Forced/precautionary landing

Investigation summary

What happened

On the morning of 14 July 2025, a Bell 206L-3 helicopter, registered VH-JMM, was being operated on multiple passenger charter flights around the Arnhem region in the Northern Territory. On board was a pilot and one passenger. 

During the fourth leg of the day at approximately 1338, while looking down and to the left out of the helicopter, the pilot heard a loud bang. The pilot saw a large bird laying between the 2 occupants, and what appeared to be serious injuries to the passenger’s upper body. The pilot reached over to the passenger to check for a pulse but was unable to feel one. Noting the passenger required immediate attention, they decided it would be better for the passenger to receive medical attention at Lake Evella Aerodrome where a police station was next to the airport.

Police, a local nurse and doctor attended to the passenger, however the passenger had succumbed to injuries. The helicopter sustained minor damage. 

What the ATSB found

While cruising at about 900 ft AMSL, the helicopter struck a white bellied sea eagle which passed through the windshield and impacted the passenger.

The pilot had limited opportunity to detect the bird as they were looking down and to the left of the helicopter’s trajectory, reducing the pilot’s ability to see the bird and change the helicopter’s flight path in time, and likely rendering the collision unavoidable under the circumstances.

The passenger was not wearing a helmet at the time, nor was there an aviation regulatory requirement for them to do so. In this case, the location of the bird strike on the passenger was such that wearing a helmet probably would not have reduced the level of injury.

Safety message

Birdstrike is an almost unavoidable and relatively common hazard for all aviation operations. While these strikes typically result in minor or no damage to an aircraft and no injuries to occupants, this is the third fatal birdstrike accident in Australia in recent years.

Pilots are reminded that maintaining effective lookout will assist in maintaining better situational awareness in flight, and also assist in providing better outcomes to see‑and‑avoid not only birds, but other airspace users. 

Additionally, pilots should maintain situational awareness, especially when flying over waterways or wetlands. It is relatively common for large birds, such as eagles, hawks, and gulls, to attack helicopters and drones, often perceiving them as threats or territorial intruders. These birds may display aggressive behaviour during nesting or breeding seasons, diving at or striking the aircraft in an attempt to drive it away. Helicopter operators should consider whether available occupant protections, such as the wearing of flight helmets and the fitment of impact-resistant aircraft windshields, are appropriate for their operations.

The investigation

The occurrence

On the morning of 14 July 2025, a Bell 206L-3 helicopter, registered VH‑JMM, was being operated by Nautilus Aviation on multiple air transport (passenger charter) flights around the Arnhem region in the Northern Territory. On board was a pilot and a passenger. 

At approximately 0928 local time the helicopter departed Gove Airport for Donydji. The pilot reported that from Donydjii, they flew to ‘Nyquist tower’[1] and then on to Mirrnatja before departing for Burrum, which would be the last stop of the day before returning to Gove (Figure 1).

Figure 1: Flight path overview

Source: Google Earth, annotated by the ATSB

The flight departed for Burrum at 1313 (Figure 2) and the pilot established a cruise altitude of about 900 ft above ground level. The pilot recalled having a conversation with the passenger about a waterway which they were flying near, and was familiar to the passenger. The pilot recalled slightly deviating off track to view the waterway. At approximately 1338, while looking down to the left out of the aircraft, the pilot recalled hearing a loud bang. 

The pilot saw a large bird laying between the 2 occupants, and what appeared to be serious injuries to the passenger’s upper body. The pilot reached over to the passenger to check for a pulse, but was unable to feel one. Noting the passenger required immediate attention, the pilot deliberated whether to land nearby and attempt resuscitation, and initially began to descend. However, considering the logistical issues with getting medical attention in a remote location, they decided it would be better for the passenger to receive medical attention at Lake Evella Aerodrome where a police station was next to the airport. 

Figure 2: Accident flight overview

Source: Google Earth, annotated by the ATSB

The pilot landed the helicopter at Lake Evella Aerodrome at approximately 1346. They stated they attempted to call emergency services on 000, however the call did not connect. They decided not to attempt the call a second time and ran to the police station for assistance instead. 

Police, a local nurse and doctor attended to the passenger, however the passenger had succumbed to injuries. The aircraft sustained minor damage (see Helicopter damage). 

Context

Pilot information

The pilot held a valid Class 1 Aviation Medical Certificate and a Commercial Pilot Licence (Helicopter). The pilot had accumulated 2,553 hours of aeronautical experience, of which 1,319 hours was on the Bell 206L.

The pilot had been with the operator since September 2024 and had regularly flown these routes to remote communities as part of their employment. 

Passenger information

The passenger was a frequent passenger on the routes operated on the day and had travelled by helicopter regularly to remote communities as part of their employment since 1995.

The pilot reported to having flown this passenger to remote communities on multiple occasions. Familiar with the aviation environment, the pilot reported the passenger would assist with monitoring for birds during flights, as they were aware they presented a hazard in flight. 

The post-mortem examination report indicated the passenger was hit between the lower jaw and the upper chest, sustaining fatal injuries to the neck, chin, lower jaw and the right side of the chest. 

Helicopter information

General

VH-JMM was a Bell Helicopter Company B206L‑3 Long Ranger, S/N 51400, manufactured in Canada in 1990. It was first registered in Australia in June 2017. The aircraft was registered to the operator in January 2024.

VH-JMM was a helicopter with two‑bladed main rotor and tail rotor systems, powered by a single Rolls-Royce 250‑C30P gas turbine engine.

At the time of the accident, the helicopter had completed 13,250 hours in service and had a current maintenance release.

Helicopter damage

The helicopter sustained damage to the passenger side windshield. There was no other reported damage to the aircraft (Figure 3).

Figure 3: Helicopter damage

Source: Northern Territory Police Force, annotated by the ATSB

Helicopter windshields

VH-JMM was fitted with standard acrylic windshields, which were not rated for impact resistance.

In 2016 Bell Helicopter Company introduced polycarbonate windshields, through a supplemental type certificate (STC) for the Bell 206 series, including the 206L. These were available as an additional option for current owners, offering higher impact resistance compared to traditional acrylic, reducing the risk of breaches from birdstrikes or other impacts. These were rated to United States regulatory requirements of a 2.2 lb (1 kg) bird traveling at V_NE (the helicopter’s never-exceed speed).[2]

Despite having a higher impact resistance than acrylic, polycarbonate windshields are more sensitive to scratches, and reportedly susceptible to clouding or hazing due to ultraviolet light exposure, resulting in loss of optical clarity and necessitating more frequent replacements.

Weather information

The terminal aerodrome forecast for the accident region forecasted clear conditions for the flight with scattered cloud above 3,500 ft and visibility greater than 10 km.

At 1530, the weather station at Elcho Island Airport, 48 km north of the accident location, recorded the wind as 4 kt from 110° magnetic. There was scattered cloud at 1,000 ft, visibility was greater than 10 km and the temperature was 23°C.

The pilot reported that the weather varied depending on where they were flying, however it was mostly clear with some areas of cloud. They reported the clouds were above their cruise height. 

Recorded data

The aircraft was fitted with a Spidertracks flight tracking unit and the pilot used OzRunways electronic flight bag software; both recorded flight data. Flight data indicated that the aircraft was cruising at 900 ft above ground level at a groundspeed of 94 kt at the approximate time the bird was struck. The data showed an initial deceleration to 86 kt groundspeed and a decrease in altitude of 50 ft, followed by a secondary decrease in altitude of approximately 150 ft (likely associated with the pilot’s consideration of whether to land). The track showed that the helicopter then climbed to 800 ft and increased groundspeed to about 70 kt (Figure 4).

Figure 4: Recorded flight track

Source: Google Earth, annotated by the ATSB

Bird information

Recovered biological specimens of the bird, including wing feathers and residue from the carcass, were found both inside the helicopter and on the passenger. Through images of the bird, the ATSB determined the species to be a white‑bellied sea eagle. 

The white-bellied sea eagle (Haliaeetus leucogaster) (Figure 5) is a large raptor commonly found in coastal regions of Australia, recognised for its distinctive white head, belly, and tail contrasted by dark greyish‑brown wings and back. Adults measure approximately 66‍–‍85 cm in length, with a wingspan of 1.8‍–‍2.2 m (Debus, 2017). Adult males typically weigh between 1.8‍–‍3 kg, while females average 2.5‍–‍4.5 kg (Marchant & Higgins, 1993). The weight and sex of the bird in this accident was unknown. 

Figure 5: Some of the bird remains retrieved from the helicopter

Source: Northern Territory Police Force

These eagles are often observed soaring over coastlines, estuaries, or inland waterways, preying on fish, seabirds, or carrion, and can reach heights of up to 1,000 m (about 3,300 ft) (Ferguson‑Lees & Christie, 2001). Their activity increases during the June to January period in Australia, which is their breeding season (Debus, 2017). 

It is relatively common for large birds, such as eagles, hawks, and gulls, to attack helicopters and drones, often perceiving them as threats or territorial intruders. These birds may display aggressive behaviour during nesting or breeding seasons, diving at or striking the aircraft in an attempt to drive it away (Washburn & others, 2015). 

Limitations of see-and-avoid

The human visual system is inherently limited in detecting small objects such as birds at distances. Hobbs (1991) notes that effective visual scanning requires systematic eye movements across the visual field, yet pilots often employ unsystematic techniques, resulting in unsearched areas. Furthermore, the cognitive process of identifying a threat, assessing its collision risk, deciding on evasive action, and executing control inputs requires time that is often unavailable in low‑altitude, high‑speed scenarios.

