Originally Posted by
DOUBLE BOGEY
Maybe I misunderstood the report but I believed the TR Pitch went to FULL POWER. Which leaves no chance of a recovery as we saw in this event. No just loss of control . The worst scenario imaginable. Its odd that all other manufacturers fit a safety mechanism, or inherent blade aerodynamics, that would "center" the servo to a reasonable, hopefully survivable pitch setting. I also understand this latent failure event exists in 109/139/169 and 189. Are there any Leonardo techs on here that could confirm this.
In the report:
The AW139 was designed to have a left-hand thread on the actuator shaft and
locking feature, to prevent it unscrewing if the actuator shaft rotated following a
bearing failure. This suggests that the failure mode had been considered as part
of the AW139 development. An AW139 bearing failure in 2012 demonstrated
that this design successfully prevented the actuator shaft from unscrewing from
the pin holder.
2012 an operator based in Qatar suffered a loss of yaw control incident on
an AW139. This was found to have been caused by a failure of the duplex
bearing (p/n 3G6430V00151). Evidence of rotation of the tail rotor actuator
control shaft confirmed that the bearing had seized at some point. However,
as the control shaft had a left-hand thread, the pin carrier had tightened onto
the actuator shaft, rather than unscrewing. This transferred the torque load
back into the bearing, forcing rotation until the bearing components became so
heavily worn that the bearing failed completely, to the extent that it no longer
provided any resistance to the movement of the hydraulic actuator. Effectively
no longer attached to the control system, the tail rotor blades moved to, and
remained at, a positive blade pitch angle of approximately 10° with no means of
changing the blade position possible by pilot action. The helicopter started to
turn under the influence of the main rotor torque couple, but the loss of tail rotor
control occurred while the helicopter was in forward flight. The reduced engine
torque demand, the vertical tail surface aerodynamically contributing to the yaw
control and the force generated by the default blade position, were sufficient to
allow the pilot to maintain forward flight and perform a ‘run on’ landing without
any additional damage occurring to the helicopter.