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Loss of tail rotor authority Warwick Qld Oct05 ATSB Report

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Loss of tail rotor authority Warwick Qld Oct05 ATSB Report

Old 1st Jul 2006, 14:14
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Loss of tail rotor authority Warwick Qld Oct05 ATSB Report

The helicopter sustained a loss of tail rotor control while carrying out a power line inspection. It made approximately 12 to 15 full rotations before landing in a paddock. The pilot and two observers on board received minor injuries. The helicopter sustained minor damage.

Examination of the tail rotor revealed that the loss of control was due to failure of the inner tab of a tang washer. The washer locked the retaining nut holding the drive fork and the tail rotor assemblies onto the transmission output shaft. The failure of the inner tab resulted in looseness of the retaining nut and the split ring becoming dislodged. Consequently, the drive fork and the tail rotor assemblies were allowed to move freely along the transmission output shaft.

The ATSB was advised that at the helicopter rebuild a new tang washer was fitted. To inspect the washer, the retaining nut would have to be removed.

The operator sent the failed tang washer to the helicopter manufacturer to determine nature of the locking tab failure. The report had not been received at the time of writing this report.

A search of the ATSB database, for the 1995 to 2005 period, revealed no records of similar tail rotor control problems. The Civil Aviation Safety Authority Service Difficulty Report database, for the same period, contained only the record of the tail rotor control problem from this occurrence.

The manufacturer advised that they have received two worldwide reports of a broken locking tab since 1988.

FACTUAL INFORMATION
On 27 October 2005, a McDonnell Douglas Hughes 369E1 helicopter, registered VH-SUV, sustained a loss of tail rotor control and started spinning, while being operated on a power line inspection near Warwick, Qld. The pilot attempted to control the spin, but the helicopter was reported to have made approximately 12 to 15 full rotations before landing in a paddock. The pilot and two observers on board received minor injuries. The helicopter sustained minor damage.

Helicopter
The helicopter was manufactured in 1990 and at the time of the occurrence had accumulated 3,543.8 hours total time in service (TTIS) and 4,348 landings since new. The helicopter was purchased overseas in a damaged condition after accumulating 3,118.35 hours TTIS since new. It was purchased and rebuilt by the operator and in February 2005 received a Civil Aviation Safety Authority (CASA) Certificate of Airworthiness. Maintenance records indicated that the helicopter had been fitted with a serviceable pitch control assembly during the rebuild (Figure 1).

Figure 1: Tail rotor pitch control assembly (see website for picture)

Since the rebuild and its return to service, the helicopter had been maintained in accordance with the maintenance requirements applicable at that time and had a valid Maintenance Release. It had flown approximately 82.2 hours since the last periodic inspection in September 2005.

Tail rotor examination
The operator examined the tail rotor. That examination found that the loss of tail rotor control was due to the failure of the inner tab of the tang washer (Figure 2). The washer locked the retaining nut holding the drive fork and the tail rotor assemblies onto the transmission output shaft. The failure of the inner tab resulted in looseness of the retaining nut that allowed the split ring to be dislodged. Consequently, the drive fork and the tail rotor assemblies were able to move freely along the transmission output shaft. That resulted in the loss of tail rotor control and the blades adopting a neutral pitch.

Figure 2: New tang washer with an undamaged locking tab and the subject tang washer with broken locking tab (see website for picture)

Tang washer
The operator reported that a new tang washer was fitted at the time of the helicopter rebuild and that a torque strip was applied across the retaining nut, tang washer and drive fork as required by the helicopter maintenance manual. The maintenance manual requires the torque strip to be applied in a location where it could be checked visually.

The operator sent the failed tang washer to the helicopter manufacture to determine the nature of the locking tab failure. At the time of issue of this report the operator had not received a response from the helicopter manufacturer.

Previous tail rotor control problems
A search of the ATSB database, for the 1995 to 2005 period, revealed no records of similar tail rotor control problems. The CASA Service Difficulty Report database, for the same period, contained only the record of the tail rotor control problem from this occurrence.

The manufacturer advised that they have received two worldwide reports of a broken locking tab since 1988.


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The helicopter was fitted with the two bladed tail rotor assembly.
Source: http://www.atsb.gov.au/publications/...200505332.aspx
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