Birds present unique challenges to the see‑and‑avoid principle due to their relatively small size, unpredictable flight paths, and speed difference compared with aircraft. Unlike aircraft, birds cannot be tracked electronically, meaning pilots must rely solely on visual identification.

Survivability

Restraints

The helicopter was fitted with 4‑point harnesses in the front seats. The pilot reported both they and the passenger had been fastened into the seats by the aircraft’s 4‑point harnesses. 

Helmets

The pilot reported wearing a flight helmet[3] and reported wearing a helmet whenever possible, noting that helmets had saved lives in the past. The pilot recalled previously having a discussion with the passenger about helmets and the benefits of them. 

The passenger was not wearing a helmet at the time, nor was there an aviation regulatory requirement for them to do so. Nautilus Aviation stated that there was no requirement for passengers to wear a helmet and the decision on their use rested with the passengers themselves or their employers.

Telstra helicopter charters

The passenger was on board the aircraft as part of their work for Telstra, a telecommunications company. Telstra reported that its employees took about 630 helicopter charters on average per year, a mix of passenger charter (transit) and aerial work.

The employer had an operational framework for chartering aircraft that addressed many risks typically associated with helicopter flights, outlining expectations for the aircraft operator. These included the requirement for the aircraft operator to perform a risk assessment ‘prior to the first flight of any new operation by the Charter operator.’ 

Telstra did not have prescribed or recommended personal protective equipment for employees travelling or working on helicopters. Telstra advised that it relied on the licenced and accredited aviation providers that it engages to advise on safety of flight aspects including the use of personal protective equipment (PPE).

Related occurrences

Global data

Birdstrikes are a recognised hazard in aviation and there are mitigators in place at certified airports, however, there are challenges when operating outside of these areas. 

A review of Australian and internation data was conducted using the Avisure serious accident database. Between 1912 and 2024, birdstrikes have resulted in 763[4] reported aviation occurrences worldwide that involved serious or fatal injuries, of which 204 were fatal. Among these fatal cases, 18 involved rotary‑wing aircraft such as helicopters. These 18 accidents comprised 13 civil and 5 military rotary‑wing aircraft (Figure 6).

Figure 6: Global birdstrike data resulting in fatalities

Data does not include this occurrence (AO-2025-039). Source: Avisure

United States data

In the United States, a total of 13,667 bird strike occurrences were reported to the Federal Aviation Administration (FAA) in operations involving aircraft (fixed-wing and rotary-wing) under 5,700 kg maximum take‑off weight from 2014 to 2024. Of these, 60 occurrences resulted in non-fatal injuries, and 11 were fatal. 

A subset of 334 occurrences involved birds striking and damaging the aircraft windshield, with 48 of these occurrences (14.4%) resulting in serious injuries and 6 (1.8%) leading to fatal injuries. 

Of the total, 3,001 occurrences involved rotary-wing aircraft, which equated to a birdstrike every 285,390 flight hours (Table 1). These included 201 recorded windshield strikes, that resulted in 28 (13.9%) serious injuries and 2 (1.0%) fatalities. 

Table 1: Reported helicopter birdstrikes comparison 2014–2024

In comparison to the most frequently struck aircraft component, the wings, with 1,171 occurrences, only 6 (0.5%) resulted in injuries including 1 with fatal injuries (0.09%), indicating that the proportion of serious and fatal outcomes from windshield strikes is unexpectedly high relative to other aircraft parts. 

Australian data

Birdstrikes in Australia

Between 2014–2024 the ATSB aviation wildlife dashboard indicated there were 17,060 reported birdstrikes reported to the ATSB across all aircraft types (including fixed‑ and rotary-wing). There were 412 reported birdstrikes during helicopter operations (Table 2), which equated to a birdstrike every 39,690 flight hours. The data did not include what component was struck unless the component was damaged, so it was not possible to determine the proportion of windshields struck that were penetrated or damaged. Of the 412 reported birdstrikes to helicopters, 17 had damage to the windshield.

Table 2: Reported helicopter birdstrikes within Australia 2014–2024

ATSB records indicate there were 2 fatal accidents in civil aircraft in Australia due to birdstrike. Additionally, there was 1 serious accident involving a bird entering through the windshield. These investigations are described in the following subsections.

Birdstrike involving Glasair Sportsman GS‑2, N666GM, near Bathurst, New South Wales, on 24 December 2015 (

)

During take-off the aircraft collided with a wedge‑tailed eagle (Aquila audax), penetrating the windscreen and causing significant damage to the propeller and engine, while also striking the pilot, who sustained serious facial injuries and was temporarily unable to see. The pilot, who was wearing a headset and spectacles (both dislodged and damaged during the impact), managed to land safely. 

Birdstrike and in-flight break-up involving a Bell 206L‑1, VH‑ZMF, near Maroota, New South Wales, on 9 July 2022 (AO‑2022‑034)

Shortly after departing from a private helipad, the helicopter was struck by a wedge‑tailed eagle (Aquila audax) just below the front left windscreen. The pilot, likely startled by the birdstrike and distracted by sun glare and a required radio frequency change, made abrupt control inputs that caused the main rotor to sever the tail boom, resulting in an in‑flight breakup and collision with terrain. The pilot, who was the sole occupant, was fatally injured.

Birdstrike and collision with terrain involving Air Tractor AT‑502B, VH‑KDR, 32 km east‑north‑east of Chinchilla Airport, Queensland, on 19 September 2022 (AO‑2022‑043)

During low-level aerial spraying at about 8 feet above ground, the aircraft was struck by a large Australian bustard (Ardeotis australis), which shattered the right windshield. The bird entered the cockpit, likely impairing the pilot’s ability to control the aircraft. The aircraft continued for approximately 310 m before colliding with terrain, resulting in the pilot being fatally injured and destruction of the aircraft.

Safety analysis

Birdstrike

Images from the accident site showed that the aircraft collided with a white‑bellied sea eagle (Haliaeetus leucogaster). The pilot had limited opportunity to detect the bird as they were looking down and to the left of the helicopter’s trajectory, so it was probably in their peripheral vision where detection of small objects is very limited. Even if they had been looking ahead at the time, they may not have been able to see the bird in time to avoid it due to the inherent limitations of the see-and-avoid principle. The closure rate to the soaring bird would have been around 94 kt and the difference in speed between them would have also made the relative trajectory almost direct. These factors further reduced the pilot’s ability to see the bird and change the helicopter’s flight path in time, likely rendering the collision unavoidable under the circumstances.

Considerations for aircraft operators and employers

Windshield impact resistance

The analysis of bird strike data highlights the significant safety risks posed to windshields. In windshield impacts in the United States, 14.4% caused serious injuries and 1.8% caused fatalities. Australia’s occurrences included 3 fatal and 3 serious injuries. Comparison of the United States and Australian data indicated that there was a higher chance of both birdstrike and the strike resulting in windshield damage per flight hour in Australia.  

There is an elevated risk for helicopter operations due to low‑altitude operations and often less robust windshield designs. This is because if a bird penetrates the windshield, it can directly impact occupants, causing injury or incapacitation of flight crew, which may lead to loss of aircraft control or further operational hazards. While advancements in windshield design, such as laminated materials and reinforced structures, have mitigated many impacts, the data highlights vulnerabilities in extreme cases. 

Manufacturers like Robinson and Bell have both released birdstrike‑rated windshields that provide higher impact resistance and significantly decrease the likelihood of objects breaching the windshield upon impact. However, these windshields have been rated to withstand a 1 kg bird strike at the aircraft’s never‑exceed speed, and the occurrence scenario involving a 3 kg bird colliding with the helicopter would likely exceed the windshield’s design limits. Nevertheless, and noting there are some disadvantages of impact‑resistant windshields, operators are encouraged to consider installing impact‑resistant windshields if operating in areas with a high probability of birdstrike.

Helmets

Helicopter pilots often wear helmets as a safety measure due to their frequent exposure to the dynamic conditions of rotary‑wing flight, where turbulence, rapid manoeuvres, and potential accidents pose risks of head injury. In contrast, passengers often do not wear helmets, as the risk is lower for occasional travellers, particularly considering the other safety measures associated with commercial passenger transport operations.

Passengers who travel frequently in helicopters fall between these 2 extremes. They are naturally exposed to a higher risk (over the occasional passenger) simply due to the increased number of flights. While the pilot reported being a helmet advocate and had previously discussed the potential benefits with the passenger, the decision whether to wear a helmet was ultimately left to the passenger’s discretion.

Helmets provide an additional layer of protection against birdstrikes, particularly in aviation scenarios like the Glasair Sportsman GS‑2 incident (AO‑2016‑001). A helmet, often equipped with a sturdy visor, can shield the face and head from small‑object impacts, reducing the risk of injury from a shattered windshield. Additionally, a helmet, especially one designed for aviation, is engineered to absorb and disperse kinetic energy from impacts with larger objects such as a bird potentially mitigating the severity of injuries like those sustained by the pilot of the Glasair, who was not wearing a helmet and suffered serious facial injuries. The helmet’s hard outer shell and padded inner liner work together to reduce the force transmitted to the skull, significantly lowering the risk of traumatic brain injuries, concussions, or skull fractures.

A helmet would not have prevented the passenger’s injuries in this case due to the impact location. Nevertheless, wearing a helmet as standard practice would provide some protection against a range of other potential hazards.

Pilot response

The pilot maintained control of the aircraft despite the sudden disruption and potential aerodynamic effects of the compromised windscreen. They promptly identified the nearest suitable landing site with access to medical facilities and executed a controlled descent and landing. 

The pilot’s effective response and adherence to emergency procedures ensured the injured passenger was positioned for immediate medical response, highlighting sound decision‑making under extreme circumstances. 

