# NTSB ERA14LA099: Bell 47D1, N93067, Pell City, AL, January 21, 2014 - Type: NTSB accident investigation - Classification: Accident · Minor injuries · Limited investigation · Final report - Date: January 21, 2014, 15:37 local (21:37 UTC) - Place: Pell City, AL - Coordinates: 33.565277, -86.303337 - Nearby events: https://openairdocs.org/map.md?lat=33.5653&lon=-86.3033&mi=10 - Phase of flight: [Approach](https://openairdocs.org/ntsb-phases/500) (OpenAirDocs' grouping) - Defining event: [Approach: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) - Highest injury: Minor injuries - Injuries: 2 minor - Status: Completed, report March 14, 2016 - Report: Approved by Office Director - NTSB office: Eastern Region Aviation - Parties: FAA FSDO, Scott's-Bell 47 Inc. - NTSB case key (mKey): 88695 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ERA14LA099 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/88695/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=88695 - Page: https://openairdocs.org/ntsb/ERA14LA099 ## Probable cause The pilot’s use of an improper clutch engagement procedure, which resulted in clutch glazing, slippage, and, ultimately, the in-flight disengagement of the clutch and a loss of rotor rpm. Contributing to the accident were the installation of a light-weight clutch assembly and a higher horsepower engine, both of which increased the tendency for clutch slippage, and inadequate inspection of the clutch assembly. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Action: incorrect action performance](https://openairdocs.org/ntsb-findings/02041015): Personnel issues › Action/decision · Pilot (probable cause); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-findings/02041015/cases.md?model=1180802), [All aircraft, 1,386 cases](https://openairdocs.org/ntsb-findings/02041015/cases.md) - [Use of equip/info: use of equip/system](https://openairdocs.org/ntsb-findings/02063015): Personnel issues › Task performance · Pilot (probable cause); cases with it: [Bell 47D1, 2 cases](https://openairdocs.org/ntsb-findings/02063015/cases.md?model=1180802), [All aircraft, 1,391 cases](https://openairdocs.org/ntsb-findings/02063015/cases.md) - [Main rotor drive: engine/transmission coupling](https://openairdocs.org/ntsb-findings/01046310): Aircraft › Aircraft propeller/rotor · Damaged/degraded (probable cause); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-findings/01046310/cases.md?model=1180802), [All aircraft, 24 cases](https://openairdocs.org/ntsb-findings/01046310/cases.md) - [Main rotor drive: engine/transmission coupling](https://openairdocs.org/ntsb-findings/01046310): Aircraft › Aircraft propeller/rotor · Incorrect use/operation (probable cause); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-findings/01046310/cases.md?model=1180802), [All aircraft, 24 cases](https://openairdocs.org/ntsb-findings/01046310/cases.md) - [Maintenance: modification/alteration](https://openairdocs.org/ntsb-findings/02062030): Personnel issues › Task performance · Maintenance personnel (probable cause); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-findings/02062030/cases.md?model=1180802), [All aircraft, 69 cases](https://openairdocs.org/ntsb-findings/02062030/cases.md) - [Maintenance: scheduled/routine maintenance](https://openairdocs.org/ntsb-findings/02062010): Personnel issues › Task performance · Maintenance personnel (probable cause); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-findings/02062010/cases.md?model=1180802), [All aircraft, 320 cases](https://openairdocs.org/ntsb-findings/02062010/cases.md) - [Inspection: scheduled/routine inspection](https://openairdocs.org/ntsb-findings/02061520): Personnel issues › Task performance · Maintenance personnel (probable cause); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-findings/02061520/cases.md?model=1180802), [All aircraft, 167 cases](https://openairdocs.org/ntsb-findings/02061520/cases.md) - [Main rotor drive: engine/transmission coupling](https://openairdocs.org/ntsb-findings/01046310): Aircraft › Aircraft propeller/rotor · Not inspected (probable cause); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-findings/01046310/cases.md?model=1180802), [All aircraft, 24 cases](https://openairdocs.org/ntsb-findings/01046310/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Prior to flight: Aircraft maintenance event](https://openairdocs.org/ntsb-events/040); cases with it: [Bell 47D1, 2 cases](https://openairdocs.org/ntsb-events/040/sequence.md?model=1180802), [All aircraft, 484 cases](https://openairdocs.org/ntsb-events/040/sequence.md) - [Prior to flight: Aircraft inspection event](https://openairdocs.org/ntsb-events/050); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-events/050/sequence.md?model=1180802), [All aircraft, 229 cases](https://openairdocs.org/ntsb-events/050/sequence.md) - [Standing-engine(s) start-up: Miscellaneous/other](https://openairdocs.org/ntsb-events/900); cases with it: [Bell 47D1, 1 case](https://openairdocs.org/ntsb-events/900/sequence.md?model=1180802), [All aircraft, 1,556 cases](https://openairdocs.org/ntsb-events/900/sequence.md) - [Approach: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) (defining event); cases with it: [Bell 