On 27 October 2005, a McDonnell Douglas Hughes 369E
1 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
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
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.