Findings

From the evidence available, the following findings are made with respect to the birdstrike involving Bell 206L‑3, VH‑JMM, 16 km west-north-west of Lake Evella Aerodrome, Northern Territory, on 14 July 2025.

Contributing factors

• While cruising at about 900 ft above mean sea level, the helicopter struck a white‑bellied sea eagle, which passed through the windscreen and impacted the passenger.

Other factors

• Despite the injuries to the passenger and the damage to the aircraft, the pilot demonstrated composure and maintained control of the aircraft, enabling a calm and controlled return to a location where medical assistance could be provided.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot
• Nautilus Aviation
• Northern Territory Police Service
• recorded data from the Spidertracks unit on the helicopter
• OzRunways.

References

Australian Transport Safety Bureau (2002). The Hazard Posed to Aircraft by Birds. Canberra: ATSB.

Debus, S. J. S. (2017). Australasian Eagles and Eagle-like Birds. CSIRO Publishing.

Ferguson-Lees, J., & Christie, D. A. (2001). Raptors of the World. Christopher Helm.

Hobbs, A. (1991). Limitations of the See-and-Avoid Principle. Canberra: ATSB. 

Marchant, S., & Higgins, P. J. (Eds.). (1993). Handbook of Australian, New Zealand and Antarctic Birds: Volume 2 - Raptors to Lapwings. Oxford University Press.

Washburn, B. E., Begier, M. J., & Wright, S. E. (2015). Wildlife strikes to civil helicopters in the United States, 1990–2011. Wildlife Society Bulletin, 39(1), 115‑120.

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• the pilot
• Nautilus Aviation
• Telstra
• Civil Aviation Safety Authority
• Northern Territory Police Force
• TSB Canada.

Submissions were received from:

• Nautilus Aviation
• Telstra

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2025   Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Investigation summary

What happened

On 11 June 2025, a Robinson R44 Raven I helicopter, registered VH-OOE, was being operated on a personal transport flight from Daly Waters Aerodrome to Wally’s Airstrip, Northern Territory, with a pilot and one passenger on board. As the helicopter neared the destination, the pilot felt the onset of severe airframe vibration. The pilot elected to conduct a precautionary landing in an area of open farmland, resulting in a hard landing. The pilot and passenger were uninjured, and the helicopter sustained minor damage.

What the ATSB found

The helicopter’s engine was found to have suffered a mechanical failure due to in-service loosening of the nuts on the connecting rod bolts, leading to separation of one of the connecting rods from the crankshaft. The reason the nuts became loose was not determined.

While there was no indication of influence on this occurrence, independent inspection of the connecting rod attaching hardware performed during the overhaul of the engine did not involve a physical torque check of the connecting rod bolts. While the inspection was not a regulatory requirement, this was a missed opportunity to verify the installation torque.

During the most recent periodic inspection the helicopter maintenance provider did not refit the spark plugs using new gaskets, as required by the spark plug manufacturer. It was also found that the Civil Aviation Safety Authority guidance on spark plug gasket fitment was inconsistent in this respect. 

What has been done as a result

The Civil Aviation Safety Authority acknowledged the inconsistent information contained within the 2 airworthiness bulletins. CASA advised that Airworthiness Bulletin AWB 20‑001 is scheduled for cancellation and Airworthiness Bulletin AWB 85-023 is to be amended to reflect current recommendations. 

The helicopter maintenance provider advised the ATSB it now installs new gaskets when refitting spark plugs.

Safety message

This incident highlights the importance of managing inflight anomalies through a comprehensive understanding of aircraft systems and the application of emergency procedures. The pilot’s timely actions following the onset of the vibrations ensured a safe outcome for the occupants and resulted in minimal damage to the helicopter.

The incident also emphasises the importance of adhering to manufacturer requirements when installing aircraft components, as well as the additional assurance provided by a thorough independent inspection of completed work.

The investigation

The occurrence

On 11 June 2025, a Robinson R44 Raven I helicopter, registered VH-OOE, was being operated on a personal transport flight with a pilot and one passenger on board. The flight was conducted under the visual flight rules,[1] and the planned route was from Daly Waters Aerodrome to Wally’s Airstrip, Northern Territory (NT) (Figure 1).

On the morning of the flight, the pilot completed their pre-flight inspection and refuelled the helicopter. Shortly after starting the engine, the pilot recalled sensing an unusual sound and vibration through the helicopter, but it resolved when the engine speed was increased. The pilot completed their pre-take-off checks, and the helicopter departed Daly Waters Aerodrome at about 0900 local time. The pilot did not recount any issues with the helicopter’s performance during the take-off, climb or initial cruise.  

Figure 1: VH-OOE flight track

Source: Google Earth, annotated by the ATSB

At about 1015, when the helicopter was about 46 km to the south‑east of Wally’s Airstrip, the pilot contacted Tindal Airport air traffic control (ATC). Several exchanges with Tindal Airport ATC took place, during which the pilot was instructed to follow a railway line and maintain an altitude not above 1,500 ft above mean sea level. The pilot complied with these instructions and continued towards their destination. At about 1020, when the helicopter was at an altitude of about 1,100 ft, the pilot felt the onset of severe airframe vibration. They recalled initially thinking the helicopter tail may had been struck but later discounted that possibility when they identified they still had directional control. The pilot was unable to diagnose the cause of the vibration and decided to undertake a precautionary landing. 

At 1020:42, and an altitude of about 1,100 ft, the pilot alerted Tindal Airport ATC that they had a ‘problem’ (Figure 2). The pilot selected a paddock for the landing that had recently been harvested of its crop and commenced a right turn towards the landing location at 1020:49. At 1020:52, they communicated that operations were not normal, and at 1021:00 they advised Tindal Airport ATC that they would be landing immediately. The pilot recalled noting the engine gauges and the rotor and engine speed indications at that time were normal.  

Figure 2: VH-OOE flight path from the onset of vibrations until landing

Source: Google Earth, annotated by the ATSB

At 1021:05, and an altitude of 700 ft, the pilot made a transmission to Tindal Airport ATC during which a low speed warning horn could be heard in the background (see Low rotor speed). The pilot did not recall hearing the horn. At about 150 ft above ground level, the pilot recalled noting a low oil pressure light on the helicopter’s caution warning panel (see Oil warning caution light). They continued the approach and, as the helicopter slowed for landing, they observed smoke blowing forward from the rear and recalled having concerns about a fire. 

The helicopter landed heavily in the paddock. The pilot recalled that the landing was probably completed ‘quicker’ and with a lower tail position than normal, due to their concerns about a fire. Once the helicopter had landed, the pilot instructed the passenger to exit and run forward. They then shut down the helicopter’s engine, and at 1021:41 advised Tindal Airport ATC that they had landed and were safe. The pilot then exited the helicopter. Both occupants were uninjured, and the helicopter sustained minor damage.  

Context

Pilot information

The pilot held a valid Commercial Pilot Licence (Helicopter) with single engine and low‑level ratings. The licence was issued on 6 June 2025 following the successful completion of a commercial pilot licence flight test in May 2025. The pilot had held a Private Pilot Licence (Helicopter) since October 2023. They also held a current class 1 aviation medical certificate valid to 6 August 2025. At the time of the incident, they had a total flying time of 194 hours of which 118 hours were on the Robinson R44.

Helicopter information

General information

The Robinson R44 Raven I is a 4-place helicopter with a 2-bladed main rotor system and a conventional 2-bladed tail rotor. VH-OOE was manufactured in the United States in 2008 and first registered in Australia in July 2008. At the time of the incident, the helicopter had accumulated 1,995 hours total time in service. 

It was powered by a Lycoming O-540-F1B5, 6-cylinder, horizontally opposed piston engine that is naturally aspirated and rated at 235 horsepower. The overhauled engine was installed in September 2022 and had operated for 291 hours at the time of the incident, with a total time of about 1,614.6 hours. The last periodic inspection was undertaken on 6 May 2025, and the helicopter had flown about 25 hours since that inspection. 

Airworthiness and maintenance history

Recent maintenance

The last periodic inspection was undertaken by Platinum Helicopters on 6 May 2025. During the inspection, the Champion REM38E spark plugs fitted to the engine were removed, inspected and then refitted by the maintenance engineer. The maintenance engineer recalled that it was not their practice to fit new spark plug washers (gaskets) when refitting the spark plugs, instead electing to use annealed[2] gaskets (see Spark plug maintenance). 

Engine overhaul

In September 2022, VH-OOE underwent a 12 year/2,200 hour inspection. During the inspection, the engine was removed and an overhauled engine was fitted to the helicopter. This engine had been salvaged from a Robinson R44, and was overhauled by South West Aviation, a CASA‑approved maintenance organisation. 

During the overhaul of the engine, additional components were used to replace some aspects, including:

• 6 new cylinder kits
• new connecting rod hardware (bolts and nuts) with parts manufacturer approval[3]
• a crankshaft that had been salvaged from a different Robinson R44.

Records show all salvaged components were inspected and tested to assess serviceability prior to fitment. Once the engine overhaul had been completed, it underwent ground runs and checks prior to being installed in VH-OOE.

Records show that independent inspections were undertaken during the engine overhaul of the engine fitted to VH-OEE. The purpose of an independent inspection is to verify that a maintenance task has been completed correctly. The inspection is undertaken by an appropriately authorised person who did not undertake the original activity. While there was no regulatory requirement for the independent inspection of maintenance work carried out on engine systems, South West Aviation had included these inspections as part of the organisation’s worksheets for engine overhaul. 