47D1, 2 cases](https://openairdocs.org/ntsb-events/330/sequence.md?model=1180802), [All aircraft, 994 cases](https://openairdocs.org/ntsb-events/330/sequence.md) - [Autorotation: Hard landing](https://openairdocs.org/ntsb-events/092); cases with it: [Bell 47D1, 5 cases](https://openairdocs.org/ntsb-events/092/sequence.md?model=1180802), [All aircraft, 2,102 cases](https://openairdocs.org/ntsb-events/092/sequence.md) ## Analysis The commercial pilot was completing a business flight in the helicopter. He reported that, while descending the helicopter for a confined off-airport landing, at 25 to 35 ft above ground level, he heard a "snap/pop" and that the engine suddenly began "over-revving." The helicopter started descending and then touched down hard, and one of the main rotor blades struck the helicopter's tail. After touchdown, the engine was still running, but the rotor blades were not moving. Examination of the transmission revealed that it incorporated a light-weight clutch assembly, which was not authorized for use in the installed transmission model. Further examination revealed that the clutch shoe running surface on the inner diameter of the clutch drum exhibited a series of abnormal wear grooves along the entire surface of clutch shoe contact and that the clutch lining of each shoe exhibited glazing, which occurs when the clutch is improperly engaged. The contact surface of each clutch shoe also contained numerous metallic slivers that were atypical of a normal in-service clutch assembly and appeared to be material that had transferred from the drum to the shoes. During the transmission examination, the pilot was asked to describe his normal engine start and clutch engagement procedure. The clutch engagement procedure described by the pilot was not in accordance with the published procedure. The pilot's procedure likely resulted in a lengthy clutch engagement and an extended period of clutch slippage during each start that led to the glazed condition of the clutch assembly. Once the glazed condition began to develop, the period of clutch slippage during each start lengthened, further contributing to the glazed condition. Ultimately, the glazed condition resulted in the in-flight disengagement of the clutch and a loss of rotor rpm. The pilot reported that, about 15 flight hours before the accident, he had a mechanic install an engine in the helicopter that produced 25 more horsepower than the previously installed engine. The installation of the higher horsepower engine would have increased the tendency for slippage of a glazed clutch assembly. The pilot also reported that the transmission had accumulated about 615 hours since overhaul. According to the helicopter's maintenance and overhaul instructions, the transmission should be disassembled every 600 hours to inspect all parts, including the clutch, for damage or excessive wear. Given that the engine had been installed about 15 flight hours before the accident, the 600-hour inspection could have been accomplished at that time. However, review of maintenance records did not indicate that the 600-hour transmission inspection had occurred, and the mechanic who installed the higher horsepower engine reported that he did not disassemble the transmission. ## Factual narrative HISTORY OF FLIGHT On January 21, 2014, about 1537 central standard time, a Bell 47D-1, N93067, operated by Milam Inc. was substantially damaged during a forced landing near Pell City, Alabama. The commercial pilot and one passenger received minor injuries. Day, visual meteorological conditions prevailed, and no flight plan was filed for the business flight, operated under the provisions of Title 14 Code of Federal Regulations Part 91, which originated at Birmingham- Shuttlesworth International Airport (BHM), Birmingham, Alabama about 1330. According to the pilot, after meeting his passenger at BHM at 1300 they flew to an area just outside of Pell City, Alabama to look at a newly installed alarm system and to take some aerial photographs of the customer's property. After flying around the property which covered approximately 100 acres, and discussing how they could take the photographs, they landed and retrieved a camera from the "chin area" of the helicopter. The pilot then discussed with the passenger how he could fly along the borders of the property so the passenger could take the photographs he desired. They then departed and flew "the plan" they had discussed, and then returned to the spot where they had landed before. While descending into the confined area for landing, at 25 to 35 feet above ground level, the pilot heard a "snap/pop" and the engine suddenly began "over-revving." The helicopter began descending, and due to the undulating terrain that was below them the pilot could only extend the flight slightly forward, and within approximately 3 seconds, he had used up all of the available rotor speed. The helicopter then touched down hard and one of the wooden main rotor blades struck the tail of the helicopter. The engine was still running but with no rotor motion. The pilot then shutoff the magnetos and battery switches, and he and the passenger exited the helicopter. PERSONNEL INFORMATION According to Federal Aviation