| 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 December 2020, the pilot of a Robinson R22 helicopter was preparing to take off from a private property near Derby, Western Australia. The helicopter landing site was covered with rubber matting made from conveyor belts as a means of limiting dust when landing (Figure 1).
As the helicopter became airborne, it moved backwards slightly due to a headwind, resulting in the heel of the skid sliding between two overlapping sheets of matting (Figure 2). As the pilot attempted to gain height, the skid did not free itself and the helicopter pitched back and rolled to the left. The pilot recognised the early onset of dynamic rollover and attempted to recover by lowering the collective.[1]While that action prevented a rollover, it resulted in a hard landing and substantial damage to the helicopter.
Figure 1: Landing site conveyor belt matting
Figure 2: Snag hazard
Source: Aircraft operator
Helicopter static and dynamic rollovers
ATSB occurrence brief AB-2020-015 included the following explanation for helicopter 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 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 action
As a result of this incident, the operator advised the ATSB they have taken the following safety action:
- removed rubber conveyor belt matting from all company helicopter landing sites
- issued an internal safety alert to all pilots regarding the incident, reminding them of the requirements to ensure company helicopter landing sites are clear of any objects that could cause dynamic rollover on take-off
- reminded pilots how dynamic rollover can occur and the appropriate recovery technique.
Safety message
This was an unfortunate outcome for an operator who in addressing one problem, the potential for ‘brownout’[2]when landing, inadvertently created another hazard that resulted in damage to the helicopter. ATSB occurrence brief AB-2020-024 illustrates the potential consequences of ‘brownout’ when operating in dry and dusty conditions.
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.
- Brownout condition: is an in-flight visibility restriction due to dust or sand in the air. In a brownout, the pilot cannot see nearby objects which provide the outside visual references necessary to control the aircraft near the ground. This can cause spatial disorientation and loss of situational awareness leading to an accident.