The worksheets for the engine overhaul stated that an independent inspection of the engine sub-assembly was completed during the engine rebuild. Figure 3 shows the sub‑assembly of the crankshaft and the connecting rods, which were secured to the crankshaft by 2 connecting rod bolts and nuts. The crankshaft has 2 dynamic counterweight assemblies fitted, which assist in removing torsional vibration during engine operation.

Figure 3: O-540 crankshaft and connecting rod sub-assembly 

Source: Lycoming O-540-F1B5 Illustrated Parts Catalogue, annotated by the ATSB

During interview, when asked about a torque check of the connecting rod nuts, the engineer who conducted the independent inspection stated they would check the torque was set correctly on the tooling that had been used, but it was not their normal procedure to physically check the torque on each nut. South West Aviation did not have a documented procedure that detailed how the independent inspection of the connecting rod hardware should be conducted.

Helicopter systems and procedures

Vibration

The Robinson R44 pilot operating handbook (POH) contained advice for the management of vibration, and stated:

A change in the sound or vibration of the helicopter may indicate an impending failure of a critical component. If unusual sound or vibration begins in flight, make a safe landing and have the aircraft thoroughly inspected before flight is resumed. 

Low rotor speed

The helicopter was fitted with a low rotor speed horn. The activation of the horn indicated that rotor speed may be below safe limits (97%). Power available from the engine is directly proportional to rotor speed. With less power the helicopter will start to sink. If the collective is raised to stop it from descending, the rotor speed will reduce even further causing the helicopter to sink faster. To restore rotor speed, the Robinson R44 POH stated that a pilot should lower the collective, roll throttle on and, in forward flight, apply aft cyclic.

Oil warning caution light

The helicopter was fitted with an oil warning caution light. The illumination of the light indicated a loss of engine power or oil pressure. The Robinson R44 POH stated the actions to take in response should be to check the engine tachometer for power loss and the oil pressure gauge. If oil pressure loss was confirmed, the POH stated the pilot should land immediately. Continued operation without oil pressure causes serious engine damage and engine failure can occur.

Spark plug maintenance

The Champion Aviation Service Manual,[4] which included recommended service, handling and reconditioning practices for Champion spark plugs stated:

Always install both new and reconditioned Champion aviation spark plugs with a new copper gasket. 

Additionally, Champion Aviation Technical Bulletin 95-11[5] stated:

Gaskets that have become too hard with normal usage won’t “hold torque” correctly, and spark plugs can come loose with disastrous results. An annealed gasket will not meet new specifications.

The maintenance engineer stated they carried out the periodic inspection in accordance with the Lycoming O-540 Operator’s Manual.[6] However, this manual, which covered both the O-540 and IO-540 engines, contained no information regarding spark plug gasket fitment. The guidelines for the installation of spark plugs were contained in Lycoming service instruction 1042 Approved Spark Plugs, which stated:

Always install a spark plug with a new gasket.    

The Civil Aviation Safety Authority (CASA) had produced 2 advisory airworthiness bulletins (AWBs) that included information on spark plug fitment. However, the advice within these 2 documents was not consistent. 

AWB 20-001 Spark Plug Care, issued in September 2001, stated:

Most modern spark plugs have a solid copper gasket that requires annealing prior to spark plug installation to ensure a tight, gas sealed fit. The maintainer should check that the spark plug has only one washer, is of correct dimensions and is annealed. If the engine is equipped with a thermocouple probe in the form of a spark plug gasket, a normal gasket is not required.

Whereas AWB 85-023 Piston Engine Spark Plug Cracking, issued in June 2021, stated:

Always install a new spark plug gasket when servicing spark plugs or installing new spark plugs. Failure to install a new spark plug gasket may result in incomplete sealing of the combustion chamber, loss of heat transfer with spark plug overheating leading to possible pre-ignition.

Meteorological information

The weather at the time of the incident, recorded at Tindal Airport around 13 km to the north of the landing site, captured a wind of between 9–13 kt from the east, clear skies and a temperature of 23°C. 

Recorded information

The helicopter was not fitted with a flight data recorder or a cockpit voice recorder, nor was it required to be. During the incident flight, data was being transmitted by the helicopter’s transponder. This data, recorded by ground-based receivers, captured the aircraft’s position, altitude, and groundspeed during the final 25 minutes of the flight. All radio communications made and received by Tindal Airport ATC throughout the flight were recorded.

Helicopter damage

The ATSB did not attend the landing site. A post-incident inspection of the helicopter was completed by a maintenance organisation located at Wally’s Airstrip, NT. This inspection identified:

• damage to the engine with scattered material within the cowling
• damaged and displaced drive belts
• impact damage to the engine oil cooler caused by engine material
• engine oil on external areas of the engine and airframe
• the skid landing gear was spread outwards (Figure 4).

The engine and a selection of components were removed for a detailed examination by the ATSB.

Figure 4: VH-OOE shortly after landing showing oil leak and smoke haze

Source: Supplied, annotated by the ATSB

Engine examination 

The engine was disassembled and examined at a CASA‑approved engine overhaul facility under the supervision of the ATSB. The examination found that the number 4 connecting rod had separated from the crankshaft journal, resulting in mechanical damage to the internal engine components and fracture of the adjacent crankcase. Both connecting rod bolts had been fractured, with one connecting rod nut missing and the other unwound (see Component examination). There were also witness marks from impact between the number 4 piston crown and cylinder head.

Prior to removal of the remaining connecting rods, the nuts were checked for torque. The check found that the number 3 cylinder connecting rod nuts were at 20 ft/lb, while numbers 1, 2, 5 and 6 connecting rod nuts were at the correct torque of 40 ft/lb. 

The number 4 cylinder spark plugs were found loosened, but the spark plug leads were attached tightly. Subsequent testing of the spark plugs found both were serviceable. Figure 5 depicts the engine prior to disassembly.

Figure 5: Engine assembly showing damage 

Source: ATSB 

Component examination 

Several components were retained from the engine disassembly and were examined at the ATSB’s technical facilities in Canberra, Australian Capital Territory. 

Extensive deformation and fracture of the number 4 connecting rod (Figure 6) and deformation of the crankshaft journal, was consistent with initial separation of the connecting rod, followed by repeated impacts to the connecting rod by the still-rotating crankshaft. 

Figure 6: Number 4 cylinder connecting rod and piston

Source: ATSB 

This resulted in significant damage to the adjacent cylinder wall, piston skirt, camshaft and the hole in the crankcase. The fractured connecting rod showed no evidence of fatigue cracking or other defect.

The number 4 connecting rod bearings were deformed due to contact with the moving internal engine components but were found to be the correct parts and did not exhibit any abnormal signs of wear. Bearings from some of the other connecting rods displayed minor surface wear, which was attributed to low engine oil volume during the final part of the flight. 

There were visibly fewer combustion deposits on the number 4 piston crown, compared to the remaining pistons. However, a considerable amount of sand-like contamination was recovered from the number 4 cylinder during engine disassembly, which was found to be chemically similar to the piston deposits. There was no evidence of destructive combustion issues such as pre-ignition or significant detonation. 

The connecting rod was secured to the crankshaft by 2 connecting rod bolts (Figure 3). Both number 4 cylinder connecting rod bolts were fractured in approximately the same location (Figure 7). The fracture surface features of both bolts and deformation of the adjacent shank were consistent with overstress failures. 

Figure 7: Cylinder number 4 connecting rod bolts

Source: ATSB

One of the cylinder 4 connecting rod bolts had no nut and heavily damaged threads. The nut was not located. The other connecting rod bolt had a partially unwound nut retained on the threads (Figure 8).[7] The exposed threads were damaged. The nut could not be further unwound by hand, likely due to impact damage. The bolts and nut material was in accordance with their specification. The extent of deformation precluded a detailed inspection of the threads; however, the threads were not stripped and the remnants of a compound consistent with thread lubricant was identified. 

Figure 8: Number 4 cylinder connecting rod bolt showing position of retained nut

Source: ATSB

The examination also identified evidence of abnormal fretting[8] wear in the number 4 cylinder connecting rod bolt holes. A comparison between the number 4 cylinder and number 6 cylinder connecting rod bolt holes is depicted in Figure 9.  

Figure 9: Number 4 cylinder connecting rod end cap bolt hole fretting wear and exemplar

Source: ATSB

The abnormal fretting wear indicated relative movement (micro-slip) between the bolted surfaces during operation, which would occur if the bolt tension was insufficient to restrain movement under normal operational loads. The missing nut from one of the bolts, and the other nut retained in an improper position on the fractured bolt, was also an indicator that the nuts had loosened in-service.

Possible mechanisms that could result in the in-service loosening of the nuts included:

• Abnormal loading or vibration from engine overspeed or abnormal combustion that could lead to bolt stretch and nut loosening.
• Variations in thread condition or installing the threads dry versus lubricated could produce a lower bolt stress than desired.
• Unintended deformation due to improper or defective parts leading to reduced bolt stress over time.
• Microscopic surface deformation and fretting at contact interfaces could reduce clamping force by a small margin over time, which could then make the nut susceptible to further loosening during service.
• Inadequate torque applied to the nuts during installation that could lead to relative movement between the clamped surfaces and nut loosening during normal operation.

Related occurrences

In 2007, the ATSB published a research and analysis report (B20070191) into aircraft reciprocating (piston) engine failures. The report examined 20 high-power[9] piston engine structural failure occurrences in Australia, between 2000 and 2005. The report focused on failures of the combustion chamber, connecting rods and crankshaft assemblies. It included several engine failure investigations, including investigation 200105866 (below).  