Administration (FAA) and pilot records, the pilot held a commercial pilot certificate with ratings for helicopter, airplane single-engine land, Airplane multi-engine land, and instrument airplane, with private pilot privileges for airplane single-engine sea. He also held a flight instructor certificate with a rating for helicopters, and repairman experimental aircraft builder certificate, with an inspection certificate for an experimental Rotorway Executive helicopter he had built. His most recent FAA second-class medical certificate was issued on April 4, 2013. He reported that he had accrued 4,430 total hours of flight experience, 1,086 of which were in make and model. AIRCRAFT INFORMATION The accident aircraft was two-bladed, single engine, light helicopter which according to the FAA registry was manufactured by Bell Helicopter Company in 1965 as a Bell model 47D-1, serial number 5501. According to maintenance records, the helicopter's most recent annual inspection was completed on June 9, 2013. At the time ofaccident, the helicopter had accrued approximately 3,980 total hours of operation. Bell 47 History and Build Records On March 8, 1946, the Bell 47 was certificated for civilian use by the Civil Aeronautics Administration. More than 5,600 were produced with the helicopter being manufactured in the United States, and under license in Italy, Japan, and the United Kingdom. Review of Bell Helicopter Company build records by Scott's-Bell 47, which purchased the type certificate from Bell Helicopter Textron in 2010, revealed that despite what was recorded in the FAA registry, the accident helicopter was not built by Bell Helicopter Company or any of its licensees. According to build records, at the time that the accident helicopter was listed as being manufactured in 1965, Bell Helicopter was manufacturing two production blocks of helicopters, with serial numbers 3958 through 4013, being built as OH-13 models for the United States Army, and serial numbers 6501 thru 6513, being built as 47G-3B-1 models, for the commercial market. Bell produced no helicopters during that time period bearing serial numbers within the gap (4014 thru 6500) which existed between the two production blocks, which would have encompassed manufacturer's serial number 5501. Further review of build records also revealed that no civilian Bell Model 47 was ever delivered bearing the serial number 5501 by Bell, nor were there any military Bell helicopter models similar to the model 47 under any year of manufacture, that would have been assigned a sequential serial number and/or military ship number by Bell or the military services, which would have consisted of the 2-digit year of manufacture, and military serial number that could have been similar to the 5501 serial number on the accident helicopter. FAA Certification Data Review of FAA Type Certificate Data Sheets (TCDS) and a search of FAA's Electronic Data Retrieval System (EDRS) revealed that with Federal Aviation Agency (Successor to the CAA, which later became the Federal Aviation Administration under the Department of Transportation) approval, a number of entities "manufactured" helicopters from Bell spare and surplus parts, which the Federal Aviation Agency issued airworthiness certificates for, and then classified as Bell 47s in the registry, though Bell had not manufactured them as the type certificate holder. These included approximately: - 20 entities who had manufactured Bell 47D-1 models. - 16 entities who had manufactured Bell 47G models. - 9 entities who had manufactured Bell 47G-2 models. Further review of FAA TCDS and FAA EDRS data indicated that a helicopter manufactured as a 47D-1 bearing serial number 5501, was listed as an aircraft eligible for designation as a 47D-1, under the name of Louis Wm. Cottriel. Identification (Data) Plate According to the FAA, Each manufacturer of aircraft, aircraft engines, propellers, and propeller blades and hubs is required to display the following information on a fireproofplate, by means of stamping, engraving, etching, or another approved method: (1) The builder's name, which may be the name of an individual, firm, co-partnership, corporation, company, association, or joint-stock association. (2) Model designation. (3) Builder's serial number. (4) TC number, if any. (5) Production certificate (PC) number, if any. (6) For aircraft engines, the established rating. (7) For aircraft engines, the date of manufacture, that is, month (two digits)/ year (four digits). (8) For aircraft engines, the engine's approved 14 CFR part 34 designations. (Part 34 discusses fuel venting and exhaust emission requirements for turbine engine-powered airplanes.) Approved designations include comply, exempt, and non-U.S. for engines specified in part 34, manufactured on or after January 1, 1984. The fireproof plate is required to be placed on a surface where it is not likely to be defaced or removed during normal service. The identification cannot be placed on an inspection plate, or removable fairing. On aircraft manufactured before March 7, 1988, the identification plate is required to be mounted on an interior or exterior location near an entrance, so that it is legible from the ground either just behind and next