Introduction
On 25 July 2024, four helicopters were involved in a multi-helicopter mustering operation along the Fitzroy River, approximately 13 km north-west of the Mount Anderson Station near Curtin, Western Australia. Two Robinson R22 helicopters, registered VH-HQH (Heli 1) and VH-HYQ (Heli 2), departed from a clearing behind the homestead, initially tracking west to join the other two helicopters before following the river. Shortly after take-off, at an altitude of about 150 ft, the two helicopters collided, resulting in an immediate loss of control. Both aircraft impacted the ground, and the pilots sustained fatal injuries.
Collision Details
A witness reported that the helicopters were traveling in a similar direction and at a similar speed before the collision. Examination of the wreckage indicated that at the moment of impact, the helicopters were almost alongside each other in an attitude consistent with level flight. Heli 2 was approximately 2 m behind and 2.5 m lower than Heli 1. This spatial arrangement was not consistent with the agreed sequencing, in which Heli 2 was to follow Heli 1. The pilot of Heli 3 reported last seeing the two helicopters passing about 80 m south of where Heli 3 was parked, with a lateral separation of less than 100 m. This suggests that prior to the collision, they were established on a parallel west-north-west track (approximately 285° magnetic) from the overnight parking area.
Wreckage and Audio Analysis
No evidence of pre-existing mechanical defects was found on either helicopter. Additionally, there was no indication that either aircraft struck flora, fauna, or a wire before the collision. The smoke observed from the lead helicopter immediately after the collision was likely caused by the damaged smoker tank spilling diesel onto the hot engine and exhaust after being struck by the main rotor of the following helicopter.
Audio analysis of a video recording made at the time of the collision (which did not capture the helicopters visually) revealed a notable reduction in audio frequency from one source shortly before the collision. Since both helicopters were operating normally, this frequency change was consistent with one helicopter maneuvering away from the microphone at a greater speed. Given that the helicopters were moving from right to left relative to the microphone at similar speeds, the turn was most likely made by Heli 1. This turn established the helicopters on converging flight paths. Approximately two seconds after the turn's completion, sounds consistent with the collision were detected.
The wreckage trajectories indicated that after the midair collision, the helicopters descended on different paths. The main debris trail was concentrated in two distinct areas, each containing the heaviest components of each helicopter, except for Heli 2's main rotor, which detached at or immediately after the collision. This suggests the helicopters were maintaining different tracks at the point of impact.
Pilot Actions
Shortly before the collision, the pilot of Heli 1 was communicating with Heli 3 and Heli 4, which were on the ground. It is plausible that the pilot of Heli 1 was looking toward those helicopters, focusing ahead and to the right, with the expectation that Heli 2 was behind. Meanwhile, the pilot of Heli 2, flying slightly lower than Heli 1, may also have looked toward Heli 3 and Heli 4. These helicopters were preparing to lift off, and the pilot of Heli 2 may have been assessing a potential collision risk. For the pilot of Heli 2, the ground helicopters would have been located to the right.
Environmental Factors
The twilight conditions were not expected to have impaired the pilot of Heli 2, as Heli 1 was higher and silhouetted against the sky. However, the effect on the pilot of Heli 1, who needed to observe Heli 2 against a background of terrain and vegetation, was uncertain. Both helicopters were equipped with standard position indication (conspicuity) lighting, which would have mitigated the visual challenge. Additionally, the helicopters had high-visibility paint schemes on their main rotor blades, but the contrast against the morning light background could not be determined.