N541SB

History of the Flight

On July 13, 1996, about 1325 Alaska daylight time, a Sikorsky CH-54A helicopter, N541SB, crashed during aero logging operations, about 16 miles north-northeast of Ketchikan, Alaska. The helicopter was being operated as a visual flight rules (VFR) local area flight under Title 14 CFR Part 133 when the accident occurred. The helicopter, registered to and operated by Silver Bay Logging Company, Juneau, Alaska, was destroyed. The certificated commercial pilot received serious injuries. The copilot, holder of a commercial helicopter certificate, received fatal injuries. Visual meteorological conditions prevailed. The flight originated at a nearby logging camp about 1308.

The operator reported that logging operations were being conducted near Shelter Cove, located in the Tongass National Forest. The helicopter, a surplus military aircraft certificated in the restricted category, was using a 200 foot long external cable to retrieve logs.

The helicopter was operating on its 4th cycle of the day and was hovering above nearby trees over an area of cut logs. Ground personnel, communicating with the pilot by radio, hooked the helicopter's cable to a metal choker cable that was cinched around 2 large logs with an estimated total weight of 18,000 pounds. The helicopter lifted the logs about 60 feet above the ground when a popping sound was heard coming from the helicopter and the tailrotor began to slow down. The helicopter was observed to spin counter-clockwise about 5 complete turns. The pilot released the load of logs from the end of the cable and commented over the radio that "we're going down". Ground personnel dropped to the ground under fallen trees. The helicopter descended toward the ground and the main rotor blades struck a 170 foot high spruce tree, separating the top 20 feet of the tree. The helicopter continued toward the ground and collided with steeply sloped terrain.

The accident occurred during the hours of daylight at latitude 55 degrees, 33.33 minutes north and longitude 131 degrees, 25.09 minutes west.

Crew Information

The pilot holds a commercial pilot certificate with a rotorcraft helicopter rating. He holds private pilot privileges with airplane single-engine and instrument airplane ratings. The pilot holds a type certificate in Boeing Vertol 107, Sikorsky S-61, and S-64 helicopters. The type certificates are limited to VFR only. In addition, the pilot holds a flight instructor certificate with a rotorcraft helicopter rating and a mechanic certificate with powerplant and airframe ratings. The most recent second-class medical certificate was issued to the pilot on March 4, 1996, and contained no limitations.

No personal flight records were located for the pilot and the aeronautical experience listed on page 3 of this report was obtained from information contained in the pilot/operator report submitted by the operator. According to the report, the pilot's total aeronautical experience consists of 7,985 hours, of which 5,060 hours were accrued in helicopters and 500 hours accrued in the accident helicopter make and model. In the preceding 90 and 30 days prior to the accident, the report lists a total of 125 and 60 hours respectively.

The copilot held a commercial pilot certificate with a rotorcraft helicopter rating. The most recent second-class medical certificate was issued to the pilot on January 25, 1996, and contained no limitations.

No personal flight records were located for the copilot and the aeronautical experience listed on Supplement E of this report was obtained from information contained in the pilot/operator report submitted by the operator. According to the report, the pilot's total aeronautical experience consisted of 850 hours, of which 100 hours were accrued in the accident helicopter make and model as second-in-command. The copilot's duties during aero logging operations include monitoring the engine and hoist gauges and recording the total turns and log weights.

Aircraft Information

The helicopter's tailrotor drive system consists of 6 individual drive shaft segments between the main rotor transmission and the intermediate gear box. A seventh shaft is positioned between the intermediate gearbox and the tailrotor gearbox that is mounted at the top of the tail boom pylon. Each of the 6 drive shaft segments are connected to each other by a coupling flange that is bolted to the aft end of each shaft. A stack of stainless steel discs, know as a Thomas coupling, is utilized to permit flexing of the drive shaft segments and is installed between the coupling flange and the aft end of each drive shaft segment. A Thomas coupling is comprised of between 16 to 22 steel discs separated by isolators. The coupling flange has a splined center section that mates to the forward splined end of the next drive shaft segment.

