What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
On logging expedition when rotor blade strcuk trees. (.4)the pilot & copilot were on an external load, aero logging operation in a military surplus helicopter, lifting an estimated load of 18,000 lbs. Ground personnel heard a popping sound, then saw the tailrotor begin to slow down as the helicopter began to yaw/spin. It descended to sloping terrain in an area of cut logs. Postcrash exam of the tailrotor drive shaft revealed a separation at the number 5 bearing position. The shaft separation exhibited evidence of high heat & melting of adjoining shaft surfaces. The bearing & its housing were not recovered. The bearing had accrued 505 hrs of service, & was 1 of 2 bearing model numbers that were in use. The bearing was manufactured in 1991, & was purchased by the operator from surplus military supplies. The supplier (of the bearing to the operator) had performed an exterior exam of the bearing & marked 'relubed 10/95' on the bearing box. Exam of the remaining tailrotor drive shaft bearings revealed evidence of low grease fill, water & glycol contamination, & wear patterns consistent with misalignment. In civilian service, the bearing was an 'on condition' part. The operator established a service life of 1,000 hours for aero logging operations. A shelf life for the bearing was not established. The manufacturer indicated the bearing should support a 5 year shelf life. Other operators of the accident helicopter in aero logging reported similar examples of low grease fill & bearing contamination from water in both available models of bearing.
As the FAA filed it
ON LOGGING EXPEDITION WHEN ROTOR BLADE STRCUK TREES. (.4)THE PILOT & COPILOT WERE ON AN EXTERNAL LOAD, AERO LOGGING OPERATION IN A MILITARY SURPLUS HELICOPTER, LIFTING AN ESTIMATED LOAD OF 18,000 LBS. GROUND PERSONNEL HEARD A POPPING SOUND, THEN SAW THE TAILROTOR BEGIN TO SLOW DOWN AS THE HELICOPTER BEGAN TO YAW/SPIN. IT DESCENDED TO SLOPING TERRAIN IN AN AREA OF CUT LOGS. POSTCRASH EXAM OF THE TAILROTOR DRIVE SHAFT REVEALED A SEPARATION AT THE NUMBER 5 BEARING POSITION. THE SHAFT SEPARATION EXHIBITED EVIDENCE OF HIGH HEAT & MELTING OF ADJOINING SHAFT SURFACES. THE BEARING & ITS HOUSING WERE NOT RECOVERED. THE BEARING HAD ACCRUED 505 HRS OF SERVICE, & WAS 1 OF 2 BEARING MODEL NUMBERS THAT WERE IN USE. THE BEARING WAS MANUFACTURED IN 1991, & WAS PURCHASED BY THE OPERATOR FROM SURPLUS MILITARY SUPPLIES. THE SUPPLIER (OF THE BEARING TO THE OPERATOR) HAD PERFORMED AN EXTERIOR EXAM OF THE BEARING & MARKED 'RELUBED 10/95' ON THE BEARING BOX. EXAM OF THE REMAINING TAILROTOR DRIVE SHAFT BEARINGS REVEALED EVIDENCE OF LOW GREASE FILL, WATER & GLYCOL CONTAMINATION, & WEAR PATTERNS CONSISTENT WITH MISALIGNMENT. IN CIVILIAN SERVICE, THE BEARING WAS AN 'ON CONDITION' PART. THE OPERATOR ESTABLISHED A SERVICE LIFE OF 1,000 HOURS FOR AERO LOGGING OPERATIONS. A SHELF LIFE FOR THE BEARING WAS NOT ESTABLISHED. THE MANUFACTURER INDICATED THE BEARING SHOULD SUPPORT A 5 YEAR SHELF LIFE. OTHER OPERATORS OF THE ACCIDENT HELICOPTER IN AERO LOGGING REPORTED SIMILAR EXAMPLES OF LOW GREASE FILL & BEARING CONTAMINATION FROM WATER IN BOTH AVAILABLE MODELS OF BEARING.
NTSB investigation
The NTSB’s investigation of this event, with its probable cause and findings. Matched to this report by N-number and date.