Honolulu, HI, USA
N369MH
Hughes 369
The pilot of the helicopter commercial air tour flight stated that the helicopter was in cruise flight at an altitude of about 1,800 ft when, about 11 minutes after takeoff, he felt "severe" vibrations and then heard a "loud bang," after which the helicopter began to shake "violently." The pilot entered a power-on autorotation and stated that the severity of the vibration caused the transponder to shake free of its mount in the instrument panel. He also stated that even small tail rotor pedal inputs significantly worsened the vibrations. The pilot conducted a partial run-on landing in a field. Examination revealed that multiple tail rotor blade and gearbox components had failed in flight, rendering the helicopter substantially damaged. The helicopter tail rotor (TR) transmission was mounted on the aft end of the tail boom, and the four-blade TR assembly mounted onto a four-arm fork that mounted on the output shaft of the TR transmission. The TR blade assembly comprised a pair of two-blade rotor assemblies that attached to the fork. A teeter bearing mounted in each fork arm, and each two-blade rotor assembly was secured in its fork arm pair by a teeter bolt that suspended it between, and was suspended by, the two teeter bearings. At least two different tail boom versions were available for the accident model helicopter. One was the original McDonnell-Douglas Helicopters, Inc (MDHI) version, and the other was an aftermarket version produced by a company called Aerometals. The accident helicopter was equipped with the Aerometals tail boom. The primary difference between the two tail boom versions was the attachment method of the TR transmission to the tail boom. The MDHI version used studs and locking nuts, whereas the Aerometals version used bolts and locking nut plates. Both versions used a total of four attach fasteners. Postaccident examination revealed that the two bolts that attached the left side of the TR transmission to the tail boom had fractured and partially pulled through their nut plates. The two right side attach bolts were damaged, but had not failed; instead, their respective mounting lugs on the TR transmission had failed. The failure of all four attachments meant that the TR assembly was retained on the helicopter by only the TR drive shaft and the pitch control linkage. Neither of those components was designed to retain the TR transmission, and the pitch control system incurred damage during the event. The TR assembly was on the verge of imminent failure. Based on the observed damage, it is likely that with continued operation, the TR would have very shortly separated from the helicopter, rendering control difficult or impossible. All four TR blades remained attached to the fork, but the outer (furthest from the transmission) blade pair remained only partially attached to the fork. The outer teeter bolt was fractured and only a portion of it was recovered. Of the two teeter bearings that were normally mounted in the outer pair of fork arms, one was absent and presumed lost in flight. The remaining outer teeter bearing had debonded from its fork arm, and both it and its fork arm seat exhibited fretting damage on their mating surfaces. The fretting indicated that there was relative motion between the bearing and its seat, caused by helicopter operation with a debonded bearing. Detailed laboratory examinations revealed that the fractured teeter bolt and the two fractured attach bolts had all failed in fatigue. The examinations also revealed several discrepancies with the repair and installation of some of the TR components, as well as some discrepancies within the applicable maintenance and inspection guidance. The teeter bearings had been improperly installed in the fork during overhaul or during maintenance by the operator. Contrary to MDHI overhaul guidance, none of the four teeter bearing installations, including the two debonded ones for the outer blade pair, displayed any evidence of the presence of either primer or scrim cloth. "Scrim cloth" was a single-ply layer of glass fabric that should have been installed at the bearing-fork mating juncture to ensure proper bonding of the adhesive that secured the bearing in its fork seat. An overhauled fork includes installed teeter bearings, and the maintenance records indicated that the accident fork was overhauled by an outside vendor. Information provided by the operator indicated that it had not replaced or reinstalled any of the bearings, and the available records did not specify the serial numbers of the bearings installed during the overhaul. However, contrary to the operator-provided information, research revealed that the operator had independently purchased at least