| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 28 June 2018 at 1125 Central Standard Time, the pilot of a Robinson R44 Raven II helicopter departed for a return charter flight from Connellan Airport, Northern Territory, to Mt Conner, NT. The originally scheduled, larger aircraft, a Bell JetRanger experienced an unserviceability, and the Robinson R44 was allocated to the flight. There were three passengers on board. When the pilot departed Connellan Airport, the helicopter was at its maximum gross weight of 1,134 kg. This was the pilot’s first trip to Mt Conner.
Mt Conner, known as Attila to the Pitjantjara people, is a mesa standing 2,818 ft above mean sea level, and 984 ft above ground level. On the day, a high-pressure weather system pushed the pressure altitude[1] slightly lower to around 2,600 ft. There was a light wind from the east-northeast. The flight to Mt Conner took just under 40 minutes, using 25 kg of fuel on the way. The landing weight of the helicopter at Mt Conner was 1,109 kg. According to flight manual performance charts, the helicopter was capable of performing an out of ground effect hover at the landing site.
The pilot wasn’t comfortable with his first approach and correctly aborted it. The second approach felt better to the pilot and he flew the helicopter at a 300 ft/min rate of descent towards the western ridgeline. At 40 KIAS[2], 50 ft above and 10 m from the ridge, the pilot noticed a sudden increase in rate of descent. The pilot thought he had entered vortex ring state[3] (VRS) and initiated a technique known as the Vuichard Recovery. This involves using full power, tail rotor thrust and lateral cyclic to move the helicopter sideways out of the downwash of the main rotor. The Vuichard Recovery technique had not been taught to the pilot, nor was his proficiency in its use tested.
At 40 knots, the helicopter’s downwash flows out behind the aircraft and VRS will not develop. The VRS recovery initiated did not arrest the rate of descent. The aircraft was now in a state whereby the power required to arrest the rate of descent exceeded the engine power available. Cancelling the lateral movement to avoid sideways contact with the ground, the pilot initially lowered the collective[4] before pulling full collective to reduce the rate of descent.
Pulling collective increases the pitch on the main rotor blades to increase lift and increases the power demand on the engine. The low RPM light and horn activated as aerodynamic drag on the main rotor blades exceed the available power from the engine, slowing the rotor; a situation known as overpitching. The aircraft landed heavily in a level attitude 20 m to the right of the helipad. The skids absorbed the impact and there were no injuries.
Safety message
Pinnacle approaches bring complexity not encountered on level ground. Particularly, a change in the way groundspeed is judged on approach. Visual references are further away than usual and do not provide adequate groundspeed reference. Approaching a pinnacle too fast will result in ground rush. The sudden availability of rate of closure cues is perceived as a rapid increase in rate of closure. The pilot must instead manage their approach through use of attitude, instruments and observation of the expansion rate of the sight picture around the aiming point.
Pinnacle approaches are an example of a sequence that requires practice and maintenance of skill. Practice allows a pilot to develop a suitable conceptual model of the sequence. This then allows the pilot to manage the current state of the aircraft against the desired state; using pre-determined gates[5] to manage the profile and detect divergence in the absence of normal visual cues. Development of the correct mental model through practice will support well-timed initiation of a correct course of action. Flight schools play a fundamental part in developing these mental models in new pilots.
Before using new techniques in flight, pilots need training in their use. Training sessions designed and conducted by pilots proficient in both instruction and the technique of interest, create a safe environment for pilots to develop new skills and test new techniques. All pilots are encouraged to discuss their needs with respect to training in new methods or techniques with their Chief Pilot prior to employing new manoeuvres in flight.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
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- Pressure Altitude: The elevation experienced relative to the International Standard Atmosphere used for calculating aircraft performance.
- KIAS: indicated airspeed expressed in knots, used by pilots as a reference for all aircraft manoeuvres.
- Vortex Ring State: The ingestion by a rotor system of its own downwash, causing loss of lift.
- Collective: 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.
- Gates: A series of targets for flight parameters used to manage aircraft performance. I.e. a desired combination of airspeed, rate of descent, altitude and power available achieved at a predetermined point. Missing a gate would result in discontinuation of the manoeuvre.