to the rear-most entrance door or on the underside of the fuselage near the tail surface. The model and serial numbers also are required to be displayed on the fuselage exterior. In regards to identification plate requirements for aircraft built from spare or surplus parts, In addition, to the requirements listed above, the following guidelines should be followed: (1) Use the builder's name as the name (the person who assembled the aircraft), not the name of the manufacturer who builds the same model of aircraft. (2) Assign any serial number if it is clear that the manufacturer who builds the same model of aircraft did not assign an identical number. A letter prefix or suffix should also be added such as a name or initials, to the serial number to provide positive identification. Examination of the accident helicopter's data plate revealed however that even though it had the basic information required, it had been modified with multiple overlays: - Starting at the top of the data plate, the name "WORLD HELICOPTERS, INC." was displayed. - Just below that was an overlaid layer on the data plate with the words "LOUIS WM. COTTRIEL, BUILDER" with an address in Mineral Wells, Texas. - Below that stamped into the original data plate was; "SERIAL NO. 5501 MODEL 47D-1." - Below that stamped into the data plate was; "DATE MFD. May 20, 1965." The "65" however was printed with the numbers "65" not aligned with the "19" on the data plate with the "5" appearing to be smudged. - Below that stamped into the data plate was; "ENGINE FRANKLIN." - Below that stamped into the data plate was; "ENGINE SERIAL NO." but no engine serial number was listed. - Another overlay was below this which covered two other stamped data blocks in the data plate.Printed on the overlay were the words "THIS HELICOPTER MANUFACTURED FROM NEW BELL SPARE AND SURPLUS PARTS." - Below that, printed on the date plate, heavily worn and almost illegible, were the words; "THIS HELICOPTER CONFORMS TO T.C. H-1 SAME AS BELL AIRCRAFT CORPORATION HURST, TEXAS MODEL 47D-1" Previous Accident History Review of NTSB records revealed that the helicopter had been involved in 5 previous accidents: - The first accident occurred on September 1, 1966 (NTSB Case No. LAX67A0026), when the helicopter was substantially damaged after the helicopter developed a high sink rate and the pilot did not attempt a power recovery. - The second accident occurred on May 20, 1982 (NTSB Case No. FTW82DA189), when the helicopter was substantially damaged during an aerial application flight, when the pilot turned downwind and the tail rotor struck a rice dike. The pilot lost control and the helicopter landed hard. During the hard landing, the main rotor severed the tail boom. - The third accident occurred on November 3, 1984 (NTSB Case No. FTW85LA043), when the helicopter was substantially damaged on a sightseeing flight when the engine lost power over rough terrain at approximately 220 feet above ground level. During the touch down after the autorotation, the helicopter rocked forward and the main rotor blades struck the tail boom. - The fourth accident occurred on November 2, 1989 (NTSB Case No. MIA09LA023), when the helicopter was substantially damaged during an aerial application flight, the pilot pulled up and held back longer to clear trees at the end of the field. The helicopter lost airspeed faster than the pilot realized and the helicopter descended and impacted trees. - The fifth accident occurred on June 27, 1992 (NTSB Case No. MIA92LA069), when the helicopter was substantially damaged after a loss of engine power while maneuvering. The pilot was unable to maintain altitude and he performed an autorotation into trees. Review of FAA Airworthiness records revealed however that only two FAA 337 Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance) forms, were on file with the FAA: - The first was filed on April 10, 1967 after the first accident and it described the description of the work accomplished to repair the helicopter and return it to service, including replacement of the center frame, repair of the tail boom, patching of the bubble (windscreen), and replacement of the engine mount, tail rotor hub, tail rotor blades, main rotor blades, transmission, and tail rotor gearbox. - The second was filed on January 12, 1985 after the third accident and it described the description of the work accomplished to repair the helicopter and return it to service, including the repair made to the tail boom. Review of the FAA EDRS records revealed no other records describing the description of work accomplished to repair the helicopter and return it to service after the other three accidents it was involved in (NTSB Case Numbers;FTW82DA189, MIA09LA023, and MIA92LA069). METEOROLOGICAL INFORMATION The recorded weather at Anniston Metropolitan Airport (ANB), Anniston, Alabama, located 22 nautical miles east of the accident site, at 1553, included: winds 300 degrees at 8 knots, gusting to 19 knots, 10 miles visibility, overcast clouds at 3,500 feet, temperature 01 degrees C, dew point - 07 degrees C, and an altimeter setting of 30.03 inches of mercury. WRECKAGE AND IMPACT INFORMATION Wreckage Examination Examination of the wreckage by a Federal