What happened
On 16 January 2015, at about 0930 Eastern Standard Time (EST), the pilot of a Robinson R22 helicopter, registered VHSSD, was conducting aerial mustering operations on a property 23 km north-east of Roma, Queensland.
The helicopter was about 100 ft above ground level (AGL), with a low forward airspeed of about 5-10 kt and the cattle moving slowly uphill, when the pilot observed the cattle start to move back down the side of the hill towards a creek. The wind was light and blowing across the path of the helicopter from the left. The pilot elected to descend along the side of the cattle and turned the helicopter towards the right.
As he did that, the helicopter turned downwind with a high-power setting and low forward speed. The pilot realised he had turned downwind and started to raise the nose of the helicopter and raise collective.[1] He then detected a high rate of descent and an incipient vortex ring state, as the helicopter started to settle into its own downwash. He attempted to fly out of the situation, lowered the collective and wound the throttle on, but had insufficient forward speed and low rotor rpm. The low rotor rpm horn sounded at about 15-20 ft AGL. The pilot tried to regain rotor rpm but the helicopter sank quickly.
The pilot then ensured the skids were level and the helicopter collided with the ground. Due to the rough surface, the helicopter bounced into the air. The pilot pulled back on the cyclic[2] control, which resulted in the tail of the helicopter being chopped off by the main rotor. The helicopter then spun around and came to rest on its side (Figure 1). The helicopter was substantially damaged, and the pilot was uninjured.
Figure 1: Damage to VH-SSD
Source: Operator
Vortex ring state
The United States Federal Aviation Administration (FAA) handbook www.faa.gov/regulations_policies/handbooks_manuals/ describes the vortex ring state or settling with power, as an aerodynamic condition in which a helicopter may be in a vertical descent with 20% to maximum power applied and little or no climb performance.
The following combination of conditions is likely to cause settling in a vortex ring state in any helicopter:
1. A vertical or nearly vertical descent of at least 300 feet per minute (fpm). The actual critical rate depends on the gross weight, rpm, density altitude, and other pertinent factors.
2. The rotor system must be using some of the available engine power, between 20-100%.
3. The horizontal velocity must be slower than effective translational lift.
A fully developed vortex ring state is characterized by an unstable condition in which the helicopter has uncommanded pitch and roll oscillations, little or no collective authority, and a descent rate that may approach 6,000 fpm, if allowed to develop (Figure 2).
Figure 2: Vortex ring state
Source: FAA
Safety message
The pilot involved in this incident was highly experienced, with over 10,000 hours total flying (helicopter) time and over 6,000 hours in Robinson R22 aircraft. He stated that the fundamental message was to always have forward airspeed before turning downwind.
This incident highlights the importance of continually assessing and reassessing the prevailing conditions and their effect on aircraft performance. The Airbus Helicopters (formerly Eurocopter) publication, Decision Making for Single-Pilot Helicopter Operations, explains some of the factors that affect pilots’ decision making.
In the ATSB investigation AO-2013-099, the pilot of the helicopter lost situational awareness during a night approach and the helicopter developed a high rate of descent with a low forward airspeed. This resulted in the onset of an incipient vortex ring state in the final stages of flight.
Aviation Short Investigations Bulletin - Issue 40
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- A primary helicopter flight control that simultaneously affects the pitch of all blades of a lifting rotor. Collective input is the main control for vertical velocity.
- A primary helicopter flight control that is similar to an aircraft control column. Cyclic input tilts the main rotor disc varying the attitude of the helicopter and hence the lateral direction.