five bearings subsequent to the installation of the overhauled fork, and that at least two of those bearings, including one that had disbonded from the fork, were installed on the helicopter at the time of the accident. The operator was unable to provide any explanation for the improper repair or why their installation of new teeter bearings was absent from the maintenance records. Subsequent to these findings, an FAA search of the operator's premises did not locate any additional overhauled TR assemblies. Contrary to the MDHI TR transmission installation guidance, paint was observed on the faying surfaces of the transmission-tail boom mounting pads. The operator had partially cleaned these surfaces during postaccident removal of the transmission before the investigative examination took place; therefore, the thickness or condition of that paint, or a reliable estimate of its effect on the joint clamp-up, could not be determined. Reduction in joint clamp up, due to compression or breakdown of the paint in the joint, particularly over time, has the potential to result in shear failure of the attachment hardware threads and/or fatigue and failure of the attach hardware, by allowing relative motion between the TR transmission and the tail boom. This condition can be aggravated by increased vibrations due to multiple sources, including but not limited to TR imbalance, disbonded or deteriorated elastomeric bearings, and improper torque of the TR transmission attach hardware. When asked, the operator was unable to provide any explanation for the improper paint application. The available evidence indicates that the failure sequence began with the disbonding of one or both of the improperly installed outer teeter bearings from their respective fork seats. This permitted increased vibration of the TR, which then caused the outer teeter bolt to rapidly fatigue and fracture. The fracture failure of the outer teeter bolt resulted in the in-flight liberation of one outer teeter bearing and a segment of the outer teeter bolt. This further increased the vibration level, which caused the failure of all four structural attach points that secured the TR transmission (including the TR) to the tail boom and resulted in the TR being retained on the helicopter only by the TR drive shaft and the pitch control linkage. Neither of those components was designed to retain the TR transmission, and likely would have failed rapidly with continued operation, resulting in loss of the TR. The pilot's decision to land as quickly as possible likely prevented the loss of the TR and subsequent loss of control of the helicopter.
HISTORY OF FLIGHTOn August 8, 2018, about 0920 Hawaii time, a Hughes MD Helicopters, Inc. 369D helicopter, N369MH, was substantially damaged when it was involved in an accident near Honolulu, Hawaii. The commercial pilot sustained minor injuries; the three passengers did not report any injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 136 revenue air tour flight. The helicopter was the lead helicopter in a flight of two that departed Honolulu International Airport (HNL), Honolulu, Hawaii about 0909. The flight proceeded east/southeast from HNL approximately along the shoreline. The pilot stated that the helicopter was in cruise at an altitude of about 1,800 ft when he felt "severe" vibrations and heard a "loud bang," after which the helicopter began to shake "violently." The pilot lowered the collective control and entered a power-on autorotation, with the intent of landing the helicopter in a grassy clearing. He radioed his intentions to his colleague in the trailing helicopter and then advised his passengers of the same. The pilot reported that small tail rotor pedal inputs significantly worsened the vibrations. The pilot made a partial run-on landing onto the targeted clearing, which was about 13 miles east of HNL. He reported that on first contact, the helicopter bounced about 1 ft into the air and that the remaining slide on the dry and rocky grass field was rougher than he expected. The helicopter came to a stop upright, and the pilot shut down the engine. The landing field was part of the grounds of a public school, and the pilot released the passengers to the care of the school staff while he examined the helicopter and coordinated with his company. The helicopter came to rest upright on its landing skids; the right skid was fractured but able to support the helicopter. Postaccident photographs indicated that multiple tail rotor blade and gearbox components were damaged, rendering the helicopter substantially damaged. Without NTSB or FAA knowledge or approval, and contrary to applicable regulations, the operator recovered the helicopter back to its facility shortly after