Aviation Administration (FAA) inspector revealed that the helicopter was substantially damaged. Approximately 4 feet of the tail section including the tail rotor had separated from the helicopter during the ground impact. The airframe and skid mounting tubes were bent; the acrylic cockpit bubble was cracked and broken. One blade of the two blade main rotor system displayed impact damage, was bent downward, and was broken about mid-span. Transmission Examination The transmission assembly, which was mounted on an adapter plate at the upper end of the engine, was a speed-reducing mechanism for driving the main rotor, cooling fan, generator, tail rotor, and rotor tachometer generator. The cylindrical case was composed of a series of castings and rings held together by through bolts. The main rotor mast support bearing was mounted in the top of the upper section. Four drive assemblies, which drive the tail rotor drive shaft, cooling fan, generator, and rotor tachometer generator, were mounted to the lower section of the case. Internally, the assembly consisted of a centrifugal clutch, freewheeling unit, a two-stage planetary reduction gear train, spiral bevel gear to drive the tail rotor drive assembly, and a bevel gear to drive the cooling fan. The centrifugal clutch relieved the starter of the rotor load during starts and permitted rotation of the rotor manually for inspection without turning the engine. The freewheeling unit made possible autorotation landings with an idling or dead engine. The generator, tail rotor drive, and rotor tachometer generator were driven whenever the main rotor was in motion (power on, or power off). This made it possible for the pilot to have complete control of the helicopter during autorotation landings. Examination of the transmission revealed that the data plate affixed to the transmission contained information that the transmission assembly was originally meant for installation in a Bell 47E. The data plate contained part number 47-620-600; the original dash number ("9") was still visible though the data plate had been over-stamped with the number "17". Transmission serial number 3346 was clearly visible in the reserved block. Review of FAA TCDS revealed that the TCDS listed Bell helicopter serial number block from 165-173 as eligible for designation as a Bell 47E, Bell records revealed however that these serial numbers were built as Bell HTL-3 helicopters and were delivered to the US Navy in 1950 and1951. Bell records indicated that only one Model 47E was actually certificated for commercial sale and it was delivered in December, 1951. After removing the transmission assembly from the engine adapter plate, free access was gained to the engine-transmission drive ring. Manual force applied to the drive ring confirmed that the drive ring was secure in place on the upper end of the engine crankshaft and it was able to transmit engine power to the main transmission. The transmission case assembly's through bolts were removed, and the upper case assembly was removed exposing the upper spider assembly. The upper spider assembly, consisting of the upper planetary gear reduction and main rotor drive flange; case assembly spacer, freewheel assembly, and lower spider assembly, were removed from the transmission assembly. These assemblies exhibited no unusual condition and the freewheel assembly operated normally. The main case assembly was inverted and supported on support fixture to facilitate removal of the clutch assembly. The clutch drum retainer nut was still locked in place with two lock-wired fillister head screws with a straight screwdriver head slot. The lock wire and screws were removed, a nut removal tool was installed and the nut removed to allow the clutch assembly to be removed from the lower sun gear assembly. The installed clutch drum was discovered to be part number 47-620-333-1. It was identified by the straight side, smooth external surface of drum. The installed clutch shoes (4 each) were discovered to be part number 47-620-610-011. They were identified by the smooth lining surface versus the grooved surface diamond pattern of other clutch shoe linings. This combination of clutch drum and shoes are used with spider 47-620-608-1, and make up clutch assembly 47-620-609-1. According to Scott's-Bell 47, this light weight clutch assembly was not authorized for use in transmission 47-600-620-17, and was the least robust of any used in the 47-620-600 series. After the clutch spider assembly, which consisted of the clutch shoes, spider and bearing were removed from the clutch drum for closer examination, it was discovered that, the clutch shoe running surface on the inner diameter of the drum displayed a series of abnormal wear grooves along the entire surface of clutch shoe contact. The grooves were estimated to be approximately .200 in. apex to apex and approximately .015-.020 in. in depth. The clutch lining of each shoe exhibited a smooth, slick and shiny surface typical of the phenomenon of "glazing." Additionally, the contact surface of each clutch shoe contained numerous metallic slivers resembling very fine wire which had become embedded in the asbestos material. TESTS AND RESEARCH Clutch Engagement Procedure During the