Their Escape The tail rotor separated when it hit the water, and the fuselage spun through one and a half right turns. The main rotor blades like- wise separated after striking the water, and the right cockpit door was torn off. The helicopter then rolled onto its left side, submerg- ing the cockpit. The crew executed its HUET drill as soon as the rotation stopped; the SIC evacuated through the missing right door and helped the PIC through the same exit. The rear door was jammed, but the crew chief was able to knock out a window to make his escape. Two other helicopters—a Sikorsky S-76 and S-61N—were on the scene to assess the possibility of using the same dip site for night operations. Neither was “equipped or able to offer direct assistance” but did report the accident to an incident controller who triggered the emergency response plan. The Aircrane’s crew used hand signals to advise that they were safe and not seriously injured.
The Takeaway The ATSB concluded that Christine entered vortex ring state, otherwise known as “settling with power,” a condition in which a helicopter descends into its own main rotor’s downwash:
The air recirculate[s] 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 lon- ger has the steady stream of air required to produce lift and the helicopter will descend despite the application of additional power.
Adding power only intensifies the downwash, so even a heli- copter as capable as a lightly loaded Aircrane can’t just muscle its way out. The pilots were trained in the Vuichard recovery tech- nique, currently considered the fastest means of escape; it uses lateral cyclic and opposite pedal to kick the helicopter sideways out of the downflow. But it still requires both lateral and vertical maneuvering room, neither of which was present at the accident site, and the crew’s accounts suggest that the accident developed almost instantaneously. The ATSB concluded that the ongoing shift in the drop sites led to an incremental shortening of the helicopter’s final approach path, eventually requiring the crew to operate it at “the upper mar- gins of allowable speed and angle of bank.” However, well- rehearsed evacuation procedures enabled them not only to survive an emergency that could easily have been fatal but to escape with- out severe injuries. Experience cuts both ways.
58 ROTOR 2020 Q3
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