Loss of control Kawasaki Heavy Industries 47G3B-KH4, VH-MTF

FACTUAL INFORMATION

At 1215 Eastern Standard Time¹ on 27 September 2004, a Kawasaki Heavy Industries, 47G3B-KH4² helicopter, registered VH-MTF, was being operated on a tourist flight with one adult and a young boy as passengers. The flight included landing on a 1 m high wooden platform in the Teepookana Forest in north-west Tasmania.

The pilot reported that as he brought the helicopter to a 1 m hover above the platform, the helicopter began to rotate slowly to the right. He unsuccessfully attempted to counter the rotation by applying left tail rotor control input. The pilot then increased engine power in an attempt to regain tail rotor control and to move the helicopter clear of the landing platform. That action had the effect of rapidly increasing the rotation of the helicopter to the right and it began to ascend, reaching about 5 m above ground level. The pilot then lowered the collective control and the helicopter impacted the ground heavily on its right side, several metres from the landing platform. The pilot and adult passenger released their seatbelts and then both assisted the young boy to exit the wreckage. The pilot and passengers received minor injuries.

The pilot described the wind conditions at the time of the accident as a headwind with an approximate strength of 8 kts. That assessment was consistent with the wind data for the Strahan area provided by the Bureau of Meteorology³. The pilot also reported that the main rotor RPM indications were normal and that the helicopter had sufficient power to complete the approach⁴. At the time of the accident, the weight and balance of the helicopter were within prescribed limits. There was no evidence that the helicopter had collided with anything during the approach.

The pilot was appropriately qualified and endorsed to operate the helicopter type and held a valid medical certificate. He was a very experienced agricultural aeroplane pilot and had obtained a commercial pilot (helicopter) licence 14 months before the accident. He had accrued at total of 292 hours in helicopters since that time; 286.4 hours of which had been in the Bell 47 helicopter type. The pilot was experienced with operations into and out of the Teepookana Forest landing platform.

The landing platform was located within a dense forest in an area that was cleared of trees but covered by 1 m high scrub. The trees closest to the clearing had been trimmed to a height of about 5 m to allow a 'fly-in, fly-out' approach. There was no requirement to conduct a vertical approach to the platform.

The helicopter's fuselage structure was deformed by the impact and the tail boom was bent in a downward direction at approximately station 100⁵. There was corresponding bending damage to the tail rotor drive shaft assembly long shaft at the same point. The operator reported that examination of the damaged tail rotor pitch control system revealed that the controls were intact and would have been capable of normal operation. All parts of the helicopter were accounted for by the operator at the accident site.

The two-blade tail rotor assembly, mounted on the right side of the tail boom, was intact and correctly attached to the helicopter. There was no evidence of rotational damage to the leading edges or tips of either blade (Figure 1). During the ground impact one blade had been bent outward at the tip and the other was bent in toward the tail rotor gearbox.

Figure 1:     Tail rotor blade damage

The helicopter's tail rotor drive shaft assembly consisted of a series of two short shafts and one long shaft that were situated on the top of the tail boom assembly. The long shaft was supported in eight hanger bearing assemblies and was secured at its front and rear by drive coupling assemblies. The operator inspected the tail rotor drive system and found that the long shaft assembly tubing was fractured and the pin situated through the front drive coupling assembly was sheared. There was also significant distortion of the corresponding pin in the shaft's rear coupling.

Inspection of the tail rotor drive system, including the drive shaft bearings, tail rotor extension housing and tail rotor gearbox, with the exception of the long drive shaft, revealed nothing that would have prevented normal operation.

ATSB specialist examination of the failed components (Appendix A) attributed the tail rotor drive shaft failure to a significant torsional overload event, leading to a loss of coupling security and the subsequent slippage, frictional heating and shear fracture of the shaft. That examination was unable to determine when the torsional overload occurred or what specific events may have contributed to it.

At the time of the accident, the helicopter had logged 72 flight hours since the issue of the current maintenance release. The last recorded maintenance carried out on the helicopter was a spark plug change on 21 September 2004, 1.0 flight hour prior to the accident. On 31 August 2004, 5.7 flight hours prior to the accident, one tail rotor blade was replaced because of delamination of the leading edge wear strip.

Information received from the operator and from the maintenance organisation indicated that there had been no known tail rotor strike or sudden rotor stoppage since the helicopter was placed on the Australian aircraft register in 1992. The helicopter's prior history was not examined.

The company operations manual contained the published normal and emergency procedures affecting aircraft operations. An appendix to the manual contained the flight check systems and operating procedures specific to each aircraft type operated by the company, with the exception of the Kawasaki-Bell 47G3B-KH4 helicopter. The company did however, make available to pilots a copy of the Civil Aviation Safety Authority approved Kawasaki-Bell 47G3B-KH4 helicopter flight manual.

