On 9 January 2015, the pilot of a Robinson R44, registered VH-YMD, was operating in support of the Northern Territory Police. As part of the operation, the pilot was asked to convey three passengers from a location on the bed of Todd River just north of Alice Springs, to another site near Alice Springs Township.
The pilot lifted off from the river bed and commenced departure over the river to follow what he assessed to be the most suitable departure route. Lift-off was normal, but as the helicopter climbed away from the river bed the pilot became aware that the main rotor RPM was decaying. In response, he overrode the governor and applied full throttle. The pilot was able to clear rocks and shrubs on the opposite side of the river, but with decaying rotor RPM he was forced to make a run-on landing on a flat area just above the river bed.
The helicopter tipped precariously during the landing. The pilot immediately lifted off again to reposition the helicopter with a level attitude, following which he shut down the engine and the passengers evacuated. The helicopter was substantially damaged during the event, particularly the left skid and the tail boom.
A number of factors related to power available and power required probably explain why the rotor RPM decayed during departure from the river bed. These factors relate to density altitude, take-off weight and possibly a tail wind component. This accident provides a reminder to pilots to carefully and accurately assess these factors before committing to any departure. When performance is likely to be adversely affected by a combination of these factors, extreme caution is warranted.
Aviation Short Investigations Bulletin - Issue 41
During the morning of 9 January 2015, the pilot of VH-YMD was operating in support of the Northern Territory Police. As part of the operation, the pilot conveyed two passengers to a site on the Todd River, just north of Alice Springs. The pilot landed on the sandy surface of the river bed where the passengers disembarked, then flew the helicopter from that location to a local landmark known as the Telegraph Station, about 3 km away. The pilot then conveyed another two passengers from the Telegraph Station to the site, and again landed on the sandy surface of the river bed, facing in a westerly direction (Figure 1).
Figure 1: VH-YMD landing site on the river bed
Source: Northern Territory Police
Soon after, the pilot was asked to convey three passengers back to the Telegraph Station, as a continuing part of the police operation. When all three passengers had boarded the helicopter, the pilot lifted off from the river bed. Lift-off was normal, and the pilot commenced departure in a southerly direction over the river (Figure 2) to follow what he assessed to be the most clear and suitable departure route from the river bed. As the helicopter climbed away from the river bed, the pilot became aware that the main rotor RPM was decaying. In response, he overrode the governor and applied full throttle.
The pilot needed to maintain height to clear the rocks and shrubs on the southern side of the river, but was acutely aware that rotor RPM would be further compromised by the application of more collective. The pilot carefully managed the collective and the helicopter cleared the rocks and shrubs, but with decaying rotor RPM, he realised that continued climb was not possible. The pilot steered the helicopter toward a flat area, just above the river bed on the southern side of the river, and conducted a run-on landing (Figure 3).
Figure 2: Photograph taken from left side of VH-YMD as it crossed the river
Source: Northern Territory Police
Figure 3: Take-off and landing area
Source: Northern Territory Police (two photographs joined and edited by the ATSB)
During the run-on landing, the left skid sank into mud and struck a rock. The left skid was substantially damaged by the impact (Figure 4) and the helicopter tipped precariously. As the helicopter came to a stop, it was on a substantial lean. The pilot reported that he considered the situation to be unsafe, so he immediately lifted off again and repositioned the helicopter on the ground with a level attitude. During the second lift off, a passenger recalled that the main rotor of the helicopter struck the limb of a tree, and that this was when damage to the tail boom occurred (Figure 4). The pilot then shut down the engine and the passengers evacuated the helicopter.
Figure 4: Damage to VH-YMD skid (left) and tail boom (right)
Source: Northern Territory Police (edited by the ATSB)
At the time of the accident, weather conditions were overcast, with showers in the area and a temperature of about 25 ºC. The wind at Alice Springs Airport (about 18 km to the south) was from the north-west at about 15 kt, and the QNH was about 1003 hectopascals. While the pilot was aware that the wind was generally a north-westerly, he assessed the wind at the site as relatively light and variable. The relative humidity at Alice Springs Airport was around 80 per cent, and the pilot reported humid conditions at the accident site.
