| 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 8 November 2019, a Robinson R44 helicopter was conducting sling load operations about 10 NM north-east of Jabiru, Northern Territory. On the first lift of the day at approximately 1430 Central Standard Time, the pilot attached a load estimated to be about 120 kilograms to the helicopter by a 30-foot sling. In the prevailing windless conditions, the pilot lifted into a high hover, began to lift the load off the ground, and continued to climb until the load was clear of the surrounding trees. Once established in the hover, at approximately 80-100 feet AGL, the pilot observed all the engine gauges to be in the normal range and the helicopter appeared to be operating normally.
As the pilot commenced the translation into forward flight over a treed area, the rotor RPM began to decay and the low rotor RPM warning horn sounded. The pilot unsuccessfully attempted to regain rotor RPM by lowering the collective[1] and increasing the throttle. In an attempt to alleviate the situation by reducing the weight on the helicopter, the pilot released the slung load. This action did not assist with the recovery of rotor RPM and the aircraft continued to descend into the trees before colliding with the ground.
Figure 1: Area of operations and wreckage
Source: Operator
Operator’s investigation
The operator has conducted an investigation into the circumstances surrounding this accident.
The investigation revealed that in the hot and humid operating conditions, a contributing factor to the accident was the pilot over-pitching during the sling load operation. The over-pitching was to such a degree that it made successful recovery in the circumstances unlikely. The operator’s investigation also stated the pilot’s decision to depart the pick-up location over a treed area when clearer areas were available also restricted the options available once the helicopter started to descend.
Over-pitching
The International Civil Aviation Organization (ICAO) manual of aircraft accident and incident investigation, chapter 15: Helicopter investigation, described over-pitching as a phenomena that happens when collective pitch is increased to a point where the main rotor blade angle of attack creates so much drag that all available engine power cannot maintain or restore normal operation rotor speed. At low rotor speed, the rotor blades bend upwards and drag increases. The high inflow angles and rotor drag quickly decay main rotor speed, which may decrease to the point where the main rotor blades stall.
Hover performance
Hover performance is essentially a product of engine power available and engine power required. The main factors affecting engine power required in a hover are helicopter weight, density of air and proximity to the ground (ground effect).
To maintain a steady high hover, lift a sling load or climb, the helicopter requires more main rotor thrust to act as lift, which in turn requires more engine power.
As air density decreases with an increase in altitude, temperature, and to a lesser degree humidity a normally aspirated engine produces less power. Additionally, if the same amount of rotor thrust is needed, the rotor blades need a higher angle of attack, which creates more drag and generates a requirement for more engine power.
When a helicopter is hovering within about one rotor diameter[2] of the ground, the performance of the main rotor is affected by ground effect. A helicopter hovering in-ground-effect requires less engine power to hover than a helicopter hovering out-of-ground-effect.
Safety action
As a result of this occurrence, the operator has advised the ATSB that they are taking the following safety actions:
The operator will produce a report for all company pilots to fully explain the circumstances surrounding this accident to further educate and train pilots of the considerations when undertaking similar operations. To increase the safety of company operations, this further training will concentrate on decision-making, helicopter performance and weather effects, over pitching and using available terrain features when approaching and departing from unprepared landing sites.
Safety message
This accident serves as a reminder that when operating helicopters from unprepared landing sites, pilots should consider the approach and departure routes available in conjunction with operational constraints, weather (particularly wind), performance available and possible emergency recovery. Time spent considering and confirming the fundamental factors of decision-making, helicopter performance and limitations and the consideration of actions in an emergency may help prevent injury to crew and damage to, or loss of, an aircraft.
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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