the accident and began disassembly for repair. More than a day after the accident, the NTSB became aware of these activities, and instructed the operator to cease repairs. AIRCRAFT INFORMATIONConfiguration & Design Information The helicopter was equipped with a single 5-blade main rotor (MR) system, a tail boom, a 4-blade tail rotor (TR) system, and a T-configuration horizontal and vertical stabilizer assembly. The TR transmission attached to the aft end of the tail boom, and the TR assembly attached to the output shaft of the TR transmission. A TR driveshaft, routed inside the tail boom, provided torque from the main transmission to the TR transmission. Pitch links and a swash plate connected the flight controls to the TR blades. At least two different tail boom versions were available for this model helicopter. One was the original Hughes Helicopters (MDHI) version, and the other was an aftermarket version produced by Aerometals. The Aerometals tail boom was approved as FAA supplemental type certificate (STC) SH5055NM. The accident helicopter was equipped with the Aerometals tail boom. The vertical stabilizer attached via 4 bolts to the aft right side of the tail boom. The horizontal stabilizer was equipped with 4 studs that inserted into holes in 4 lugs atop the vertical stabilizer and was secured by nuts on the studs. The original Hughes Helicopters (MDHI) tail boom had an aft-facing threaded steel stud anchored in each of the tail boom pads of the cast-aluminum TR transmission attachment frame. The Aerometals tail boom eliminated the studs, incorporated a machined aluminum TR transmission attachment frame, and installed four self-locking nut plates (MS21075L4) forward of the attachment frame mounting pads. Through-bolts were installed through the TR transmission lugs and tail boom attachment frame and into the self-locking nut plates. The steel bolts (MS21250) were 1/4-28 standard aircraft hardware. The bolts and nut plates were cadmium plated. The four-blade TR comprised of two, two-blade TR blade assemblies mounted 90° from one another. Each TR blade assembly comprised a central hub with a TR blade attached to each end. A tension-torsion strap pack was installed inside each TR hub. The TR blade assemblies were referred to as the "inboard" and "outboard," where "inboard" referred to the TR assembly closest to the TR transmission. A four-arm fork, with two pairs of arms arranged 90° apart, referred to as the "inboard fork" and "outboard fork," served as the mount for the two TR blade assemblies. The fork installed directly onto the TR transmission output shaft. Each fork arm incorporated a machined conical receptacle near its end, with an elastomeric "teeter" bearing nested and secured in each receptacle. Each teeter bearing comprised an assembly of several alternating concentric cones of metal and elastomer, with an outer metal shell, and a central, axially oriented hollow metal cylinder that served as a bolt hole for the bearing. All components of each bearing were bonded together to form a single unit. Each TR blade assembly was mounted in one pair of fork arms. It was suspended by its two teeter bearings, secured by a fork (or "teeter") bolt that extended through the TR hub, and through the bearing near each end of the fork bolt. The nickel alloy tension fork/teeter bolts (369A1602-3) and their nuts (VCU0001) were MDHI parts. (see Figures 1 through 3) Figure 1. Overview of TR Assembly Figure 2. TR Forks and Blades Figure 3. TR Hub and Forks Fastener Torque Guidance "Drag torque" is the term applied to the baseline torque value obtained when running a nut onto a bolt, or a bolt into a nut plate. Drag torque is unique to each bolt and nut/nut plate combination. FAA Advisory Circular 43.13-1B (Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair) states that for the installation of all torqued fasteners, the drag torque values are to be determined and recorded for each fastener combination, and that specific value is to be added to the specified installation torque value of the fastener. This drag torque procedure was to be used for installation of both locking and non-locking hardware. According to the MDHI Maintenance Manual (MM) Torque Maintenance Practices section, the allowable drag torque range for 1/4-28 hardware was 3.5 to 30 in-lbs. The MM states that the following requirements governing torque loads apply throughout the manual except where otherwise specifically indicated. Values apply to cadmium-plated bolts, cadmium-plated nuts coated with molybdenum disulfide (MoS2) Manufacturer applied lubricant must not be removed nor additional lubricant added. Bolts, nuts and surfaces they bear on must be clean, dry and free of lubricant except as stated in