The pilot commented that he believed that the accident resulted from a combination of a relatively heavy take-off weight, the prevailing conditions, and limited departure options because of surrounding terrain and obstacles. The pilot indicated that, with the benefit of hindsight, he should have taken two trips to move the three passengers, rather than attempt to take off with three passengers on board. He believed that he would have been able to complete the departure safely at a lower take-off weight.
Power required and power available
A number of factors related to the power required and the power available warrant consideration in understanding the probable reasons for which the pilot experienced decaying main rotor RPM during departure from the river bed, as the helicopter moved out of ground effect and transitioned into forward flight. These factors include density altitude, take-off weight and the wind component.
- Density altitude. Increasing density altitude adversely affects helicopter performance through the combined effects of reducing the power available and increasing the power required. Considering elevation and temperature, and barometric pressure in the area, the density altitude at the accident site would have been around 4,000 ft. High relative humidity would have had the effect of further increasing the density altitude.
- Take-off weight. Increasing the take-off weight increases the power required. The greater lifting force demanded of the main rotor, and the requirement to counter the associated increased torque effect with the tail rotor, both contribute to an increased power requirement. The pilot estimated the weight of the helicopter at the time of the accident to be less than the maximum permitted take-off weight, however subsequent calculations by the operator using actual data, indicated that the take-off weight was marginally above the maximum permitted take-off weight.
- Wind component. Taking off with a tailwind component increases the power required because of the diminished or delayed influence of translational lift. Additionally, a tailwind or crosswind component may require greater tail rotor force to maintain directional control during departure, which places an increased power demand on the engine. Although the pilot commented that the wind seemed light and variable prior to departure, the helicopter may have encountered a tailwind component as it climbed away from the river bed during the accident flight.
The following references discuss factors affecting helicopter performance, and provide some guidance to pilots regarding the associated considerations:
- A ‘Good Aviation Practice’ booklet titled Helicopter Performance, produced by the Civil Aviation Authority (CAA) of New Zealand. The booklet is available via the CAA website.
- The Federal Aviation Administration (FAA) Helicopter Flying Handbook(chapter 7 deals with helicopter performance). The handbook is available on the FAA website.
The Robinson R44 Pilot’s Operating Handbook includes a number of important safety tips and notices. Pilots (particularly those who fly Robinson helicopters) are encouraged to carefully reflect on these safety tips and notices – the tips are suggestions intended to improve safety, while the notices have been issued as a result of various accidents and incidents. The safety tips and notices are available in the R44 Pilot’s Operating Handbook on the Robinson Helicopter Company website under the Publications tab. Two Safety Notices with relevance to this accident are Safety Notice 10 (Fatal accidents caused by low RPM rotor stall) and Safety Notice 24 (Low RPM rotor stall can be fatal). One safety tip with particular relevance to this accident is:
Never allow rotor RPM to become dangerously low. Most hard landings will be survivable as long as the rotor is not allowed to stall.
Three other ATSB investigation reports that identified helicopter performance and low main rotor RPM as possible factors include AO-2013-203, 200600979 and 199900833. These investigation reports are available on the ATSB website.
This accident provides a reminder of the effect on helicopter performance of density altitude, weight, and possibly wind. Pilots are encouraged to carefully and accurately assess these factors before committing to any departure. Careful assessment of these factors is essential to ensure that an adequate performance margin is maintained, particularly under high density altitude conditions, when the helicopter is near its maximum take-off weight, or where the direction of departure is downwind. When performance is likely to be adversely affected by a combination of these factors, extreme caution is warranted.
- The collective is a primary helicopter flight control that simultaneously affects the pitch of all blades of the lifting rotor. Increasing collective increases blade pitch, which increases the lift force generated by the blades. Increasing the collective also increases drag on the rotor blades, which can only be overcome by increasing power.
- A run-on landing is a landing where the helicopter lands with forward speed.
- Damage to the tail boom is consistent with damage that could be expected if the main rotor struck the tail boom.
- QNH is the altimeter barometric pressure subscale setting used to provide an altimeter indication of height above mean sea level in that area.
- Ground effect refers to the apparent improvement in helicopter performance near the ground which results from a modification of the airflow through the main rotor due to the interaction of that flow with the ground beneath.
- In this context, torque effect is the reaction of the helicopter to the torque applied by the main rotor. This effect is countered by the tail rotor.
- Translational lift is the additional lift resulting from induced airflow through the main rotor as a result of forward airspeed (oncoming flow of air through the main rotor).