Accident Overview
On October 11, 2013, about 1244 Pacific daylight time, a Bell 206L-1 helicopter, registration N206KK, was destroyed during a forced landing following an in-flight loss of engine power and subsequent fire. The helicopter was operating under Title 14 Code of Federal Regulations Part 91 as an aerial observation flight, with a company visual flight rules flight plan filed. The cross-country flight originated from Yuma, Arizona, around 1200, with an intended destination of El Cajon, California. The airline transport rated pilot and two passengers were not injured. Visual meteorological conditions prevailed.
Pilot Report and Emergency Response
According to the pilot's written statement to the National Transportation Safety Board (NTSB) investigator-in-charge, during cruise flight he heard a loud pop followed by an immediate loss of engine power. He lowered the collective and entered an autorotation. The left front door opened, and the cabin filled with black smoke. The helicopter landed upright in an open area, and the pilot and passengers egressed. Upon exit, the pilot saw fire originating from about 2 feet aft of the right forward door.
Wreckage Examination
Examination by a Federal Aviation Administration (FAA) inspector revealed that the fuselage was mostly consumed by fire. All primary components were located at the accident site. The wreckage was recovered to a secure location for further examination. On October 21, 2013, representatives from Bell Helicopter, Rolls-Royce, Blackhawk Helicopters, and the FAA examined the wreckage at the operator's hangar. Little of the aluminum structure, magnesium components, or plastic/composite material remained. Cockpit instruments and controls were consumed by fire. An approximate 4-foot section of the tail boom, including the tail rotor and vertical fin, was not damaged by fire. The tail rotor turned and changed pitch freely, with no evidence of impact damage or operational failure.
Main rotor blades were largely consumed by fire; blade grips on the main rotor mast were fire damaged. The main rotor transmission housing was consumed, exposing the gearing. A few feet of each main rotor blade tip remained, exhibiting fire damage but no impact damage. The engine showed evidence of intense fire and the magnesium auxiliary gearbox housing was consumed. High-energy debris from the turbine section was observed. Several gears from the accessory gearbox were found within debris, but not all could be accounted for. The helical torquemeter gear and power output gear were found; the idler gear and N2 tachometer gear were not located. The centrifugal breather gear and two unidentifiable idler gears were found melted in a pool of solidified aluminum. The fuel pump/oil pump gear was thermally damaged. The spur adapter gearshaft remained in place, also thermally damaged. No engine accessories were located in the recovered debris.
Engine Examination
On November 13, 2013, the engine was examined at Rolls-Royce's facility near Indianapolis, Indiana, under NTSB supervision. The exhaust collector and duct had impact marks and holes consistent with high-energy debris. High-energy debris penetrated the starboard-side compressor discharge tube. Several exit holes on the exhaust collector were in line with gearbox-mounted accessories. The compressor module was disassembled.
The number 2 1/2 bearing was visually inspected with no operational damage noted. The number 1 bearing was dry with no operational damage. The number 2 bearing was also dry with no operational damage. The compressor bleed valve housing and diaphragm were consumed by fire; the remaining spring and poppet valve showed no operational damage. The compressor shroud housing was removed, exposing the impeller with no evidence of foreign object damage or contact.
The helical power train drive assembly, the only intact accessory gearbox component, was examined. It included the housing, pinion gear, and number 3 and 4 bearings. The pinion gear coupling was bound within the pinion gear. The number 3 bearing was intact, dry, and coated with powdery residue. The number 4 bearing separator cage was fractured, with rotational scoring on the outer race surface. The number 4 bearing rollers appeared exposed to intense heat; one roller was flattened and out-of-round, consistent with skidding damage.
The number 5 bearing retaining snap ring was dislodged, and the outer race had impact damage. Two fragments of the race liberated upon removal, exposing ball bearings with no operational damage. The turbine to compressor coupling was fractured in two locations, with the forward end stuck within the power turbine outer shaft and the aft end jammed within the power turbine rotating labyrinth seal. Fracture surfaces were consistent with overload.
The combustion outer case and inner liner showed no failure or abnormal combustion. The 1st stage turbine nozzle shield was undamaged, with no metal splatter. The number 8 bearing sump cover exhibited contact with the turbine tie bolt and lock nut. The number 8 bearing was dry but showed no operational failure. The bearing lock nut was backed off and loose within the bearing chamber; impact marks on the sump cover were consistent with the lock nut being centered in place on the tie bolt. The tie bolt was fractured, with both fragments showing tension overload signatures.
