Pilot Debrief

BELL 206-L4 near Anaconda, MT — 2024-04-26

Final reportWPR24FA132
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Date
2024-04-26
Location
Anaconda, MT, USA
Aircraft
BELL 206-L4
Registration
N988B
Category
Helicopter
Highest injury
Fatal
Fatalities
1

Probable cause

A total loss of engine power due to a loss of cooling oil to the turbine-to-compressor coupling shaft and subsequent fracture of the shaft at an altitude too low for the pilot to complete a successful autorotation. Contributing to the accident was carbon buildup in the piccolo tube screen and nozzles and the disintegration of the spur adapter gearshaft O-rings for reasons that could not be determined.

Contributing factors

Contributing to the accident was carbon buildup in the piccolo tube screen and nozzles and the disintegration of the spur adapter gearshaft O-rings for reasons that could not be determined.

NTSB narrative

The pilot of the helicopter was performing aerial application operations. Security video showed the helicopter approach the loading truck for a third load of fertilizer and complete an onload of fertilizer before departing. The helicopter reached about 150 ft above ground level (agl) and 40 kts groundspeed when it rotated about 180° to the left and descended, consistent with an emergency autorotation. The helicopter impacted an area of flat terrain in a mostly level attitude. Examination of the wreckage revealed no anomalies with the airframe or flight controls that would have precluded normal operation. Examination of the engine revealed that the gas producer turbine rotor did not turn when the N1 rotor was rotated. The N2 rotor was continuous from the 4th-stage power turbine rotor to the output driveshaft, but an audible rubbing or scraping sound was heard when it was rotated. The engine was disassembled and the turbine-to-compressor coupling shaft was found fractured into three pieces. Coking was observed in the forward and aft spline locations of the turbine-to-compressor coupling shaft and between the turbine-to-compressor coupling shaft and the power turbine outer shaft. Extensive coking was noted upon removal of the power-turbine-to-pinion-gear coupling shaft. The two O-rings of the spur adapter gearshaft, which manage oil distribution in the turbine-to-compressor coupling shaft, were not present in their designated grooves. Coked material was found that restricted oil flow in one orifice of the piccolo tube and blocked a second orifice of the oil jet to the No. 3 bearing. Analysis of the coked material revealed fluorocarbon rubber signatures consistent with O-ring material. The engine manufacturer stated they were not aware of any previous instances of O-rings disintegrating. The initiating event for the disintegration of the O-rings could not be determined. Maintenance records indicated that the spur adapter gearshaft, where the O-rings would normally be located, was last accessible when the engine was overhauled about 5 years (1,414.1 flight hours) before the accident. According to the engine manufacturer, a damaged or wrong part number O-ring (or a missing O-ring) may allow cooling oil flow to leak back into the gearbox rather than flow between the concentric shafts. The reduced oil flow between the shafts is not sufficient to cool the shafting below oil carboning temperatures, as evidenced by the finding of coked carbon material in the area of the fractured compressor coupling shaft. Carbon deposits on the outside diameter of the turbine-to-compressor coupling and the inside diameter of the power turbine inner shaft and turbine-to-compressor coupling can build up until rub occurs, causing interference between the shafts, resulting in frictional heating and ultimate failure, which subsequently resulted in a total loss of engine power. It is likely that the carbon buildup in the piccolo tube screen and nozzles reduced cooling oil flow to the turbine-to-compressor coupling and the turbine inner shaft that caused the shafts to operate at a higher temperature than the carboning limits of the oil, allowing coke to build up between the shafts. The reason for the carbon accumulation in the piccolo tube screen and nozzles was not determined. The helicopter’s Height-Velocity performance chart indicated that, in general, pilots should avoid operations below 600 ft agl and below 65 kts, when above 4,150 lbs gross weight, and 500 ft agl and below 45 kts when below 4,150 lbs gross weight. Operations within these parameters reduce the likelihood of completing a successful autorotation. Practice 180° autorotations are not recommended below 700 ft agl. Given the helicopter’s altitude and speed at the time of the engine power loss, the pilot likely had insufficient altitude from which to establish an autorotation and perform a successful landing following the loss of power. The pilot’s toxicology results indicate he had used the sedating antihistamine medication diphenhydramine. Although caution must be used interpreting the diphenhydramine level measured in postmortem subclavian blood, the level indicates a reasonable probability that the pilot was experiencing some associated sedation or psychomotor impairment at the time of the accident. However, given the lack of clear evidence for any deficiency of the pilot’s preflight or inflight performance, and the altitude at which the sudden total loss of engine power occurred, it is unlikely that the pilot’s use of sedating antihistamine medication contributed to the accident.

NTSB coding

Evidence available

  • Video
  • Photos
  • 19 docket documents
View NTSB final reportView NTSB docket

Docket documents19

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