What happened
On 12 November 2016, a Robinson R44 helicopter, registered VH-YMJ, departed from a landing site on the Riversleigh Station property for the return leg of a private sight-seeing flight to Adels Grove aircraft landing area (ALA), Queensland. On board the helicopter were the pilot and three passengers.
The pilot used the helicopter to ferry two groups of people from Adels Grove ALA, to a dirt road landing site near a river in the morning for swimming, and then ferried the first group back to Adels Grove after lunch. At about 1420 Eastern Standard Time (EST), the pilot attempted the last planned departure of the day from the landing site for the return flight. The take-off direction followed the road, which was a south-south-east direction. The pilot reported that there were no power performance issues in the hover, but then during the initial climb, at about 100–130 ft above ground level (AGL) and between 15–20 kt airspeed, the helicopter started to experience a loss of performance. The helicopter started to descend and the pilot advised there was insufficient engine power to prevent the descent.
As the helicopter approached the ground, the pilot raised the collective lever[1] to cushion the landing. The right skid of the helicopter landed first, on the side of the road and the left skid landed off the side of the road below the right skid, which resulted in the helicopter rolling onto the left side before coming to rest (Figure 1). One passenger received minor injuries and the helicopter was substantially damaged.
Figure 1: VH-YMJ accident site
Source: Operator
Terrain and weather conditions
Riversleigh Station is located in the north-west quarter of Queensland, about 200 km south-west of the southern corner of the Gulf of Carpentaria. The countryside along the river in the vicinity of the accident site was described by the pilot as hilly with plateaus and escarpments and tall gumtrees.
The pilot reported that the weather conditions started fine in the morning, but changed late in the morning with a hot wind, which was gusting in strength and varying in direction. Wind strength varied from 0–20 kt, and wind direction varied between south and south-east. The temperature was about 38 °C, the elevation of the landing site was about 430 ft above mean sea level (AMSL), and the QNH[2] was about 1010 hPa. There were also rain showers in the vicinity.
Aircraft performance
The maximum take-off weight (MTOW) published for the Robinson R44 in the rotorcraft flight manual is 1,088 kg. The weights of the occupants and estimated fuel on board at the time of the accident indicate the all-up weight (AUW) of the helicopter was about 1,041 kg. With an estimated elevation of 430 ft, QNH of 1010 hPa and maximum height on take-off of 130 ft, the pressure altitude was about 650 ft when the pilot noticed there was insufficient power to continue the climb. The pilot reported that they started the take-off with about 10 kt of head wind and the helicopter had just passed through translational lift[3] and was at about 15–20 kt before the descent started.
If the wind speed dropped during the initial climb, then the helicopter could have been below translational lift at 100-130 AGL when the descent started. In this case the helicopter would have been in the hover out of ground effect[4] (HOGE) flight regime (Figure 2). At the reported temperature and AUW, this would place the helicopter at the limit of the hover altitude for the power available (point B, Figure 2).
In comparison, the hover in ground effect (HIGE) performance chart indicated the helicopter could maintain a hover at about 4,500 ft pressure altitude at the AUW and 38 °C.
Air density
The power produced by the engine and the lift produced by the helicopter rotors are influenced by air density. Low atmospheric pressure[5] and hot and humid conditions decrease air density. A decrease in air density decreases power available from the engine, but increases the power required for rotor thrust, because a larger angle of attack[6] is required from the rotor blades to produce the same lift.
Tail rotor and demand for power
The helicopter tail rotor is an anti-torque device, which is controlled by the tail rotor pedals to increase or decrease the angle of attack of the tail rotor blades. The engine provides the power for the tail rotor drive. Therefore, an increase in demand for anti-torque ‘bleeds off’ engine power. When the relative wind is directly in front of the nose of the helicopter, the helicopter airframe behaves like a weathervane, holding the nose of the helicopter into the wind and reducing the requirement for anti-torque. However, if the wind strikes the helicopter from the right side, this will increase the demand for left tail rotor pedal to maintain heading, which will bleed off engine power.
Figure 2: R44 HOGE performance
Source: Manufacturer, annotated by ATSB
Previous incidents
Previous ATSB reports of Robinson R44 helicopters descending with insufficient power in low airspeed and low air density (high density altitude) conditions include the following:
- Collision with terrain involving a Robinson R44, VH-HLB (AO-2014-154)
- Collision with terrain involving a Robinson R44, VH-UGC (AO-2013-203)
- Collision with terrain 10 km west of Gunpowder Mine, Qld, 21 February 2006, VH-HBS (200600979)
Safety analysis
The AUW of the aircraft was below the published MTOW and within the published limits for HIGE operations. At the time of take-off, there were no unusual noises or vibrations, the engine was delivering power to the rotors and the rotor speed did not decrease below limits. The pilot estimated they had a 10 kt headwind component, but that the wind was gusting in strength and variable in direction. Therefore, it is likely that the forced landing was the result of insufficient power for the prevailing environmental conditions at the helicopter’s AUW.
Two possible scenarios for insufficient power are a shift in wind direction to the right side of the helicopter or a decrease in head wind strength. In the first scenario, a shift in wind direction to the right would demand more left tail rotor pedal to maintain take-off heading and decrease the power available from the engine for main rotor thrust. In the second scenario, a decrease in wind strength just after translational lift would place the helicopter inside the HOGE flight regime and at the limit for the take-off AUW and temperature.
Findings
These findings should not be read as apportioning blame or liability to any particular organisation or individual.
- The forced landing was the result of power required to continue flight in excess of power available for the take-off AUW, temperature and wind conditions.
- The helicopter was within the published maximum take-off weight limit.
Safety message
This incident highlights the effect of high AUW, high air temperature and gusting wind conditions on the R44 helicopter’s performance. In particular, the combination of high AUW and high air temperature increase the power required and decrease the power available, which can lead to a significant difference between the HIGE and HOGE performance. In addition, the pilot reported that it is important to keep a close eye on changing wind conditions as they had never previously experienced a similar loss of performance.
Aviation Short Investigations Bulletin - Issue 57
Purpose of safety investigationsThe objective of a safety investigation is to enhance transport safety. This is done through:
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- 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.
- QNH: the altimeter barometric pressure subscale setting used to indicate the height above mean seal level.
- Translational lift is additional lift gained from induced airflow through the main rotors gained from forward airspeed. It reduces the power required for rotor thrust and becomes noticeable as the helicopter experiences about 12–14 kt of airspeed.
- Out of ground effect: helicopters require less power to hover when in ‘ground effect’ then when out of ‘ground effect’ due to the cushioning effect created by the main rotor downwash striking the ground. The height of ‘ground effect’ is usually defined as more than one main rotor diameter above the surface. This is 33 ft for the R44.
- Standard atmospheric pressure is 1013 hPa.
- Angle of attack is the angle between the rotor blade chord line and resultant relative wind. A larger angle of attack produces more drag and therefore requires more engine power.