ATSB investigation 200105866

On 14 December 2001, a Piper PA31-350 aircraft, registered VH-JCH, was in cruise flight at 8,000 ft when the flight crew noticed that the propellers went out of synchronisation. Adjustments were made to correct the problem but were unsuccessful. Following right engine speed fluctuations, the crew shut the engine down, feathered the propeller and conducted a single‑engine landing. 

During the subsequent disassembly of the engine, the crankshaft was noted to have fractured at the number 6 connecting rod journal, and the number 6 connecting rod big end had separated from the crankshaft and impacted the camshaft. The separation of the number 6 big end permitted the piston to strike the top of the combustion chamber with sufficient force to deform the top of the piston. 

The number 6 connecting rod disconnection from the crankshaft was due to the loosening of the nuts on the connecting rod bolts, and eventual loss of one nut. Evidence of nut loosening, leading to fretting wear damage, was observed on the bolt threads and the connecting rod cap bolt hole locations. The reason for the loosening of the number 6 connecting rod nuts could not be determined. 

The damage of these components was almost identical to the damage noted in the engine from VH-OOE.

Safety analysis

The ATSB examination of the engine components determined that the engine failure resulted from mechanical damage caused by the separation of the number 4 cylinder connecting rod from the crankshaft. 

The initiating factor of the separation was almost certainly the in-service loosening of the connecting rod nuts of the number 4 cylinder. This was evidenced by the fretting wear in the connecting rod bolt holes, which was illustrative of engine operation after a loss of bolt tension, allowing relative movement between the bolts and holes. The absence of one of the associated nuts, and the opposite one mostly unwound was also evidence of the nuts loosening prior to the engine failure. There was also an absence of fatigue cracking of the number 4 bolts or connecting rod that might otherwise account for the component fractures and separation of the connecting rod. 

Of the possible mechanisms identified that could have led to the connecting rod nuts loosening:

• Abnormal loading or vibration: there was no evidence of engine overspeed, or of piston melting or structural damage consistent with severe abnormal combustion. There was no evidence that the spark plugs, found loose during the disassembly, had any negative impact on the engine performance.
• Variation in thread condition and lubrication: this could not be fully assessed due to thread damage and the absence of one of the nuts.
• Improper or defective parts: the connecting rod bolts and nut material was correct; however, a full assessment of their original condition was not possible.
• Embedding (microscopic deformation): it is possible that the initial bolt tension reduced by a small margin due to microscopic deformation of the clamping or thread surfaces, which would then make the nut more susceptible to further loosening during service.
• Inadequate installation torque: like the above, it was possible that the nuts were slightly under-torqued during installation and progressively loosened during the subsequent 291 hours of operation. The number 3 connecting rod nuts being found at the incorrect torque value during the engine disassembly further supports this scenario.

Given most of the possibilities above could not be definitively ruled out, the reason for the nuts loosening was ultimately not determined. 

Despite this, it was identified that during the overhaul of the engine fitted to VH-OOE, the independent inspection of the engine sub-assembly did not involve a torque check of the connecting rod nuts. While there was no evidence of influence on this occurrence and while the inspection was not a regulatory requirement, the ATSB considered it a missed opportunity to positively verify the installation torque.

Additionally, during the engine examination, both spark plugs in the number 4 cylinder were found to be loose. The reason for the loose spark plugs was not determined and, as above, there was no evidence identified to indicate influence on the engine failure. However, it was identified that during the most recent periodic inspection, the helicopter maintenance provider did not refit the spark plugs using new gaskets as required by the engine and spark plug manufacturer. 

On the same subject, the Civil Aviation Safety Authority guidance on spark plug gasket fitment was inconsistent. Airworthiness Bulletin AWB 20-001 stated that annealed gaskets could be used, whereas Airworthiness Bulletin AWB 85-023 stated new gaskets must be used in all circumstances. 

The unusual sound and vibration noted by the pilot during engine start was possibly a precursor to the eventual failure inflight, however the vibration disappeared when engine speed was increased. In response to the onset of severe vibration inflight, the pilot assessed the controllability of the helicopter and noted there were no abnormal engine indications at that time. In accordance with the Robinson R44 POH, the pilot conducted a precautionary landing in a suitable location. They also communicated the issue to Tindal Airport ATC, which increased the likelihood of a timely emergency response had one been necessary.

During the late stages of the approach, the low rotor speed warning horn and low oil pressure caution light activated. Both indicated a reduction in power, almost certainly due to the mechanical failure, resulting in less power than normal to arrest the rate of descent in the final stages of landing. This, in combination with the pilot’s concern about a possible fire and recollection of landing ‘quicker’ than normal, likely resulted in the helicopter landing heavily which spread the landing gear skids.

Findings

From the evidence available, the following findings are made with respect to the engine failure and forced landing involving Robinson R44, VH-OOE, 13 km south of Tindal Airport, Northern Territory, on 11 June 2025.

Contributing factors

• In-service loosening of the connecting rod nuts resulted in the eventual separation of the connecting rod from the crankshaft and the mechanical failure of the engine. The reason for the nuts loosening was not determined.
• During the engine overhaul, the torque on the connecting rod nuts was not physically checked as part of the independent inspection of the engine assembly. This was a missed opportunity to verify that the installation of the connecting rod nuts had been completed correctly.

Other findings that increased risk

• During the most recent periodic inspection, the helicopter maintenance provider did not refit the spark plugs using new gaskets as required by the spark plug manufacturer. This increased the risk of loosened spark plugs, insufficient heat transfer and pre-ignition.
• The Civil Aviation Safety Authority guidance on spark plug gasket fitment was inconsistent. Airworthiness Bulletin AWB 20-001 stated that annealed gaskets could be used, whereas Airworthiness Bulletin AWB 85-023 stated new gaskets must be used in all circumstances. The inconsistency in this guidance could have led to incorrect procedures being performed which were not in accordance with spark plug maintenance requirements.

Safety actions

Safety action by Civil Aviation Safety Authority

The Civil Aviation Safety Authority acknowledged the inconsistency between Airworthiness Bulletin AWB 20-001 (that stated that annealed gaskets could be used) and Airworthiness Bulletin AWB 85-023 (that stated new gaskets must be used in all circumstances) and advised the ATSB that AWB 20-001 will be cancelled and AWB 85‑023 will be amended to reflect current recommendations.

Safety action by Platinum Helicopters

Platinum Helicopters advised the ATSB that new spark plug gaskets are now fitted each time spark plugs are reinstalled. 

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot of VH-OOE
• the engine overhaul organisation
• the maintenance provider for VH-OOE
• Civil Aviation Safety Authority
• the aircraft manufacturer
• the engine manufacturer
• the PMA parts manufacturer
• Airservices Australia
• the Bureau of Meteorology.

References

Australian Government (1988), Civil Aviation Regulations 1988 (Commonwealth), reg 42G. AustLII. https://classic.austlii.edu.au/au/legis/cth/consol_reg/car1988263/s42g.html 

Australian Government (2021), Aircraft Reciprocating-Engine Failure: An Analysis of Failure in a Complex Engineered System, Australian Transport Safety Bureau, Canberra, ACT. /publications/2007/b20070191

Civil Aviation Safety Authority (2025). Airworthiness Bulletin 20-001. Retrieved from https://www.casa.gov.au/aircraft/airworthiness/airworthiness-bulletins/spark-plug-care

Civil Aviation Safety Authority (2025). Airworthiness Bulletin 85-023. Retrieved from https://www.casa.gov.au/aircraft/airworthiness/airworthiness-bulletins/piston-engine-spark-plug-insulator-cracking 

Lycoming Engines Operator’s Manual 4^th edition 2006, O-540, IO-540 Series

Lycoming Engines Overhaul Manual, Direct drive engines 1974

Lycoming Engines Parts Catalogue 2009, O-540-F1B5

Robinson Helicopter Company 2024, R44 Pilot’s Operating Handbook, section 10, p.10-2

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• the pilot of VH-OOE
• Civil Aviation Safety Authority
• the maintenance provider
• the engine overhaul organisation
• Textron Lycoming
• Robinson Helicopters
• National Transportation Safety Board (NTSB).

Submissions were received from:

• the pilot of VH-OOE
• the maintenance provider
• the engine overhaul organisation
• Textron Lycoming
• Robinson Helicopters.

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. About ATSB reports ATSB investigation reports are organised with regard to international standards or instruments, as applicable, and with ATSB procedures and guidelines. Reports must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2025  Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Investigation summary

What happened

At 1021 on 7 April 2025, a Bankstown Helicopters Robinson R44 helicopter, registered VH‑EWM (EWM), with a pilot and 2 passengers on board, departed from Bankstown Airport, New South Wales, for a local scenic flight around Sydney Harbour. Shortly after 1028, as EWM was entering the Parramatta River helicopter lane behind an EC120 helicopter, the occupants of EWM experienced a sudden onset of turbulence followed by an uncontrolled descent. 

In response, the pilot applied full collective, which resulted in a low rotor speed condition as the helicopter descended towards the water. The pilot was able to manoeuvre the helicopter and complete a forced landing on the river shoreline.

What the ATSB found

The ATSB found that it is likely that EWM entered the rotor wake from a preceding heavier EC120 helicopter, which resulted in the control difficulties, an uncontrolled descent, low rotor speed warning and the forced landing. 

What has been done as a result

Following review of the draft report, the Civil Aviation Safety Authority undertook proactive safety action to improve existing guidance about helicopter wake vortices in Advisory Circular 91-16. The updated advisory circular was released on 17 July 2025 and can be found at the link: AC 91-16 v1.2 - Wake turbulence.