The oil supply fitting for the 6/7 bearing and number 8 bearing were coated with blackened oil; all passages and filter screens were clear. The number 6 and 7 bearings were dry with no operational failure.
The exhaust collector was separated from the turbine module. The nozzle had impact damage on several vane trailing edges; the inner web had separated and deformed from contact with the power turbine outer shaft. The 4th stage turbine rotor blades and shroud were separated, fracture signatures consistent with overload. The rotor's aft curvic coupling was smeared; the forward face of the rotor's web had rotational scoring.
The 3rd stage turbine rotor shroud was separated, and all turbine blades were separated about mid span, consistent with overload. The 3rd stage turbine nozzle had impact damage on several vanes; the forward curvic coupling was smeared. The 2nd stage turbine rotor was intact, but the stub shaft portion fractured in overload; turbine blades had impact marks on trailing edges. The 1st stage turbine rotor was intact, with impact and rub damage to leading edges of all blades. The 1st stage turbine nozzle had impact damage along trailing edges.
Metallurgical Examination
Components were examined by Rolls-Royce's Failure Analysis Laboratory. The number 4 bearing, power train pinion gear, and turbine shaft to pinion gear coupling were oxidized from the engine fire. The turbine shaft to pinion gear coupling rotated in the power train pinion gear consistent with spline disengagement. The number 4 bearing separator was fractured through one roller pocket at three locations; the separator fragment was retained. Surface rail fractures were oxidized, and a specific fracture mode could not be identified. All but one roller were cylindrical with no skidding damage; the roller from the fractured pocket had a flat spot consistent with skidding damage.
The outer ring raceway was distorted; rail surfaces were worn from contact with the separator after fractures. The raceway was distorted and inner diameter surface severely worn, consistent with the inner ring having spun on the power train pinion gear. Both rings were oxidized from the post-landing fire. A cross-section through the number 4 bearing inner ring revealed wear, distortion, and radial cracks extending inward from the raceway. The entire microstructure was thermally damaged, consistent with spinning on the pinion gear and post-landing fire. Cross-sections through the outer ring and flat-spotted roller showed wear and distortion, consistent with a skidding roller. Semi-quantitative XRF analysis determined the outer ring and a roller were AMS 6491 (M-50 steel) and the inner ring was AMS 6278 (M-50 NIL steel), as required. The number 4 bearing showed evidence of spinning on the power train pinion gear.
According to the Rolls-Royce representative, the generated heat caused expansion and softening of the aft end of the power train pinion gear, resulting in disconnection from the turbine shaft-to-pinion gear coupling splines. This disconnect suddenly removed the load on the power turbine, leading to an overspeed of the 3rd and 4th stage turbine wheels and subsequent separation of their shrouds and blades, some of which were liberated from the engine.
Oil filters retained by the operator's mechanic were examined. Back-flushing and analysis of particle contaminants revealed chemical makeup consistent with two alloys: AMS9310 low alloy steel and M50 NIL. The pinion gear is manufactured from AMS9310, and M50 NIL is used in bearing races.
Maintenance Records
Review of helicopter maintenance records showed that in June 1996, the gearbox logbooks indicated compliance with CEB 72-3193 Rev-2 by Omni Helicopters (on a different engine, CAE-895018). This CEB installed number 3 and 4 bearings, part number 23030917. The number 4 bearing, serial number JJ-01236, and number 3 bearing, serial number JJ-01235, were installed. In October 2000, the gearbox underwent a sudden stoppage/hard landing inspection while owned by Omni Helicopters. Ownership transferred to JBI Aviation in 2001, which sent the gearbox to Arrow Aviation for the required inspection. During that inspection, logbooks indicated compliance with CEB 72-3193 Rev-3, but CEB A-72-3217 should have been incorporated, which requires replacement of number 3 and 4 bearings with new part number 23066678. There was no indication that any bearings were changed at that time.
In April 2010, the turbine module was overhauled at AeroMaritime. Work order paperwork specified no CEBs were accomplished during overhaul. However, a gearbox logbook entry dated April 17, 2010, stated the number 5 bearing was replaced to comply with CEB 72-3158. There was no mention of compliance with CEB A-72-3217. During engine disassembly, the number 3 bearing was found updated to part number 23066678, but it could not be determined when. The number 4 bearing found installed had the same serial number and part number as installed in 1996.
CEB-A-72-3217 specifies that all Rolls-Royce Model 250-C30/C30S engines, serial numbers CAE-890001 through 890888, must have the number 3 and 4 bearings replaced with new part number 23066678 no later than December 31, 2002. This CEB was released January 15, 1998, with the latest revision (4) released October 4, 2001.