The forward end of each drive shaft segment has a splined shaft that is inserted into the adjoining coupling flange. Just aft of the splines is a polished bearing surface where a support bearing is installed. The support bearings are pressed onto the drive shaft, over the splines, and onto the bearing surface. The bearings are installed in a bearing support housing that incorporates a viscous vibration dampner between the bearing and the housing. The bearings are retained in the housing by a snap ring on either side of the bearing. The bearing support housings are attached to the upper surface of the tail boom. The bearings are lubricated with Mobilgrease 28 lubricant. The normal amount of grease (by weight) in a bearing is between 4 and 6 grams or between 30 to 50 percent.

The operator utilized temperature sensitive tape applied to each bearing housing as indicators of bearing heat. In addition, the helicopter's crew chief took periodic temperature readings from each bearing housing by using a hand held, laser temperature sensor that measured temperature readings while the helicopter was running. The bearings and housings were part of every turn-around inspection and bearing temperatures were hand checked immediately following a shut-down of the helicopter.

The helicopter maintenance records indicate that a vibration was apparent in the airframe on July 9, 1996. The crew chief for the helicopter reported the pilot commented about a medium to high frequency vibration. An inspection revealed 6 discs of the number 7 tail rotor drive shaft segment's Thomas coupling were cracked. The coupling was replaced and in addition, an inspection of the intermediate tail rotor gear box revealed 3 slivers of metal. A serviceability inspection of the intermediate gear box was completed on July 11, 1996, and the helicopter was returned to service.

On the date of the accident, the crew chief reported the pilot again commented about a vibration. A reinspection of the number 7 drive shaft revealed a slight bow in the alignment of the shaft. An adjustment of the shaft reduced the vibration. Following the completion of the first logging cycle of the day, the pilot reported the vibration was a higher frequency than the previous occasion. Following the completion of the second cycle, the helicopter was shut down and the crew chief conducted a visual and hand inspection of the aircraft with no unusual results.

After lunch, the pilot flew the third cycle of the day and he commented that the vibration was barely being felt. The pilot landed for fuel and an inspection was conducted while the helicopter was running. The crew chief inspected the tail rotor bearings by a hand and visual inspection of the bearing housings. The crew chief did not notice any signs of grease splattering on the tail boom adjacent to the bearing housings. The helicopter then departed on the 4th cycle of the day. The accident occurred during the 7th turn of the cycle, about 17 minutes after the last inspection.

Telephone conversations with former pilots of the helicopter revealed a concern about the vibration in the helicopter. One pilot reported previous movement of the number 5 tail rotor bearing within its housing, including migrating of the bearing beyond the snap ring that normally retained the bearing in the housing.

The operator reported that the drive shaft bearings were part number SB1111-105. The tail rotor bearings had accrued the following hours in service: Number 1; serial number B449-01022: 849.6 Number 2: serial number B449-01679: 329.7 Number 3: serial number B449-04132: 329.7 Number 4: serial number B449-04146: 329.7 Number 5: serial number B449-00942: 505.4

The operator reported that the accident helicopter may use either a SB1111-105 or a SB1111-3 bearing for the tailrotor drive shaft. The -105 tailrotor bearings are normally an "on condition" part when used on civil aircraft. For aero logging applications, the operator established a 1,000 hour life limit on the -105 bearings. The -3 bearings have a life limit of 500 hours that is specified in the military overhaul manual.

The number 5 bearing, positioned between the number 5 and number 6 drive shaft segment, was purchased in a lot of 9 SB1111-105 bearings from the U.S. Government as surplus items at a Defense Reutilization and Marketing Region (DRMR) sale in San Diego, California, about 2 years prior to the accident. The bearing was purchased "as is" by the Marsell Bearing Company, Fallbrook, California. The Marsell Company reported that the bearing was packaged in its original "Fafnir" box and bore the date of 1992. The company did not keep any records of the precise date of purchase nor was any record made of the serial numbers of each bearing.

On November 2, 1994, the number 5 bearing, again as a lot of 9 SB1111-105 bearings, was purchased from Marsell Bearing by the Aero Independent Bearing Company, Inc., Sun Valley, California. Aero Independent Bearing reported that the bearing was visually inspected for any apparent defects. This included an inspection ...