Safety message

Flight tests have demonstrated that helicopter wake turbulence is comparatively larger and less predictable in its behaviour than for aeroplanes of the same weight. Helicopter rotor vortices can descend, remain level or climb, and the duration of their persistence can increase significantly in conducive weather conditions. The United States Helicopter Safety Team website recommends remaining 3 rotor disks clear of a hovering or taxiing helicopter and allowing 3 NM and/or 2 minutes for the rotor wake from a preceding helicopter to dissipate.

The investigation

The occurrence

At 1021 local time on 7 April 2025, a Bankstown Helicopters Robinson R44 Raven 1 helicopter, registered VH‑EWM (EWM), with a pilot and 2 passengers on board, departed from Bankstown Airport, New South Wales for a local scenic flight around Sydney Harbour. Bankstown Tower air traffic control (TWR) cleared EWM to depart via ‘Choppers West’, which was a standard procedure for helicopters departing to the north when runway 29 was active at Bankstown. 

The pilot reported that they climbed to about 1,000 ft above mean sea level.[1] The pilot’s plan was to join the Parramatta River on the west side of the Ryde Bridge and descend to 500 ft to follow the helicopter lane[2] along the south side of the river to Sydney Harbour (Figure 1).

Figure 1: Key locations

Source: Google Earth, annotated by the ATSB

About 1 minute after EWM departed, an Airbus EC120B helicopter departed Bankstown, also following the Choppers West departure. Shortly after the EC120 departed, TWR advised the EC120 pilot that there was ‘R44 traffic 1 NM ahead’, to which the EC120 pilot reported that they had the traffic sighted. Bankstown TWR then advised the pilot of EWM that they were not receiving their transponder data, which the pilot acknowledged. The pilot of EWM then turned their transponder off and on in an attempt to transmit transponder information, but no data was received from it throughout the incident flight.

As the 2 helicopters tracked north towards the Parramatta River, the EC120 flew to the west of EWM and passed it before reaching the river. The EC120 then turned right to join the Parramatta River helicopter lane, tracking towards the Ryde Bridge and Sydney Harbour, and passed over the Ryde Bridge at a recorded radar altitude of 600 ft. 

The pilot of EWM reported that they descended the helicopter to 500 ft as they approached the river. Just before the pilot turned EWM right to join the helicopter lane, another larger helicopter (the EC120) suddenly appeared in front of them (Figure 2). The pilot of EWM estimated the EC120 was about 500‍–‍600 ft (150‍–‍180 m) in front of them and about 100 ft above them. While there was no recorded altitude for EWM, primary radar data indicated that EWM entered the lane about 9 seconds behind the EC120.[3] Primary radar data for EWM was lost about 10 seconds later, just after 1028, indicating it had descended below radar coverage.

Figure 2: Primary radar return (left) and loss of primary radar return (right) for VH‑EWM

Source: Airservices Australia, annotated by the ATSB

The pilot of EWM made a radio broadcast that they were entering the helicopter lane as they crossed the Ryde Bridge behind the EC120. They then experienced what they described as very strong turbulence from a vertical motion in the atmosphere. A passenger later described it as ‘like heavy turbulence … rolling left and right’ followed by ‘diving towards the water’.

The pilot noted that the helicopter was descending through 400 ft and responded by raising the collective lever.[4] However, the helicopter continued descending towards the water as it tracked behind and below the EC120. A passenger recalled the pilot announced ‘brace for impact’ as the helicopter approached the water. The pilot applied full collective to avoid the water, which caused the rotor speed to decay sufficiently for the low rotor speed warning horn to activate. They also reported feeling that they could not escape what they believed to be the rotor wake from the EC120. The pilot then sighted a suitable forced landing area at Cabarita Park and, using the helicopter’s remaining airspeed and rotor speed, manoeuvred the helicopter to the shoreline for a landing.

Following the landing, the pilot rolled the engine throttle back to idle and proceeded through their after‑start checks and confirmed normal operations on the ground. The pilot then conducted a hover check and again confirmed normal operations. The pilot attempted radio contact with their operations base but received no reply. They then conducted a return flight to Bankstown without further incident. 

Context

Pilot information

The pilot held a commercial helicopter pilot licence, issued on 26 November 2024, with a single‑engine helicopter class rating and low‑level rating. The pilot held a class 1 aviation medical certificate with no restrictions and expiration date of 30 May 2025. The pilot had accumulated about 112 hours flying experience and the incident flight was the pilot’s first commercial flight.

Helicopter information

The incident helicopter, EWM, was a piston‑engine 2‑bladed Robinson Helicopter Company R44 Raven 1 with a maximum take‑off weight of 1,089 kg. The weight and balance data provided by the operator indicated it was within limits for the flight. 

The Airbus EC120B was a turbine-engine 3‑bladed helicopter with a maximum take‑off weight of 1,715 kg. Therefore, the EC120 was about 57% heavier than EWM at their respective maximum take‑off weights.

The maintenance release for EWM indicated the helicopter was operated by Bankstown Helicopters in the operational category of Part 133 Air Transport. The maintenance release current at the time of the incident was issued on 3 April 2025 at 4,349 hours total time in service with an expiry date of 3 April 2026 or 4,400 hours. A maintenance test flight was certified on the maintenance release as conducted on 3 April with ‘nil defects evident.’ 

After the incident, the operator’s maintenance organisation inspected the helicopter and found no defects. As the flight hours remaining on the helicopter were close to the next overhaul, the operator elected to remove the helicopter from service and have the maintenance organisation complete the overhaul. 

Meteorological information

The METAR[5] recordings for Bankstown Airport at 1000 and 1030 indicated that the wind was westerly at a speed of 9 kt at 1000 and 7 kt at 1030. No cloud was detected. These conditions were consistent with the Bankstown Airport forecast for 8 kt westerly winds. The pilot reported their assessment of the weather was 5 kt of variable wind and CAVOK[6] conditions, but when they encountered the turbulence over the Ryde Bridge it felt like 40 kt of wind.

Rotor wake turbulence

In 1996, the United States Federal Aviation Administration (FAA) produced a report on the subject of Flight test investigation of rotorcraft wake vortices in forward flight. They used a laser doppler velocimeter to measure the vortices and small probe aircraft to test the actual flying conditions. Smoke generation was used to visualise the wake vortices for the probe aircraft. Their investigation concluded that:

• The measured vortex circulation diminished with decreasing airspeed for helicopter airspeeds below 40 knots. At these lower speeds, the wake vortex structure begins to break down and changes to a distinct downwash.
• Vortex duration depends strongly on ambient weather conditions and a variance of 300% was observed on those days most conducive[7] to vortex persistence and duration compared with those observed on typical days.
• Typically, helicopters with higher gross weight, larger rotor diameters, and larger numbers of rotor blades generated vortices of larger core diameters.
• Probe tests revealed that helicopter vortices did not descend in the same predictable manner as for fixed‑wing aircraft. Some vortices descended; some remained level; and some initially descended, levelled off, and then ascended above the altitude of the generating helicopter.

Figure 3: Rotor wake vortices visualised with smoke generators

Visualisation of the wake vortices behind an S‑76A helicopter in forward flight with smoke generators from the FAA (1996) flight tests. Source: Reddit

Meiris (n.d.) provided an article for the United States Helicopter Safety Team website, on the subject of Avoiding helicopter wake turbulence. The article referenced the FAA 1996 flight test report and provided the following recommendations:

As a result of these findings and the studies conducted regarding helicopter downwash in a hover, a few guidelines have been developed to increase awareness around helicopter wake turbulence:

• For hovering flight or a hover taxi, stay three rotor diameters away.

• For forward flight, a minimum of 3 nm [NM] separation is recommended, especially from larger helicopters. The investigation we discussed previously discovered that even at 3nm [NM], the planes encountered uncommanded pitch and roll oscillations.

• Leave 2 minutes for the rotor vortices to dissipate behind a helicopter in forward flight.

Related occurrences

The French Bureau of Enquiry and Analysis for Civil Aviation Safety investigation BEA2019-0234, Accident to a paraglider involving the Airbus - EC135 - T2 PLUS registered F-HTIN, examined a fatal paraglider accident in 2019. The paraglider’s wing collapsed after encountering the rotor wake from an Airbus EC135 helicopter, which drifted with the wind from the helicopter’s flightpath onto the paraglider (Figure 4).

Figure 4: Simulation of rotor wake drifting onto the paraglider

Source: YouTube – Bureau of Enquiry and Analysis for Civil Aviation Safety, annotated by ATSB

The 2022 United States National Transportation Safety Board investigation WPR22LA072 found that the pilot of a Cessna 120 attempted a go‑around about 20 seconds behind the passage of a Bell UH‑1H helicopter. During the go‑around the Cessna encountered wake turbulence, resulting in a loss of control and collision with terrain (Figure 5). The report indicated light wind conditions of 4 kt at the airport.

Figure 5: Loss of control accident from rotor wake

Source: YouTube – Aviation Safety Network, annotated by ATSB

Safety analysis

Primary radar data and the pilot’s report indicated that EWM entered the Parramatta River helicopter lane and passed over the Ryde Bridge about 9 seconds behind and slightly below the EC120 helicopter. At this point, EWM encountered heavy turbulence, an uncontrolled descent and a low rotor speed when the pilot applied full collective to avoid a collision with the water.

The uncontrolled descent and low rotor speed condition resulted in the pilot conducting a forced landing on the shoreline of the Parramatta River. 