transmission examination, the pilot was asked to describe his normal engine start and clutch engagement procedure. The pilot reported that engine speed was maintained in the range of 1500-1600 RPM and that it wasnormal for the clutch engagement period to be lengthy. In a normal engine start and clutch engagement procedure the pilot would accelerate the engine to 1900-2100 RPM, adding throttle to maintain engine RPM as the load from the accelerating rotor increased. When the increasing main rotor RPM aligned with the 1500-1600 engine RPM radial on the dual tachometer, he would reduce throttle slightly to match the engine and rotor RPM needles and then accelerate the engine to 2100 RPM and continue the warm up. The pilot stated that under certain circumstances when he considered the clutch engagement too lengthy, he would select one magneto off in order to further reduce the engine RPM to accelerate the main rotor to clutch engagement speed. According to the Bell 47D-1 Flight Manual, however, the procedure for clutch engagement was to idle the engine at 1,500 to 1,700 rpm until the oil pressure reached 40 psi minimum, and then to run the engine at 1,700 to 1,800 rpm until the clutch was fully engaged which was apparent when the tachometer needles were synchronized. Engine rpm should then be increased to approximately 2,200 rpm to prevent clutch slippage and held there until the oil temperature reached a minimum of 40 degrees Celsius. Engine Replacement According to the pilot, about 15 flight hours before the accident, he had the engine replaced. The helicopter had originally had a 210 horsepower Franklin 6V-335 engine in it which was getting "tired." In June of 2013 though, he bought a 235 horsepower Franklin 6V-350 from a mechanic in Brewton, Alabama who then removed the old engine for him and installed the new one. When asked if the mechanic had done any upgrades to the drivetrain, or upgrade or replace the clutch during the installation, the pilot advised, that he did not believe so. When asked if the mechanic had filed an FAA form 337, he stated that "I do not know if he did." According to the mechanic, he upgraded the helicopter from the Franklin 6V-335 to the 6V-350 per the service instruction. There were no requirements to upgrade any drive train components. He advised that there were other helicopters that had been upgraded to a 6V-350 and that the same installation could be installed in the Bell 47G models as well, that the only difference in the airframes was the dual fuel tanks and synchronized elevator on the 47G, and that looking through registry information on the 47D-1s, there was an array of different engine installations throughout the years. He advised that, during the install and annual inspection, "I did not disassemble the transmission as there was no requirement within the Bell 47 maintenance manual nor the service instruction to do so." He further advised that, as a flight instructor, he was very surprised when he watched the pilot, as to how hard he would engage the clutch and that there were only a few parts within the transmission that were meant to fail due to undo stress, one being a set of shear bolts that retain the mastspline adapter, and the other the wear of the clutch shoes. The mechanic also stated that, he filled out two copies of FAA Form 337 and provided the customer with them and the flight manual supplement, in a white letter envelope when he picked up the aircraft. However, he added that "according to FAR Appendix A to part 43 this was not considered a major alteration or repair." 600 Hour Transmission Inspection The pilot reported that the transmission had accumulated approximately 615 hours since overhaul. According to the Bell 47D-1 Maintenance and Overhaul Instructions, every 600 hours, the transmission was supposed to be disassembled to inspect all parts for damage or excessive wear, including the clutch to check the "Shoes for glazed or excessively worn linings and worn pins or bushings; drum for dimensional tolerances, out-of-round, scoring, or evidence of excessive heating. Since the Franklin 6V-350 engine, had reportedly been installed approximately 15 flight hours before the accident, it would appear that normal maintenance scheduling would have the 600 hour inspection accomplished concurrently. Review of maintenance records however did not indicate that the 600 hour transmission inspection had occurred. Supplemental Type Certificates According to the FAA, installation of a different engine is considered to be an airframe alteration and requires a Supplemental Type Certificate (STC), issued by the FAA approving a product (aircraft, engine, or propeller) modification. The STC defines the product design change, states how the modification affects the existing type design, and lists serial number effectivity. It also identifies the certification basis listing specific regulatory compliance for the design change. Information contained in the certification basis is helpful for those applicants proposing subsequent product modifications and evaluating certification basis compatibility with other STC modifications. Review of FAA approved STCs indicated that two STCs permitted the installation of the Franklin 6V-350-A in