05/23/2017
Aircraft: HUGHES 369D, registration: N920JP
Injuries: 1 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 commercial pilot of the helicopter was trimming trees on a power line right-of-way when the externally-mounted saw blades jammed. He climbed the helicopter out of the area and elected to return to the landing zone (LZ) to have the saw blades cleared. As he began a forward transition directly to the LZ, the helicopter yawed to the right. He initially corrected the situation with left pedal inputs. While maintaining a heading into the wind, he felt a “thump” and heard a “pop” sound, and the helicopter began to spin to the right out of control. The engine continued to run throughout the event. The helicopter settled into trees as the pilot attempted to cushion the landing with collective control inputs. The helicopter subsequently impacted the ground.

An examination of the wreckage revealed a spiral fracture in the tail rotor control torque tube that connected the left and right seat pedals. Metallurgical examination of the torque tube revealed that it failed in overload due to torsional stresses. A design review by the helicopter manufacturer’s engineering department revealed that the torque tube met all airworthiness standards and design criteria. 

It was apparent that, based on the pilot’s comments and the fracture characteristics of the torque tube, it fractured in flight, immediately before the loss of helicopter control. Although no airframe or foreign obstructions were found in the tail rotor control system, it is possible that a momentary jam existed, though the source could not be determined despite a thorough examination of the wreckage. Although the tail rotor pitch control was replaced about 25 hours of time in service before the accident, and a tail rotor control rigging check was required at that time, aircraft damage prevented an evaluation of the tail rotor control rigging condition at the time of the accident.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
A momentary jam in the tail rotor control system from an undetermined source, resulting in a torsional fracture of the tail rotor control torque tube and a loss of helicopter control.

On October 30, 2015, about 1158 eastern daylight time, a Hughes 369D, N920JP, collided with terrain while transitioning to cruise flight near Marion, South Carolina. The commercial pilot was not injured, and the helicopter was substantially damaged by impact forces and a small postcrash fire. The helicopter was operated by Rotor Blade LLC under the provisions of 14 Code of Federal Regulations Part 133 as an external load (aerial saw) flight. Day, visual meteorological conditions prevailed, and no flight plan was filed. The local flight originated from Marion County Airport (MAO), Mullins, South Carolina about 1130.

According to reports provided by the operator, the pilot was trimming trees on a power line right-of-way when the saw blades jammed. He climbed the helicopter out of the area and elected to return to the landing zone (LZ) to have the saw blades cleared. As the pilot began a forward transition directly to the LZ, the helicopter yawed to the right. He initially corrected the situation with left pedal inputs. While maintaining a heading into the wind, the pilot felt a "thump" and heard a "pop" sound, and the helicopter began to spin to the right, out of control. The engine continued to run throughout the event. The helicopter settled into the trees as the pilot attempted to cushion the landing with collective control inputs. The helicopter collided with the ground and came to rest. A small postcrash fire ensued and the pilot extinguished it with a fire bottle.

An inspector with the Federal Aviation Administration responded to the accident site and examined the wreckage. The helicopter came to rest in a wooded area. The main rotor blades received structural damage from impact forces. The fuselage received structural damage and the tail boom was severed from impact forces.

The wreckage was recovered to the operator's facilities for further examination. The helicopter was configured to be flown from the left cockpit seat. The right seat collective and cyclic control sticks were removed; however, the right pedals were installed without foot rests. Examination of the tail rotor control torque tube linking the left and right seat pedals revealed that the tube was cracked in a spiral pattern. The torque tube was removed from the helicopter and forwarded to the NTSB Materials Laboratory for examination. Due to extensive airframe damage, tail rotor control rigging at the time of the accident could not be established. No foreign or airframe obstructions were found that would account for a jam in the tail rotor control system.

The tail rotor control torque tube was examined at the NTSB Materials Laboratory. The examination of the tube piece with a 5X to 50X stereo-zoom microscope revealed that the fracture initiated at a through-hole in the tube and helically propagated around approximately 90 percent of the circumference of the tube. The angle of the crack path was approximately 45 degrees relative to the longitudinal axis of the tube. The fracture surfaces exhibited through-thickness shear lips consistent with overstress fracture. The helical nature of the fracture path was consistent with torsional loading.

According to the manufacturer, the through-holes in the torque tube were not utilized on the 369D. The holes were designed for the installation of a friction bracket on MD 500N helicopters. When installed on the 369D, the holes were left open.

After the metallurgical examination of the tail rotor control torque tube, MD Helicopters engineering personnel conducted a design analysis of the torque tube. Their study found that the torque tube met all applicable airworthiness and design criteria.

The Helicopter Flight Log Reports showed the pilot's pedal support bracket, part number (PN) 369A7505-8, was reported broken on July 17, 2014. A maintenance logbook entry describing the work performed was not found. However, company inventory documents show the pedal support bracket and the pilot's tail rotor control torque tube, PN 369H7531-13, were provided as replacement parts. The helicopter had accrued about 1,380 hours of time in service since the torque tube was installed.

On October 17, 2015, or about 25 hours of time in service prior to the accident, the tail rotor pitch control was removed and replaced with an overhauled unit. According to the manufacturer, the removal and replacement of the tail rotor pitch control required that the rigging of the tail rotor controls be checked.

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