Occurrence
A Bell 206B helicopter, registration C-GXNM (serial number 1111), with a pilot and sole occupant, was conducting a visual flight rules (VFR) flight from Dryden to Red Lake, Ontario. This was the first leg of a journey to transport the pilot and equipment to a contract in the Northwest Territories. The start-up and climb-out were normal, and the helicopter was established on course at 2,500 feet above dense trees and a river. A loud but muffled bang was heard near the rear of the helicopter. The aircraft lurched, and the engine-out warning sounded. The pilot commenced an autorotation to a clearing approximately one-half mile to his left. As he approached the clearing, he realized that the rate and angle of descent would result in landing in a flooded area, so he extended the glide to reach a field. The helicopter touched down with a forward speed of 10 to 12 miles per hour and pitched tail-up as it slid to a stop in an upright attitude on the high skid gear. The tail boom and tail rotor assembly were severed from the helicopter by the main rotor blades. The pilot was not injured.
Engine Examination
The engine was removed from the helicopter. A preliminary teardown determined that the effects of oil starvation of the number 6 and 7 bearings resulted in heat damage to adjacent components and a decoupling of the turbine and compressor sections. The power turbine scavenge sump was dry, and no ferrous material had transferred to the chip detectors to warn of the impending engine failure. The partially disassembled turbine and oil samples were forwarded to the TSB Engineering Branch for detailed analysis.
Investigation Findings
The design of the tee-fitting and standpipe assembly is conducive to the formation of carbon between the inner wall of the tee-fitting and the standpipe of the 6/7 bearing pressure oil nozzle, where oil becomes trapped. A hard carbon deposit had built up in this area of the tee-fitting. The carbon build-up was considered excessive for the hours of operation since the completion of the heavy maintenance inspection.
The carbon particle that blocked the 6/7 bearing pressure oil nozzle displayed characteristics similar to the composition of the carbon that had formed inside the base of the tee-fitting, and the particle had a uniform curved surface that compared with the inside diameter of the tee-fitting wall. Therefore, it is likely that the carbon originated in the tee-fitting, then broke loose and moved to block the nozzle. Although the manufacturer's data indicate that build-up of carbon in this area has not previously resulted in engine failure, it is concluded that it likely did in this case.
An inspection of pressure oil connector tee-fittings from four independent turbines revealed that carbon deposits had formed in the base of three of the four tee-fittings. The three tee-fittings had been in service for 1,000 hours or more; therefore, they would have been subjected to the drill-cleaning procedures outlined in the manufacturer's 300-hour inspection three or more times.
The current inspection and cleaning instructions may not ensure complete removal of carbon from the recessed area inside the base of the tee-fitting. Solvents may not dissolve all carbon, and reaming of the outflow port with a hand-held drill bit will not reach the area that needs to be cleaned. Considering the degree of carbon build-up in the base of the tee-fitting and the short operating time since accomplishment of the DIL 155 inspection, it is probable that the carbon was not fully removed during the overhaul process.
The engine began making smoke 12.8 flight hours after the overhauler completed a turbine heavy maintenance (DIL 155) inspection/overhaul. The overhauler and the operator's apprentice replaced the number 5 carbon seal, removed carbon from the scavenge strut and from the scavenge sump, and motored the engine to ensure that adequate oil flow was provided to the sump, but did not remove and clean the 6/7 oil pressure nozzle and tee-fitting as specified in the manufacturer's Operating and Maintenance Manual.
The turbine had been overhauled less than 14 flight-hours earlier, and the scavenge strut had been cleaned within the last flight-hour. The operator did not complete a power turbine scavenge flow test and did not remove and clean the pressure oil and scavenge components, as outlined in the 300-hour inspection procedures.
A build-up of hard carbon in the tee-fitting of this engine was considered excessive for the hours of operation since the completion of the heavy maintenance inspection. It is probable that the carbon was not fully removed during the overhaul process. A particle of carbon blocked the 6/7 bearing pressure oil nozzle, and the engine failed as a result of the lack of lubrication and the excessive heat damage in the vicinity of the number 6 and number 7 bearings. It is likely that the hard carbon that blocked the oil flow originated in the tee-fitting.
The design of the tee-fitting and standpipe assembly is conducive to the formation of carbon in an area between the inner wall of the tee-fitting and the 6/7 bearing pressure oil nozzle. The manufacturer's procedures do not specifically identify the area within the base of the tee-fitting in which carbon builds, nor do the procedures identify how to inspect this area that is hidden from view. Tee-fittings from three of four independent turbines that were inspected had developed carbon deposits in the base of the tee-fittings, although they would have been subject to cleaning three or more times during 300-hour inspections.
Safety Action
An Aviation Safety Information letter was sent to Transport Canada outlining the particulars of the carbon build-up inside the base of the pressure oil tee-fitting as identified during this investigation.