Bell 47 helicopters. The first was STC SH479SW, and was issued to the Franklin Engine Company. It permits installation in Bell 47 series helicopters under Type Certificates (TC); H-1, 2H-1, and 2H-3. The second was STC SH529SW which permits installation in the Bell 47D-1 helicopter (TC H-1). This STC is currently owned by Scott's-Bell 47. According to the FAA, after installation of an STC, any flight manual supplements are supposed to be inserted in the flight manual, a signed original of the STC is supposed to be provided to the owner operator, and a copy is supposed to be forwarded to the closest FAA Flight Standards District Office (FSDO) within 48 hours. Examination of FAA records did not reveal however a record of either STC SH479SW or STC SH529SW being filed by the mechanic, and examination of the accident helicopter's flight manual did not reveal evidence of an approved Rotorcraft Flight Manual Supplement that contained thelimitation, procedures, and performance data applicable to either STC SH479SW or STC SH529SW. Examination of images taken of the cockpit also did not reveal evidence of placards or instrument markings applicable to the installation of either STC. ADDITIONAL INFORMATION Clutch Glazing According to Scott's-Bell 47, the condition of the clutch assembly was such that remedial maintenance action would have been required. Once the glazing condition begins to develop, the period of clutch slippage tends to become extended, further contributing to the glazing condition. A properly maintained and operated Bell 47D-1 would normally exhibit full clutch engagement within 10-15 seconds after engine start. The only remedy for clutch glazing is to remove the transmission from the aircraft, remove the clutch assembly, remove the glazing from the clutch shoes, and repeat the clutch shoe – drum mating procedure as described in the maintenance manual. Serious glazing would require replacement of the clutch shoes. In the case of the accident helicopter, the metal slivers in the clutch shoe lining material also would have required replacement of the shoes, and the grooved condition of the clutch drum would have rendered it non-repairable. Operations Safety Notice In order to improve safety, on May 15, 2014, Scott's-Bell 47 released an Operations Safety Notice (OSN 47-14-11) to remind owners and operators of the Model 47, that approved transmission configuration, proper clutch engagement, and aircraft maintenance, are essential to continued helicopter flight safety, and to remind pilots to follow the approved flight manual procedures during engine start, and to pay particular attention to the normal duration of the clutch engagement, in order to identify any deterioration in performance, and to highlight the potential need for remedial action. ## Aircraft - Registration: [N93067](https://openairdocs.org/aircraft/N93067), registered now, S/N 5501, built 1965 (OpenAirDocs) - Reported make and model: BELL 47D1 - Registry model: [Bell 47D1](https://openairdocs.org/model/bell-47d1) (registry code 1180802) (OpenAirDocs) - Registry engine: [Franklin 6V4 series](https://openairdocs.org/engine/franklin-6v4-series) (registry code 27033, reciprocating) (OpenAirDocs) - Category: Helicopter - Serial number: 5501 - Year built: 1965 - Airframe hours: 3980 - Engines: FRANKLIN 6V-350-A 235 horsepower reciprocating - Damage: Substantial damage - Flight: Business, Part 91 - Operator: Milam Inc - Registered owner: Milam Inc - Airworthiness certificate: Standard, Normal - Last inspection: Annual, June 9, 2013 - Departed: BHM, Birmingham, AL - Destination: None, Pell City, AL - Airport: None, Grass Covered Clearing ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 58; injuries: minor; flight instructor, commercial; airplane ratings: single-engine land, multi-engine land, single-engine sea; instrument ratings: airplane; medical: class 2, with waivers/limitations; last flight review November 29, 2012; seat: left; restraint: lap only; flight hours: 4,430 total, 1,086 in this make and model, 4,330 as pilot in command, 56 in the last 90 days, 15 in the last 30 days, 1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 300° at 8 knots, gusting 19 - Visibility: 10 statute miles - Lowest ceiling: 3,500 ft overcast - Temperature: 34 °F, dew point 19 °F - Altimeter: 30.03 inHg - Observed at: KANB, 22 nm away ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20140121001109A](https://openairdocs.org/incident/20140121001109A), Jan 21, 2014: Bell 47D1, Pell City, AL, Substantial damage. Helicopter was descending, according to the pilot's statement, and at approximately 25' to 35' AGL he heard a loud snap/pop along with an immediate… ## Other investigations on N93067 (5) From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. - [NTSB MIA92LA069](https://openairdocs.org/ntsb/MIA92LA069), Jan 27, 1992: Bell 47D1 (N93067), Cross City, FL, substantial damage. Probable cause: PARTIAL LOSS OF ENGINE POWER DUE TO UNDETERMINED REASONS AND UNSUITABLE TERRAIN ENCOUNTERED DURING THE AUTOROTATIVE LANDING. - [NTSB MIA90LA023](https://openairdocs.org/ntsb/MIA90LA023), Nov 2, 1989: Bell 47D1 (N93067), Loxahatchee, FL, substantial damage. Probable cause: FAILURE OF THE PILOT TO MAINTAIN ROTOR RPM AFTER PULLING UP FROM A SWATH RUN. A RELATED FACTOR WAS THE PRESENCE OF TREES. - [NTSB FTW85LA043](https://openairdocs.org/ntsb/FTW85LA043), Nov 4, 1984: Bell 47D1 (N93067), Terlingua, TX, substantial damage - [NTSB FTW82DA189](https://openairdocs.org/ntsb/FTW82DA189), May 20, 1982: Bell 47D-1 (N93067), Harrisburg, AR, substantial damage - [NTSB LAX67A0026](https://openairdocs.org/ntsb/LAX67A0026), Sep 1, 1966: Bell 47D1 (N93067), Quartz Hill, CA, substantial damage ## Docket (18 files) The files NTSB released with the investigation on February 4, 2016, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=88695. 