Arcadia, IA, USA
N169AA
BELL HELICOPTER TEXTRON 206B
The pilot reported that he was completing an aerial application flight about 6 ft above the corn field when the engine sustained a loss of power. The helicopter descended into the corn and impacted the field. A small postcrash fire ensued near the engine and was extinguished. Postaccident examination revealed all external air, fuel, and oil line connections were tight except for the pneumatic air line (Pc line) B-nut connecting the power turbine governor (PTG) to the fuel control unit (FCU). The Pc line was not connected to the FCU and a gap was noted between the B-nut and FCU union. Residual torque paint was identified; however, there was no new-effective torque paint identified on any B-Nut. It appeared the B-nut had backed off the FCU fitting. This Pc is one of several that use air pressure to communicate fuel scheduling demands between the PTG and FCU. The security of this line is necessary for proper engine operation. Operation of the engine without the B-nut properly connected would result in the FCU reducing fuel to a sub-idle condition, with the engine not capable of producing power for continued flight. There were no other mechanical functions or failures that would have precluded normal operation. Maintenance was performed on the helicopter 23 days and 12 flight hours before the accident. It is likely that the maintenance personnel did not properly torque this B-but or apply torque stripe paint, which resulted in a disconnection of the B-nut and a loss of engine power.
On August 1, 2022, about 0915 central daylight time, a Bell 206B helicopter, N169AA, was substantially damaged when it was involved in an accident near Arcadia, Iowa. The pilot sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he was completing an aerial application run about 6 ft above the corn field when the engine sustained a loss of power. The helicopter descended into the corn and impacted the field. A small postcrash fire ensued near the engine and was extinguished. The pilot stated that the helicopter recently had significant maintenance performed and he had only flown it about 12 hours since then. Postaccident examination of the helicopter revealed thermal damage and soot noted on the top of the engine and cowling with fire retardant residue noted. The engine compressor turbine (N1) system was not examined. The engine power turbine (N2) system was locked and the No. 5 bearing area of the exhaust collector was buckled. The fourth stage turbine wheel appeared normal when viewed from the exhaust collector but was covered in fire retardant residue. A fuel sample was taken from the aircraft fuel filter bowl; the fuel was clear and appeared normal. The fuel filter element was clear of contamination. An oil sample was taken from the aircraft-mounted scavenge oil filter bowl and was dark brown in appearance. The filter element was clear and some minor non-ferrous debris was noted in the bowl, consistent with carbon particles. The pending bypass button was not extended. Linkages from the collective to the PTG and from the throttle twist grip to the FCU were secure. The FCU throttle input lever was rigged correctly and contacted the minimum and maximum stops when the throttle was rotated to the cut off and fly position respectively. The PTG rotated through about 20° when the collective was moved from full down to full up. All external air, fuel, and oil line connections were at least finger tight except for the Pc line B-nut connecting the PTG to the FCU. The Pc line was not connected to the FCU and a gap was noted between the B-nut and FCU union (Figure 1). Residual torque paint was identified; however, there was no new-effective torque paint identified on any B-Nut. This Pc line is one of several that use air pressure to communicate fuel scheduling demands between the PTG and FCU. Figure 1. Disconnected Pc line B-nut that connects the PTG to the FCU. The only anomaly observed that would have precluded normal engine operation was the disconnected B-nut on the Pc line. On July 9, 2022, about 23 days before the accident flight, a maintenance facility completed 100-hour engine and airframe inspections, 300-hour engine and airframe inspections, a 12-month airframe inspection, an engine oil change, and many other inspection items. Additional maintenance work included removing the turbine assembly for an over-temperature inspection and turbine outlet temperature indicator reset due to the over-temperature event. During a postmaintenance engine start attempt, the fuel management system failed to properly control the temperature. The engine start was aborted with no exceedances noted. A fuel nozzle was removed, and a newly overhauled fuel control unit was installed. A postmaintenance ground run was completed. The logbook noted that an independent control check was performed for security and safety of, among other things, the fuel control linkage at the control lever and the droop compensator at the governor control arm. On July 10, 2022, a postmaintenance ground run and test flight were completed and the helicopter was returned to service. On July 12, the helicopter was flown from the maintenance facility in Langley Regional Airport (CYNJ), British Columbia, Canada, to Bellingham International Airport (KBLI), Bellingham, Washington. Helicopter total time was 13,248.3 hours. According to Rolls-Royce, the B-nut that was found loose is difficult to access when other tubes are installed. Rolls-Royce designed, developed, and released a special tool to access this B-nut. On October 24, 2016, it was released to the field via commercial service letter (CSL) 1273. The CSL provides details on how to manufacture the tool locally and the tool is also available for purchase. Operators are advised that torque paint/slippage indication is to be applied to all B-nuts per the Rolls-Royce operation and maintenance manual (OMM). Strict adherence to the specified manuals is essential to the safe and reliable operation of the engine. Failure to maintain the engine in accordance with the OMM and supporting customer publications can result in a decrease in airworthiness of the engine.
Maintenance personnel’s failure to properly torque a fuel control unit B-nut, which resulted in a loss of engine power at low altitude and an impact with terrain.
Source: NTSB Aviation Accident Database
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