N369MH

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 Tra...