requirement above. Turning (drag) torque required to install self-locking nut or bolt up to point of final tightening must always be added to final torque value specified or the maintenance instruction, as applicable. Aerometals Installation Guidance for TR Transmission Aerometals document AMI-19, "INSTRUCTIONS FOR CONTINUED AIRWORTHINESS, 369X23500-505, -507, TAILBOOM ASSEMBLIES" was released in July 2000. The document included the following paragraphs: This manual provides maintenance instructions for Aerometals 369X23500-505 and 369X23500-507 tailboom assemblies. These instructions are for installation, removal, inspections and intermediate levels of maintenance. No repairs are authorized that are not addressed in this manual. With the exception of the tail rotor gearbox attachment hardware and the additional inspections outlined in this manual, the Aerometals tailboom assemblies are installed, inspected, and repaired in accordance with the tailboom installation, inspection and repair procedures identified in the MDHI Basic Handbook of Maintenance Instructions for the Model 369D/E/FF helicopters. The AMI-19 document specified the following procedures for installation of the TR transmission: (1) Apply primer (MIL-P-23377 or MIL-P-85582) to the four gearbox mounting bolts. While the primer is still wet, install bolts and washers through the transmission mount holes and into the aft tailboom frame. (2) Torque bolts to 100-110 in-lbs (11.3-12.4 N-m) and apply torque stripe paint. (3) Between 2 and 10 hours of helicopter operation (to allow parts to seat), check the torque of each MS21250-04026 bolt by applying 100 in-lbs (11.3 N-m). Reapply torque stripe paint. If any movement of any bolts occurred, this procedure must be repeated after 2 to 10 hours of helicopter operation. The AMI-19 document does not specify that drag torque be applied to the final torque values, however, AC 43-14B and the MDHI MM provide clear guidance as to the applicability of drag torque in this application and would be in accordance with normal practices. MDHI did not participate in or contribute to either the Aerometals design or the Aerometals installation and maintenance procedures. Aerometals Inspection Guidance According to AMI-19, the Aerometals tail boom was limited to a service life of 10,300 hrs. The document cited the following inspection criteria: The 100-Hour/Annual and Conditional Inspection requirements for tailbooms identified in the original helicopter manufacturer's Basic Handbook of Maintenance Instructions for the Model 369D/E/FF helicopters are still applicable and are required for the Aerometals 369X23500-505 and 369X23500-507 tailboom assemblies. Additionally, the Aerometals AMI-19 document did cite mandatory inspections of the tail boom at 7,300; 8,300; and 9,300 hrs that required removal of the TR transmission; such inspections would result in TR transmission re-installation that would necessitate the previously-cited attach hardware torque checks. MDHI Installation Guidance for TR Transmission Chapter 63-25-10 of the MDHI Maintenance Manual (MM) for installation of the TR transmission stated: NOTE: Ensure all paint and sealant is removed from mating surfaces. Remove excessive sealant, as required, from transmission to gain clean mounting surfaces. Ensure that no gap in sealant coverage exists around the transmission bearing cover assembly. (3). Apply primer (CM318) in holes and on the grip area of the mounting studs. Install tail rotor transmission or tailboom extension while primer is still wet. (4). With assistance, support transmission and shaft in line for minimum deflection of coupling and install tail rotor drive shaft and transmission as a unit (Ref. Sec. 63-15-10). NOTE: Record the drag torque for each nut and its location on the transmission in the helicopter for later use. (a). (369D/E) Secure tail rotor transmission to four tailboom mounting studs with washers and nuts. Torque nuts to 75—95 inch-pounds (8.47—10.73 Nm) plus drag torque. (5). Apply torque stripe paint. (6). Between 2 and 10 hours of helicopter operation (to allow parts to seat), check the torque of each mounting nut as follows: (a). Use the drag torque as measured and apply a torque load of 75—95-inch pounds (8.47—10.73 Nm) plus the noted drag torque (noted for each individual nut) to each mounting nut of the transmission. (b). Re-apply torque stripe paint. Paint is prohibited from the joint in order to maintain joint clamping force. Reduction in joint clamping force, due to compression or breakdown of paint in the joint has the potential, especially over time, to allow relative motion between the TR transmission and the tail boom, which can result in fatigue fracture of the attach hardware. In cases