With reference to tail rotor failures, that flight manual stipulated:

1. Immediately execute an autorotative descent and maintain an airspeed of 34 KIAS at least.
2. Execute a normal autorotative descent and landing.

The flight manual did not contain any specific advice for pilots in response to a tail rotor drive failure when hovering.

Information in the company operations manual regarding pilot response to a tail rotor drive failure in another piston-engine helicopter (Robinson R44) included:

LOSS OF TAIL ROTOR THRUST DURING HOVER

1. Failure is usually indicated by right yaw which cannot be stopped by applying left pedal.
2. Immediately roll throttle off into detent spring and allow aircraft to settle.
3. Raise collective just before touchdown to cushion landing

The generally accepted procedure for pilot actions in the event of a tail rotor failure is to quickly roll off the throttle or snap close the throttle and perform a hovering autorotation⁶,⁷,⁸,⁹ For example:

The likely worst place for loss of tail rotor thrust to happen is in the hover, and the reaction is quite simple - get rid of the engine power and land the helicopter from a hovering engine failure condition. Easy to do on those machines that have throttle(s) on the collective¹⁰.

1. The 24-hour clock is used in this report to describe the local time of day, Eastern Standard Time (EST), as particular events occurred. Eastern Standard Time was Coordinated Universal Time (UTC) + 10 hours.
2. The Kawasaki Heavy Industries, 47G3B-KH4 helicopter is a single pilot/single flight control helicopter manufactured under licence from Bell Helicopters. It is commonly known as the KH4 helicopter and is a derivative of the Bell 47.
3. Given that the pilot positioned the helicopter into wind during the approach and landing, the risk of loss of tail rotor effectiveness (LTE) was negligible.
4. There were no external conditions that would have placed the pilot at risk of overpitching or drooping the main rotor.
5. Positioned 100 inches aft of the datum. The datum was located 2 inches forward of the rotor mast centre-line.
6. Coyle, S. (2003). Cyclic & collective - More art and science of flying helicopters. Mojave, CA: Helobooks, pages 341and 342.
7. Federal Aviation Administration. (2000). Rotorcraft flying handbook (FAA-H-8083-21).  Washington, DC: FAA.
8. Newman, R. (1999). Helicopters will take you anywhere: A manual for helicopter pilots. Mentone, Vic: The Helicopter Book Company.
9. Becker, M. (1997). Mike Becker's helicopter handbook. Noosaville, QLD: Becker Helicopters Australia.
10. The Kawasaki-Bell 47G3B-KH4 helicopter had a throttle of this design.

ANALYSIS

The circumstances of the accident were consistent with a loss of tail rotor thrust following the failure of the tail rotor drive shaft as the helicopter entered the hover.

The time at which the damage to the drive shaft occurred was not able to be determined. However, given the absence of rotational damage to the tail rotor blades, it is unlikely that it occurred during the accident flight.

The action of the pilot in increasing engine power when faced with the loss of tail rotor thrust was inappropriate and exacerbated the situation.

Examination of a failed helicopter tail rotor shaft coupling assembly, Kawasaki Heavy Industries 47G3B-KH4

1 FACTUAL INFORMATION

1.1 Investigation brief

Accident event

On 27 September 2004, as the Kawasaki KH 4 helicopter was approaching to land, the pilot reported that the helicopter commenced an uncommanded right yaw motion that could not be arrested by tail rotor control inputs.  Upon increasing power, the rate of yaw and rotation also increased, with the helicopter revolving approximately five times before the pilot reduced power and main rotor collective, allowing the helicopter to settle to the ground where it rolled onto its right side.  The three occupants exited the helicopter, having sustained minor injuries.

Examination

During the post-accident investigation of the helicopter, the owner reporting finding the tail rotor drive shaft fractured at the point where it adjoined the forward coupling.  The tail rotor had impacted the ground, however the damage sustained by the blades showed no evidence of rotation under power.  The fractured drive shaft and both forward and rear couplings were recovered from the accident site by the aircraft owner and submitted to the Australian Transport Safety Bureau (ATSB) for technical examination to assist in the investigation of the occurrence.

1.2 Inspection

Coupling design

The helicopter tail rotor coupling assembly employed a tapered clamping nut arrangement bearing upon the outer circumference of the shaft tube. For rigidity in the clamped locations, an internal sleeve was fitted and secured with adhesive injected between the sleeve and tube bore.  A single machine pin passed transversely through the coupling, tube and sleeve to provide for the positive positional security of the components.  Figure 11 illustrates the assembly as a sectional view.

Figure 1: Tail rotor drive shaft coupling, point of failure indicated

Forward coupling and shaft fracture

Upon initial receipt, the tail rotor drive shaft was confirmed as failed and separated at the point where it entered the forward coupling socket assembly (refer to figure 2). The fractured end of the shaft remained within the coupling, requiring removal by boring of the securing through-pin ends and pressing of the shaft stub out of the coupling (refer to figure 3).