7.1 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 659 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436366&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Witness Statements and Interview Summaries (item 2, .PDF, 225 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436368&FileExtension=.PDF&FileName=Witness%20Statements%20and%20Interview%20Summaries-Master.PDF - Satellite Image of Accident Area and Wreckage Diagram (item 3, .PDF, 320 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436370&FileExtension=.PDF&FileName=Satellite%20Image%20of%20Accident%20Area%20and%20Wreckage%20Diagram%20-Master.PDF - Weather Reports and Records - KANB METARs and TAFs (item 4, .PDF, 439 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436372&FileExtension=.PDF&FileName=Weather%20Reports%20and%20Records%20-%20KANB%20METARs%20and%20TAFs-Master.PDF - Excerpts from N93067 Maintenance Records (item 5, .PDF, 808 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436373&FileExtension=.PDF&FileName=Excerpts%20from%20N93067%20Maintenance%20Records-Master.PDF - Mechanic's Statement Including Mechanic's Supporting Documentation (item 6, .PDF, 1.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436374&FileExtension=.PDF&FileName=Mechanic%27s%20Statement%20Including%20Mechanic%27s%20Supporting%20Documentation-Master.PDF - Supplemental Type Certificate Number SH479SW (item 7, .PDF, 40 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436375&FileExtension=.PDF&FileName=Supplemental%20Type%20Certificate%20Number%20SH479SW-Master.PDF - Supplemental Type Certificate SH479SW - Rotorcraft Flight Manual Supplement for Bell 47G (item 8, .PDF, 105 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436377&FileExtension=.PDF&FileName=Supplemental%20Type%20Certificate%20SH479SW%20-%20Rotorcraft%20Flight%20Manual%20Supplement%20for%20Bell%2047G-Master.PDF - Supplemental Type Certificate Number SH529SW (item 9, .PDF, 72 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436379&FileExtension=.PDF&FileName=Supplemental%20Type%20Certificate%20Number%20SH529SW-Master.PDF - Supplemental Type Certificate SH529SW - Rotorcraft Flight Manual Supplement for Bell 47D-1 (item 10, .PDF, 69 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436380&FileExtension=.PDF&FileName=Supplemental%20Type%20Certificate%20SH529SW%20-%20Rotorcraft%20Flight%20Manual%20Supplement%20for%20Bell%2047D-1-Master.PDF - Excerpt from Bell 47D-1 Flight Manual - Warm Up and Ground Test (Clutch Engagement) (item 11, .PDF, 62 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436390&FileExtension=.PDF&FileName=Excerpt%20from%20Bell%2047D-1%20Flight%20Manual%20-%20Warm%20Up%20and%20Ground%20Test%20(Clutch%20Engagement)-Master.PDF - Excerpt from Bell Helicopter Maintenance and Overhaul Instructions (Trouble Shooting - Transmissions) (item 12, .PDF, 104 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436391&FileExtension=.PDF&FileName=Excerpt%20from%20Bell%20Helicopter%20Maintenance%20and%20Overhaul%20Instructions%20(Trouble%20Shooting%20-%20Transmissions)-Master.PDF - Excerpt from Bell Helicopter Maintenance and Overhaul Instructions - 600 Hour Transmission Inspection (item 13, .PDF, 109 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436392&FileExtension=.PDF&FileName=Excerpt%20from%20Bell%20Helicopter%20Maintenance%20and%20Overhaul%20Instructions%20-%20600%20Hour%20Transmission%20Inspection-Master.PDF - Excerpts from Bell Helicopter Maintenance and Overhaul Instructions (Assembly - Centrifugal Clutch) (item 14, .PDF, 177 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436393&FileExtension=.PDF&FileName=Excerpts%20from%20Bell%20Helicopter%20Maintenance%20and%20Overhaul%20Instructions%20(Assembly%20-%20Centrifugal%20Clutch)-Master.PDF - Excerpts from Bell Helicopter Corporation Model 47 IPC (item 15, .PDF, 674 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436394&FileExtension=.PDF&FileName=Excerpts%20from%20Bell%20Helicopter%20Corporation%20Model%2047%20IPC-Master.PDF - Statement of Party Representatives to NTSB Investigation (item 16, .PDF, 250 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436395&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Master.PDF - Operations Safety Notice (OSN 47-14-11) (item 17, .PDF, 193 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436396&FileExtension=.PDF&FileName=Operations%20Safety%20Notice%20(OSN%2047-14-11)-Master.PDF - Photo Array (item 18, .PDF, 1.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40436397&FileExtension=.PDF&FileName=Photo%20Array-Master.PDF From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.