| 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 23 April 2020, the student pilot of a Robinson R44 helicopter was returning to Townsville Airport, Queensland, from a solo training navigation exercise.
As the pilot returned the helicopter to the parking position near the hangar and was in the process of landing from the hover, he momentarily lost directional yaw control. At this time, one of the helicopter’s skids was in contact with the ground while the collective[1] was in a raised position. This resulted in an unstable airframe that commenced a roll about the skid.
Despite the efforts of the pilot to recover, the helicopter continued to rollover and came to rest on its side resulting in substantial damage. The pilot was uninjured in the accident.
Figure 1: Helicopter post-accident
Source: Townsville Airport
Figure 2: Helicopter’s bent tail boom and broken main rotor blade
Source: Townsville Airport
Helicopter static and dynamic rollovers
Static rollover occurs when a helicopter is pivoted about one of its landing skids or wheels and the helicopter’s centre of gravity passes outside the in-contact skid or wheel. Once in this position, removal of the original force that raised the helicopter to that angle will not stop the helicopter from rolling further. This angle is termed the ‘static rollover angle.’
A rotors-running helicopter resting with one landing skid or wheel on the ground may, without appropriate pilot input, commence rolling. Under certain circumstances, this roll cannot be controlled and the helicopter rolls over. This condition is known as ‘dynamic rollover’ and is a function of the interaction between the:
- horizontal component of the total rotor thrust (or lift) acting about the point of ground contact
- weight of the aircraft, initially acting between the helicopter’s skid landing gear or wheels. This second, counter-rolling moment decreases the greater the roll.
Recovery from dynamic rollover is by smoothly lowering the collective lever while controlling any tendency to roll in the opposite direction with cyclic[2] to re-establish the helicopter’s weight evenly on the ground. In general, the application of smooth collective inputs is more effective in avoiding rollover issues than using the cyclic control.
Safety message
This accident highlights the importance of smooth and controlled flight control inputs in the critical phases of flight. While a helicopter is in contact with the ground and before its full weight is applied to the landing gear, it is subject to various influences such as the possibility of a rollover. A thorough understanding of the principles of, and contributing factors to, both static and dynamic rollover and the recovery methods are essential to conducting safe helicopter lift-offs and landings.
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.
- Cyclic: a primary helicopter flight control that is similar to an aircraft control column. Cyclic input tilts the main rotor disc, varying the attitude of the helicopter and hence the lateral direction.