CEN16FA171 · McDonnell Douglas Helicopter 369E

The helicopter was being used to transport personnel and equipment in support of a power line construction project. The helicopter departed the landing zone (LZ) and transported two linemen from one power line tower to another using a 50 ft long line. When the linemen detached from the long line, the helicopter proceeded to the east where it hovered for 2 to 3 minutes. The linemen requested that the helicopter return to the tower to pick up equipment and then return to the LZ. The helicopter approached the tower, and, when the long line was nearing their reach, the linemen noticed the helicopter's sound change, and it descended suddenly. The helicopter veered to the right away from the tower, and the main rotor blades slowed noticeably as the helicopter descended into the trees and impacted terrain. The linemen climbed down from the tower and heard the helicopter's engine still producing noise, so one of them pulled the emergency fuel shutoff valve and turned the battery off.

A postaccident examination of the helicopter revealed damage to the main rotor blades and main rotor hub consistent with sudden stoppage at low rotor rpm. The tail rotor exhibited damage consistent with no rotation during impact. The engine was removed from the airframe and connected to an engine test stand for a functional test, but it would not start after several attempts. The power turbine governor (PTG) was removed, and its main drive shaft was found fractured. The original PTG was replaced with a new PTG. With the new PTG installed, the engine started normally, produced rated horsepower, and met production test specifications with no anomalies noted.

Examination of the PTG revealed that a portion of the drive shaft remained embedded in the spindle of the spool bearing assembly. The fracture surface features of the shaft were consistent with overstress. The internal elements of the spool bearing assembly were seized and would not rotate. The ball bearings and spacers were found coated with voluminous, powdery, black particulate consistent with oxidized metallic wear debris, and no grease was observed. The ball retainers were fragmented, the inner surfaces were found coated with a powdery, black particulate consistent with oxidized metallic wear debris, and no grease was observed. The inner bearing surfaces were rough and frosted, consistent with three-body abrasive wear. The examination indicated that the fractured PTG drive shaft was the result of a spool bearing that seized due to a lack of lubrication.

In 2008, a service bulletin (SB) and commercial engine bulletin (CEB) were issued by PTG and engine manufacturers, respectively, that called for replacement of the dual-spool bearing, the type installed in the accident PTG, with a single-spool bearing. The dual-spool bearing had experienced 23 previous failures that had led to either engine oscillations, uncommanded engine acceleration, or a loss of engine power. Although the SB and CEB called for replacement of the accident PTG's dual-spool bearing not later than 750 hours after the PTG was installed new, the accident PTG had accumulated 1,048.7 hours since new when the accident occurred, and the SB and CEB had not been completed. As stated in the operator's Federal Aviation Administration (FAA) approved operations specifications, the operator was required to comply with FAA Airworthiness Directives but was not required to comply with manufacturer's service bulletins. It is likely that had the SB and CEB been completed, the PTG would not have failed and the engine would not have lost power.

Verbatim NTSB analysis and probable cause from the NTSB dataset