The incident occurred under relatively calm wind conditions and EWM operated in a serviceable condition for the return flight. Subsequent maintenance inspections of the helicopter found no fault. Furthermore, EWM passed overhead the Ryde Bridge in sufficient proximity to a preceding heavier 3‑bladed helicopter to be subject to a rotor wake induced upset. Therefore, the ATSB concluded that the sudden onset of turbulence and uncontrolled descent were likely the result of EWM encountering rotor wake turbulence from a preceding EC120 helicopter. 

Findings

From the evidence available, the following findings are made with respect to the wake turbulence encounter and forced landing involving Robinson R44, VH-EWM, about 15 km north‑east of Bankstown Airport, New South Wales, on 7 April 2025. 

Contributing factors

• It is likely that the incident helicopter entered the rotor wake from a preceding heavier helicopter, which resulted in control difficulties, an uncontrolled descent, low rotor speed warning and a forced landing.

Safety actions

Safety action by the Civil Aviation Safety Authority

Following review of the draft report, the Civil Aviation Safety Authority undertook proactive safety action to improve existing guidance about helicopter wake vortices in Advisory Circular 91-16. The updated version of the advisory circular was released on 17 July 2025 and can be found at the link: AC 91-16 v1.2 - Wake turbulence.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• Airservices Australia
• Civil Aviation Safety Authority
• the operator and maintenance organisation for VH-EWM
• the pilot and passengers of the incident flight

References

Bureau of Enquiry and Analysis for Civil Aviation Safety. (2021). Accident to a paraglider involving the Airbus - EC135 - T2 PLUS registered F-HTIN on 11 May 2019 at Le Conquet (Finistère). https://bea.aero/fileadmin/user_upload/BEA2019-0234.en.pdf

Federal Aviation Administration. (2023). Aeronautical information manual. http://www.faa.gov/air_traffic/publications

Federal Aviation Administration. (1996). Flight test investigation of wake vortices generated by rotorcraft in forward flight (DOT/FAA/CT-94/117). https://apps.dtic.mil/sti/tr/pdf/ADA318103.pdf

Meiris, J. (n.d.). Avoiding helicopter wake turbulence. https://ushst.org/avoiding-helicopter-wake-turbulence/

National Transportation Safety Board. (2022). Aviation investigation final report (WPR22LA072). Investigation docket https://data.ntsb.gov/Docket?ProjectID=104480

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• Civil Aviation Safety Authority
• the maintenance organisation for VH-EWM
• the operator and pilot of the incident flight.

No submissions were received.

Purpose of safety investigations The objective of a safety investigation is to enhance transport safety. This is done through:  identifying safety issues and facilitating safety action to address those issues providing information about occurrences and their associated safety factors to facilitate learning within the transport industry. It is not a function of the ATSB to apportion blame or provide a means for determining liability. At the same time, an investigation report must include factual material of sufficient weight to support the analysis and findings. At all times the ATSB endeavours to balance the use of material that could imply adverse comment with the need to properly explain what happened, and why, in a fair and unbiased manner. The ATSB does not investigate for the purpose of taking administrative, regulatory or criminal action. Terminology An explanation of terminology used in ATSB investigation reports is available here. This includes terms such as occurrence, contributing factor, other factor that increased risk, and safety issue. Publishing information Released in accordance with section 25 of the Transport Safety Investigation Act 2003 Published by: Australian Transport Safety Bureau © Commonwealth of Australia 2025   Ownership of intellectual property rights in this publication Unless otherwise noted, copyright (and any other intellectual property rights, if any) in this report publication is owned by the Commonwealth of Australia. Creative Commons licence With the exception of the Commonwealth Coat of Arms, ATSB logo, and photos and graphics in which a third party holds copyright, this report is licensed under a Creative Commons Attribution 4.0 International licence. The CC BY 4.0 licence enables you to distribute, remix, adapt, and build upon our material in any medium or format, so long as attribution is given to the Australian Transport Safety Bureau.  Copyright in material obtained from other agencies, private individuals or organisations, belongs to those agencies, individuals or organisations. Where you wish to use their material, you will need to contact them directly.

Investigation summary

What happened

On the afternoon of 11 September 2024, the pilot of a Kawasaki 47GB3‑KH4, registered VH‑BEU and operated by Katherine Helicopters, was conducting a scenic flight over Nitmiluk (Katherine) Gorge, Northern Territory with 2 passengers on board. 

About 13 minutes into the flight, while entering the mouth of the gorge, the pilot reported experiencing an engine power loss and lack of response from the engine. Due to inhospitable terrain in the area, the pilot identified a clear landing spot some distance away and attempted a forced landing at that location, during which time the aircraft collided with terrain.

The pilot and both passengers were uninjured in the incident, however, the aircraft was substantially damaged.

What the ATSB found

Several possibilities were considered during the investigation in relation to the reported engine power loss. While the ATSB did not conduct a physical inspection of the engine in this instance, a post‑incident inspection revealed that a large crack had developed in the engine exhaust pipework. Being a turbocharged engine, the escape of exhaust gases through the crack during operation has likely resulted in an engine power loss during flight due to the loss of boost pressure.

The ATSB also identified that the pilot was unable to cushion the landing during termination of the autorotation, likely due to low main rotor RPM, resulting in the helicopter colliding with terrain.

Safety message

Helicopter operations over harsh terrain offer limited safe emergency landing options. Autorotation to a suitable area in such circumstances requires accurate helicopter positioning and energy maintenance (airspeed and rotor RPM). This makes thorough and regular training in emergency procedures crucial for pilots operating in these demanding environments. The prompt action taken by the pilot, in identifying a suitable landing site, was instrumental in ensuring the safety of all personnel on board.

The investigation

The occurrence

On 11 September 2024, the pilot of a Kawasaki Heavy Industries 47GB3‑KH4, registered VH‑BEU and operated by Katherine Helicopters, was conducting scenic flights over Nitmiluk (Katherine) Gorge, Northern Territory.

During the morning, the pilot conducted the required daily checks on the helicopter and completed a passenger flight over the gorge (Figure 1). After returning to base, the pilot refuelled the helicopter and completed short, uneventful ferry flights to Katherine Museum and return.

Figure 1: Nitmiluk (Katherine) Gorge scenic flight overview 

Source: Google Earth, annotated by the ATSB

That afternoon, the pilot met 2 passengers at the operator’s base, and then completed the flight manifest including the passengers’ weight details, based on verbal information provided by the passengers. The pilot carried out their pre-departure checks, including a flat‑pitch check,[1] which were all normal. A safety briefing was also undertaken with both passengers, which reportedly included:

• use of seatbelts
• in‑flight procedures
• operation of the doors and emergency exits
• actions to be taken in case of an emergency.

At around 1451 local time, the helicopter departed and after making a turn towards the north, the pilot gradually climbed to about 1,000 ft above mean sea level (AMSL), proceeding north‑easterly along the Katherine River in the direction of the gorge.

About 10 minutes later, the pilot climbed further to a height of about 1,400‍–‍1,500 ft AMSL to fly neighbourly[2] while approaching the escarpment bordering Nitmiluk National Park (Figure 2). The pilot then arranged separation with the pilot of a second helicopter in the area, using the common traffic advisory frequency (CTAF).

Figure 2: View from helicopter of Nitmiluk National Park

Source: Passenger photograph, annotated by the ATSB

Approximately 3 minutes later, as the helicopter was entering the mouth of the gorge, the pilot noticed that the engine did not respond when they increased the throttle, and that the helicopter was decelerating and losing height. Due to being overhead undulating rocky terrain, the pilot commenced a right turn in search of a suitable landing site, in case they had to conduct a landing (Figure 3).

The pilot reported that the ensuing events occurred extremely quickly. While turning around, the helicopter kept slowing and the engine did not respond to throttle movements. They noticed a significant change in the engine noise, described as a low‑pitched ‘whir’ sound, and reported feeling stiffness in the pedals. Not knowing what was wrong with the engine, the pilot lowered the collective[3] and entered autorotation (see the section titled Autorotation). 

During the descent, the pilot glanced at the instruments and noted that the engine RPM needle was split[4] from the main rotor RPM needle and again tried unsuccessfully to increase the throttle. The pilot later advised that the main rotor RPM was well below the green arc on the gauge, and the engine RPM was just below idle. The pilot reported identifying a landing site in a clearing about 400‍–‍500 m to the west of their location and broadcast a MAYDAY[5] call on the CTAF. 

The helicopter proceeded downwind towards the clearing, attaining a maximum groundspeed in excess of 83 kt. Given the prevailing wind, that equated to an airspeed of about 70 kt, which was higher than the recommended autorotation speed for the helicopter type (see the section titled Autorotation). However, during the final approach to the selected landing site, the groundspeed, and given the crosswind approach, also airspeed was reduced to about 50 kt.

Figure 3: Helicopter flight path with identified elevation and ground speed

Source: OzRunways data on Google Earth, annotated by the ATSB 

The passengers reported that during the descent, communication from the pilot was minimal. They were not advised by the pilot to brace for impact; however, upon hearing the MAYDAY call and seeing the pilot attempt an unexpected landing, both passengers suspected that an emergency situation had developed, and chose not to interrupt the pilot or distract them from their actions.

The pilot reported that, on nearing the landing site, they attempted to flare the helicopter but there was minimal flare effect, which they assessed was most likely due to low main rotor RPM. Consequently, the pilot attempted a run‑on landing, during which the helicopter collided with terrain, damaging its tail rotor/shaft and skids (Figure 4).

Figure 4: Helicopter accident site

Source: Operator 

After landing, the pilot shut off the fuel, magnetos and battery. After checking on the passengers, the pilot disembarked the aircraft, ensured it was safe to exit with the main rotor blades still rotating, and then instructed the passengers to evacuate the aircraft. The pilot of the other helicopter operating in the area repeated the MAYDAY call on the area frequency and then circled overhead the accident site. A rescue crew arrived onsite shortly after and the pilot and both passengers were evacuated uninjured.

