The Australian Transport Safety Bureau has determined that a Sikorsky S-64 Skycrane crashed into a dam near Jericho, Victoria, because a shortened final approach caused the rotors to enter a vortex ring state.
Skycrane N173AC Christine crashed into the dam on 28 January 2019 whilst refilling to fight a fire front in the Victorian high country. Three crew on board were injured and the aircraft substantially damaged after it sank into the water.
The ATSB investigation into the accident established that, over the course of the day’s operation, the helicopter’s approach path to the dam was incrementally shortened. The length of the final approach was considerably shorter than earlier approaches.
"ATSB investigators determined that it was likely that the final tight approach path was at the upper margins of allowable speed and angle of bank," the ATSB has stated. "This would have required a steep flare on arrival and likely resulted in the rapid onset of vortex ring state.
Vortex ring state occurs when a helicopter descends into its own downwash. Air recirculates back up and over the rotors instead of flowing down and away. This causes the same parcel of air to circulate around the rotor. As a result, the rotor system no longer has the steady stream of air required to produce lift and the helicopter will descend despite the application of additional power.
“When performing aerial work, it is easy to accept incremental changes that gradually reduce margins," ATSB Director Transport Safety Stuart Macleod said. "While these changes often increase efficiency, it is worth checking how much a sequence has deviated from earlier versions and re-evaluating elements if they appear less stable.
“Helicopters excel in confined areas, but are vulnerable when operating within them. In this case, the shape of the dam and surrounds of the site reduced the opportunity for recovery. Periodic reassessment of confined areas, and approach and departure profiles should be done throughout the duration of an operation.”
The ATSB has attributed the crew's survival to their helicopter underwater escape training (HUET).
The full investigation report is on the ATSB website.