Figure 2: Forward drive shaft coupling after disassembly

Figure 3: Drive shaft stub after removal from coupling

The failure of the securing through-pin at both protruding ends (refer to figure 4) was evident after removal of the shaft stub. The morphology of both fractures was typical of ductile shear overload under transverse loading (shaft twisting) conditions.

Figure 4: Fractured through-pin from the forward coupling.  Note also the scoring from post-fracture rotation of the shaft

Figure 5: Spiral scoring on the gripped section of the shaft, adjacent to the fracture

Circumferential scoring of the shaft surfaces to either side of the through-pin indicated subsequent rotation of the shaft inside the coupling after separation. The shaft had fractured approximately 48 mm from the coupling end, exposing the end 18mm of the internal reinforcing sleeve. The last 12 mm of the shaft before the fracture showed deep spiral scoring where the tapered grip segments normally clamped upon the surface (refer to figure 5). The shaft fracture surfaces had been marred and damaged by continuing contact after separation and presented no appreciable evidence of the failure mode (refer to figure 6).

Figure 6: Damaged shaft fracture surface

Figure 7: Torsional distortion of the drive shaft adjacent to the point of failure. Note the elongation of the small hole

The opposing fracture and section of the shaft that extended from the forward coupling (refer to figure 7) showed extensive scoring, discolouration and galling, consistent with the damage noted inside the taper coupling bore (refer to figure 8). The examination also noted the torsional distortion of the material around a small adhesive bleed hole in the shaft wall (refer to figure 7 also). In a similar manner to the opposing section, the fracture surface had been heavily damaged by post-failure interference and presented little information of value.

Figure 8: Extensive galling and metal adhesion inside the clamping section of the forward coupling

Rear coupling

The rear tail rotor drive shaft coupling was a similar design to the forward unit.  The rear coupling showed no significant evidence of slippage of the shaft within the clamped section. The securing through-pin had not failed, however upon removal it presented with appreciable opposing axial bending around the points where the pin passed through the assembly (refer to figure 9).

Figure 9: Through-pin removed from the rear coupling, showing axial distortion typical of a significant torsional overload

2 ANALYSIS

The ATSB examination confirmed the failure and separation of the tail rotor shaft at the point of engagement with the forward drive coupling, approximately 48 mm from the forward end of the shaft.  While the shaft fracture surfaces were damaged beyond allowing any interpretation of the original failure mode, the twisting and distortion of the tube material at either side of the fracture was evidence of the shaft having sustained transient torsional overloading conditions.  Similarly, the shear failure of the coupling through-pin and the subsequent shaft rotation inside the coupling was a further indication that the assembly had carried, or sustained torsional loads of a magnitude well above the design allowable limits.  Mirroring the torsional overload along the load path was the distorted through-pin from the rear coupling.
On the basis of the damage sustained by the forward coupling and engaged shaft, it was evident that the failure had proceeded in two distinct stages.  Initially, the transient torsional overload event had overcome the clamping friction and caused the shear failure of the through-pin on the forward coupling.  Once the pin had failed, the shaft was then able to slip and rotate within the coupling, where it was likely that the galling damage generated between the coupling bore and shaft surface led to the 'screwing' action that pulled the shaft further into the coupling and produced the surface damage that ultimately led to the shaft fracture at that point.  While the fracture surfaces were damaged, it was probable that the shaft fracture mode was one of ductile torsional shear.  The transverse plane of fracture supports this.

Contributory events

During normal flight and ground operation, the helicopter tail rotor shaft should not sustain any transient torsional loads beyond those imposed by normal engine power changes and/or tail rotor pitch movements.  To produce the overload failure and damage to the coupling pins, the tail rotor system must have at some time, been exposed to conditions or events capable of producing a significant increase in the rotational resistance of the assembly.  Gross mechanical failures within the tail rotor gearbox, tail rotor impacts, or drive shaft bearing seizures remain as possibilities in that regard.
The reported loss of tail rotor effectiveness and the absence of rotational damage to the tail rotor upon ground impact was consistent with the drive shaft coupling slippage and rotation developing during the landing approach.  While the coupling pin failure must have been a precursor to the slippage, there was no physical or reported evidence to suggest when that failure may have occurred or what events may have contributed to it.

3 CONCLUSIONS

On the basis of the investigation findings, the following conclusions could be drawn:

1. The helicopter tail rotor drive shaft had sustained damage consistent with a significant torsional overload event and subsequent rotational slippage and separation of the shaft at the forward coupling.
2. The accident scenario and damage sustained was consistent with the slippage and separation of the shaft during the helicopter's landing approach.
3. The factors contributing to the initial overload event could not be conclusively established.

1. Diagram provided by Kawasaki Heavy Industries Ltd, assembly reference 47-640-052-39 (Shaft Assembly)