CEN16FA171

The failure of the power turbine governor's dual-spool bearing due to a lack of lubrication, which resulted in a complete loss of engine power and subsequent impact with terrain.

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.

The engine was shipped to a Rolls-Royce facility and connected to an engine test stand for a functional test. Several attempts to start the engine were made, but the engine did not start. The fuel system was checked, and fuel was noted throughout the system, up to and including the fuel nozzle where normal spray patterns and pressures were observed. The fuel control unit was removed and replaced with a new fuel control unit; subsequent engine start attempts were unsuccessful. The governor servo pressure (Py) line between the PTG and fuel control unit was removed, and its fittings were capped off to test operation of the PTG. A successful engine start was made in this condition. The PTG was removed, and its main drive shaft was found fractured. The original PTG was replaced with a new PTG, and the original fuel control was reinstalled on the engine. With the new PTG installed, the engine started normally, produced rated horsepower, and met production test specifications with no anomalies noted. On August 9, 2016, the PTG, which was designed and manufactured by Honeywell, was disassembled and examined at a Honeywell facility under the auspices of the NTSB. A functional performance test could not be performed due to the internal damage. The examination revealed that the governor pressure (Pg) lever clevis fork was bent, and the spool bearing assembly was loose within the drive body cavity (figure 6). The drive shaft guide post was fractured and trapped within the spool bearing bushing. Metallic debris was found within the interior of both the drive body and the drive body cover. The internal bearing elements of the spool bearing assembly were seized and would not rotate. One flyweight was bent and did not pivot freely. The drive shaft was found fractured at the guide post and at the drive spline. The PTG was sent to the NTSB Materials Laboratory, Washington, DC, for further examination. Figure 6 – Internal view of the PTG On September 16, 2016, the NTSB examination of the PTG revealed that a portion of the fractured drive shaft remained embedded in the spindle of the spool bearing assembly. The fracture surface features of the shaft were consistent with overstress. The outer cap of the spool bearing assembly was removed, and the ball bearings and spacers were found coated with voluminous, powdery, black particulate. Much of the powder fell from the assembly upon removal of the cap. No grease was observed. Disassembly of the bearings revealed that the ball retainers were fragmented, the inner surfaces were found coated with a powdery, black particulate, and no grease was observed. The inner bearing surfaces were rough and frosted. Figure 7 shows the disassembled pieces of the spool bearing. Figure 7 – Disassembled spool bearing with labels 14 CFR Part 133 Operations Specifications The Operations Specifications for Rotor Blade, LLC, as approved by the FAA, states: The owner or operator of the aircraft identified in the certificate holder or operator' s aircraft listing is primarily responsible for maintaining that aircraft in an airworthy condition as required by 14 CFR 91.403(a) and Part 39. PTG Information According to the component maintenance manual, the model AL-AA2 PTG is an element of the engine fuel controlling system. The function of the governor is to maintain the speed of the power turbine (N2) by resetting the main fuel control; the PTG supplements the main fuel control. This resetting establishes the gas producer speed (N1) required to supply N2. The PTG is mounted on the accessory case and senses N2 speed through reduction gearing. When an N2 off-speed condition is sensed by the PTG, it supplies a signal to the fuel control to change N1 speed to eliminate the off-speed condition. A complete description of the PTG is available in the public docket for this accident. In 2003, Honeywell introduced a dual-spool bearing for the PTG to lower cost of ownership and commonize the design. The bearing installed in the accident PTG was the dual-spool bearing. The dual-spool bearing replaced the legacy design single-spool bearing; the legacy design had no previous service issues. Honeywell reported that the dual-spool bearing had experienced a total of 23 field failures before this accident. The spool bearing failures led to either engine oscillations, uncommanded engine acceleration, or a loss of engine power. Honeywell Service Bulletin (SB) GT-73-344 Honeywell issued SB GT-73-344, Revision 2, on October 30, 2008, to replace the bearing assembly on PTGs used on Rolls-Royce 250 series engines in order to increase PTG reliability. The SB applied to several PTG models including the AL-AA2 model on the accident engine. Revision 1 was issued March 7, 2008. Rolls-Royce Commercial Engine Bulletin (CEB) 1402 Rolls-Royce issued CEB 1402 on April 21, 2008, to increase PTG reliability by incorporating a new bearing assembly. The CEB referenced Honeywell SB GT-73-344 and specified compliance times. The SB and CEB were issued after the accident PTG was installed new, and the SB and CEB were applicable to it. According to the CEB compliance times, the accident PTG's dual-spool bearing should have been replaced with a single-spool bearing no later than 750 hours TT since new. As a result of this investigation, Rolls-Royce issued a Commercial Service Letter (CSL), revision 1, on November 11, 2016, to remind customers that there are engines operating in the field that have not complied with CEB 1402 and other CEBs. The CSL recommends that customers should review the referenced CEBs to determine if they are applicable to their engine. The CSL also states: "Rolls--Royce has been involved in investigations where failure of the user to comply with the identified bulletins resulted in an uncommanded engine power reduction. It is the owner/operator's sole responsibility to comply with the identified bulletins within the specified timeframe or risk a potential for loss of aircraft or loss of life. Rolls-Royce is not responsible for an owner/operator's failure to comply." Manitowoc County Coroner's Office, Fond du Lac, Wisconsin, completed an autopsy on the pilot, and the cause of death was blunt force trauma to the head and chest. The pilot was wearing an MSA LH250 flight helmet during the accident. The Bioaeronautical Research Laboratory at the FAA's Civil Aerospace Medical Institute conducted toxicology testing, which revealed the presence of amlodipine and atorvastatin and was negative for other substances. Amlodipine (brand name Norvasc) is a prescription medication used to treat high blood pressure. Atorvastatin (brand name Lipitor) is a prescription medication used for lowering high blood cholesterol. The pilot had previously reported these medications to the FAA. The pilot's logbooks were not found during the course of the investigation. Company flight log reports revealed that the pilot flew the accident helicopter from March 8 to May 3, 2016, for a total of 67 hours. The pilot also flew the accident helicopter for an estimated 2.5 hours on the morning of the accident. The MD 369E features a fully articulated five-bladed main rotor system with anti-torque provided by a four-bladed semi-rigid tail rotor. The helicopter was configured to be flown from the left pilot seat (figure 3). Figure 3 – Preaccident photograph of the helicopter The helicopter had accumulated 7,688.4 hours total time (TT) at the time of the accident. A review of the maintenance records revealed an annual inspection was completed on December 4, 2015, at 7,522.6 hours TT. At the time of the accident, the engine, a Roll-Royce 250-C20B, had accumulated 1,048.7 hours TT since new. On April 3, 2016, at 987.4 hours engine TT, a 150-hour engine inspection was completed, and a fuel control tube was replaced. Also replaced during the maintenance work were the engine combustion case, combustion liner, and engine bleed valve due to a power