where more severe vibrations are present, the relative motion between the TR transmission and the tail boom can result in shear fracture of the attachment threads. Increased vibrations and increased component relative motion can be due to multiple sources, including but not limited to TR imbalance, disbonded or deteriorated elastomeric bearings, and improper torque of the TR transmission attach hardware. MDHI Inspection Guidance for TR Transmission Attach Hardware The MDHI MM contained sections for 100-hour/Annual and 300-hour inspections. The 100-hr inspection required a torque check of the fasteners. However, the specified torque values are not included in the inspection section of the MDHI MM but are part of the tail rotor transmission installation section of the MDHI MM. The appropriate torque values for the Aerometals tail boom are included in the installation section of AMI-19. The 300-hr inspection required removal of TR drive shaft, which necessitated removal of the TR transmission. MDHI Installation Guidance for Teeter Bearings & Bolts The MDHI Component Overhaul Manual specified the procedures for installation of the teeter bearings into the fork. The mating surfaces of the bearings and fork seats were to be coated with primer. After the primer was dry, a single ply of glass fiber fabric (referred to as a "scrim cloth") was to be installed on the bearing mating surface, and adhesive then used to secure each bearing, with its scrim cloth, to the fork. The purpose of the scrim cloth was to control the thickness of the adhesive, which promoted a consistent bond strength and prevented excessive squeeze-out of the adhesive during the installation and curing process. The adhesive was a two-part epoxy, gray in color. Each 2-blade TR assembly attached to the fork via a teeter bolt, which installed through the two teeter bearings and the TR hub. The bearings were designed to be preloaded on installation, which was accomplished by proper shimming and teeter bolt tension. The shimming procedures were described in detail in the MM. Proper teeter bolt tension was obtained by tightening the nut to elongate the bolt a specified amount more than its zero-torque length. The specified elongation range was from 0.008 to 0.011 inches, and the bolt must not be used if the bolt was elongated more than 0.011 inches for any reason. There was no FAA or MDHI-specified bolt life limit; replacement was based upon on-condition or if the fork bolt is elongated over 0.011 inch, for any reason. The 100-hr and 300-hr inspections required examination of the teeter bearings for debonding of their internal (metal-to-elastomer) bonds, bonds to the fork arms, and clamp up of the assembly by verifying the radial molded ridges on each bearing when teetering the blades stop to stop by hand takes place. If ridges assume continuous curved shape, bearings are intact, bearing bonded to the fork and clap up of the fork, elastomeric bearings and TR hub assembly. The inspection also required a torque check of the teeter bolt and confirmation of the absence of any axial or radial play in the bearing. In addition to the 100-hr and 300-hr inspection, a daily preflight visual inspection of the tail rotor drive fork elastomeric bearings is included in the MDHI Rotorcraft Flight Manual. Preflight Checks of the Tail Rotor The MD 369D Rotorcraft Flight Manual (RFM) directs, as part of the daily preflight checks, inspection of the tail rotor drive fork elastomeric bearings as follows: Tail rotor drive fork elastomeric bearings (if installed): NOTE: Check bearing for general condition. Elastomeric bearings are suspected of being unserviceable if rubber deterioration or separation, or a vibration is noted. Evidence of light swelling, pock marks and crumbs are surface conditions and are not indications of bearing failure. CHECK -_ _ Apply teetering force by hand to tail rotor blades (stop-to-stop). Check for fork-to-bearing bond failure. Failure is indicated by any motion between outer bearing cage and fork (bearing turns in fork). CHECK -_ _ Teeter blades stop-to-stop. Observe four radial molded ridges on each bearing as teetering takes place. If ridges assume continuous curved shape, bearings are intact. Discontinuity in molded ridges indicates bearing failure. Teeter Bearing Serial Numbers Lord Corporation was a manufacturer of the teeter bearings specified by MDHI for installation in the TR fork. The Lord part number for these bearings was LB2-1056, and Lord seri
The operator's improper installation of the tail rotor (TR) teeter bearings, which resulted in cascading in-flight failures of the TR components and attach hardware.
Source: NTSB Aviation Accident Database
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