| 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 3 August 2020, a Robinson R44 helicopter was conducting fire surveillance and abatement operations about 50 NM east of Jabiru, Northern Territory. In the early afternoon, the helicopter landed and shutdown in a pre-arranged temporary landing site waiting to pick up three ground personnel. At about 1700 Central Standard Time, once the personnel and their equipment was secured, the helicopter was started and brought into a hover. After checking the power available, a vertical take-off and climb was conducted until the helicopter was approximately 75 ft above ground level when a translation to forward flight commenced. The direction chosen was the most suitable available when considering terrain and wind direction.
As the pilot commenced the translation into forward flight over a treed area, the helicopter initially maintained height but as the pilot increased collective[1] the rotor RPM began to decay. The pilot increased the throttle and lowered the collective in an attempt to regain rotor RPM. As the helicopter descended, the collective was again raised in an attempt to arrest the descent, however the low rotor RPM warning horn sounded. A further attempt to recover the rotor RPM by lowering the collective was unsuccessful leaving a forced landing as the only option available for the pilot.
A landing site was selected in the treed area and the helicopter settled onto the ground resulting in a heavy landing. The main rotor blades struck several branches as the helicopter came through the tree canopy resulting in minor damage to the blades. After securing the helicopter, the pilot and ground personnel exited the helicopter and moved a safe distance away. The heavy landing activated the inertia switch on the emergency locator transmitter.[2]
Figure 1: The helicopter in situ after the forced landing
Source: Operator
Operator’s investigation
The operator conducted an investigation into the circumstances surrounding this accident that revealed several contributing factors that are summarised below:
- The aircraft was within the maximum take-off weight limits however it was close to the performance limit for an out of ground effect[3] hover in the prevailing weather conditions.
- The afternoon weather conditions had changed from both previous days and earlier that day. This was the first afternoon in approximately four months that there was a tropical build up with variable and quickly changing winds as opposed to the consistant south easterly winds associated with the dry season. The afternoon was hotter and more humid than previous days.
- The helicopter was over-pitched to a degree that successful recovery in the circumstances was not possible.
Over-pitching
Over-pitching is 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 operating rotor speed. At low rotor speed, the rotor blades bend upwards and drag increases further, which may decrease to the point where the main rotor blades stall. More information can be found in The International Civil Aviation Organization (ICAO) manual of aircraft accident and incident investigation, chapter 15: Helicopter investigation.
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 or climb vertically, 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[4] 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:
- It was identified post occurrence that there were some training and recency issues that were addressed particularly in the correct recovery actions for a low rotor RPM situation and the technique used to translate into forward flight from a vertical take-off or an OGE hover.
- The operator has limited the number of ground personnel to a maximum of two providing a larger power margin available for operating in these situations.
- All pilots have undertaken a flight review to identify and analyse any skill or knowledge deficiencies.
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, 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 the event of performance limitations or 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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- 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.
- Emergency locator transmitter (ELT): a radio beacon that transmits an emergency signal that may include the position of a crashed aircraft, and is either impact or manually activated.
- 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.
- The Robinson R44 main rotor diameter is 33 ft.