Context

Pilot information

The pilot held a commercial pilot licence (helicopter) and a class 1 aviation medical certificate. At the time of the incident, they had accumulated about 380 hours total aeronautical experience. All the pilot’s training and most of their flying experience has been obtained operating Robinson R44 helicopters. They had been flying for Katherine Helicopters for about 3 months. 

While at Katherine Helicopters, the pilot received their Bell 47 endorsement after undergoing about 3 hours of training with an instructor that included the conduct of emergency procedures. The pilot also performed about 10 hours of in command under supervision (ICUS)[6] training with the operator.

Operator information

The operator was a helicopter tour operator based in Katherine, Northern Territory. It operated 2 Bell/Kawasaki 47 Helicopters and offered tour flights throughout northern Australia. 

Previous incidents 

The pilot had been involved in another engine failure incident on a second helicopter of the same type with the operator in July 2024, wherein an engine cylinder failed. The pilot completed a successful autorotation and there were no reported injuries. While this matter was reported to the ATSB, it was not investigated.

In addition to the incident above, the other helicopter operated by the operator was involved in a third off field landing, arising from an engine fault, in February 2024. 

Helicopter information     

The helicopter was a Kawasaki Heavy Industries 47G3B‑KH4, manufactured in 1969 in Japan by Kawasaki Heavy Industries and first registered in Australia on 20 June 1996. At the time of the accident, VH‑BEU had about 7,495 hours total time in service and had flown about 40 hours since its last periodic service inspection. 

Engine and turbocharger system

VH‑BEU was powered by a Textron Lycoming TVO‑435, 6‑cylinder, vertical direct drive, horizontally opposed, turbocharged engine. The turbocharger increases the density of the carburettor inlet air to maintain the available power as altitude increases. 

During operation, the exhaust gases expelled from the 3 cylinders on either side of the engine are merged into a single pipe, through which the gases are diverted either to the turbocharger or to the exhaust bypass valve (waste gate), or both (Figure 5).

As the engine power is increased, oil pressure builds up in the exhaust bypass valve assembly and the waste gate in the exhaust system is closed. This diverts the exhaust gas to the turbocharger turbine wheel, compressing the intake air and increasing the available engine power output.

Figure 5: Schematic diagram of engine and turbocharger operation

Source: Lycoming Operator’s Manual, annotated by the ATSB

Maintenance history

As part of the investigation, the ATSB requested the maintenance logs and component log cards for the aircraft. Although the provided information was incomplete, the maintenance history was able to be inferred from the supplied documents as follows:

• Lycoming engine TVO‑435, S/N: L‑751‑52 was installed February 2000 at 6,752 airframe hours. The engine component control card showed a number of zero‑hour components at the time of engine installation, indicating that this engine was likely either a new or overhauled unit. Lycoming Service Instruction No. 1009BE specified a 12‑year, 1,000 hour time between overhauls (TBO). This interval was correctly annotated in the engine log card.
• The engine was removed on 24 February 2014 (+14 years) at 7,088 airframe hours (+336 engine hours), and was bulk stripped in lieu of being overhauled, to satisfy the requirements of CASA air worthiness bulletin (AWB) 85‑5 Issue 1.[7] The same engine was refitted on 20 March 2014.

The information supplied to the ATSB showed that prior to November 2022, the aircraft flew only limited hours, having recorded 425 airframe hours in 22 years (at an average of 19.3 hours/year). Depending on how these hours were flown, the aircraft may have required special maintenance actions and/or storage procedures. If these actions were not taken, corrosion, or contamination of components may affect serviceability. No record of storage or preservation penalty maintenance was identified in the provided records. 

Autorotation

When an engine failure occurs in a single‑engine helicopter such as the Kawasaki 47GB3‑KH4, the pilot must immediately lower the collective and enter autorotation to reduce rotor drag sufficiently to maintain normal rotor RPM. This is a power‑off manoeuvre wherein the engine is disengaged from the main rotor and the rotor blades are driven solely by the upward flow of air through the rotor during descent. The most common reasons for an autorotation are an engine or drive system failure. If the engine fails, the freewheeling unit[8] automatically disengages the engine from the main rotor, allowing it to rotate freely. The tail rotor, still driven by the main rotor transmission, continues to provide yaw control via the anti‑torque pedals.

The United States Federal Aviation Administration Helicopter Flying Handbook noted that the rate of descent during autorotation is influenced by various factors, including:

• bank angle
• density altitude
• gross weight
• main rotor RPM
• trim condition
• airspeed.

Pilots control the autorotative descent rate using airspeed and main rotor RPM. Airspeed is managed with the cyclic[9] pitch control, similar to normal powered flight. The descent angle can range from vertical to a minimum angle for maximum horizontal range. The rate of descent is highest at zero airspeed, reaches its minimum at approximately 50‍–‍60 kts (depending on the helicopter and conditions), and increases again at higher speeds.

During an autorotative landing, the kinetic energy associated with the helicopter’s airspeed and rotating main rotor blades are used to arrest the descent and cushion the landing. Termination of the autorotation usually involves initially flaring the helicopter to reduce the airspeed, rate of descent and, if necessary, increase the rotor RPM. The degree of flare effect is influenced by both the airspeed at the time aft cyclic is applied, as well as the rate of cyclic application.

Autorotative terminations at very low airspeeds are more challenging than those performed at the minimum rate of descent airspeed as they offer minimal flare effect. In that case, cushioning the landing using the stored rotor energy requires more precise collective application.

The Kawasaki 47G3B-KH4 flight manual stated that, in the case of an engine failure, the pilot must:  

execute a normal autorotative descent and establish a level attitude prior to ground contact. At a height of approximately 10 feet above the ground, apply collective pitch in sufficient quantity to stop descent as ground contact is made. The best descent speed is 55 mph [48 kt].

Meteorological information

At the time of incident, visibility was in excess of 10 km, with scattered cloud[10] well above the aircraft operating height, no precipitation and an east‑south‑easterly wind at 11 kt.

The meteorological conditions reported by the pilot at the time of incident were consistent with the Bureau of Meteorology observations, which recorded a temperature of 36°C and dew point[11] of 11°C at the incident location. 

The process of vaporising fuel in a carburetted engine can cool the airflow sufficiently to permit ice formation in the carburettor throat that restricts airflow to the engine. According to the Civil Aviation Safety Authority Carburettor icing probability chart, the probability of carburettor icing, based on the prevalent temperature and dewpoint at Katherine Gorge was on the outer limit of the ‘light icing – cruise or descent power’ zone. Of note however, the turbocharger compressor used in the Kawasaki 47G3B‑KH4 heats the inlet air to the carburettor, significantly reducing the potential for carburettor icing. 

Operating weight

The passenger manifest completed by the pilot prior to take‑off recorded the empty weight of the aircraft, the pilot and both passengers, plus fuel. An allowance of 100 kg was made for the weight of fuel. The passenger weights were only verbally requested, and not physically checked by the pilot. However, the calculated total take‑off weight was well below the maximum take‑off weight so any inaccuracy in passenger weights was unlikely to have resulted in the aircraft being overloaded. 

Aircraft fuel 

The operator conducted an examination of the fuel in the aircraft following the incident and reported to the ATSB that there was adequate fuel remaining in the tank. 

The operator also stated that the aircraft had been refuelled with premium 130 octane fuel by the pilot prior to take‑off, and that a post‑incident examination of the fuel quality did not reveal any signs of contamination or leakage.

Engine examination

Following the incident, the operator started the Lycoming TVO‑435 engine and observed that the wastegate did not operate. Thereafter, they removed the engine and delivered it to a third‍ party maintenance facility for examination. As depicted in Figure 6, the post‑incident inspection of the engine revealed the presence of an approximately 6.5 cm crack on the right exhaust collector, with no other defects identified. 

The exhaust system of the aircraft engine attains extremely high operating temperatures. The consequent widening of the crack due to heat expansion would result in the escape of exhaust gases from the defective section, resulting in inadequate drive pressure for the turbocharger compressor and reduced power output. Information provided by the engine manufacturer supported that conclusion. As such, exhaust gas bleed from the crack before the turbocharger resulted in the wastegate remaining closed. 

The ATSB did not undertake a metallurgical analysis of the exhaust pipe, however, exhaust systems are generally prone to metal fatigue over time due to continuous vibration under corrosive conditions and a cyclical pattern of constant heating and cooling with extreme thermal fluctuations.[12]

While it could not be established when the exhaust crack occurred, the maintenance facility that conducted the post‑accident engine examination assessed that, based on the location and appearance of the defect, it was likely not impact‑related. They further stated that, as the helicopter did not have an engine cowl, the leaking exhaust gases would not have left a residue. This would have made the crack more difficult to detect during a pre‑flight inspection when the engine was cold.

Figure 6: Cracked exhaust collector pipe

Source: the maintenance facility that conducted the engine strip down inspection, annotated by the ATSB

Safety analysis

Engine power reduction

The sequence of events described by the pilot and passengers were consistent with a loss of aircraft engine power during flight, necessitating an emergency landing. The ATSB considered the following potential reasons for the reduction in engine power:

• weather conditions, including carburettor icing
• fuel contamination or starvation
• handling‑related issues
• engine and/or associated systems defect. 

The evidence gathered by the ATSB during the investigation indicated that the 3 initial possibilities were unlikely. 

However, based on information from the engine manufacturer and the personnel that conducted the post‑accident engine examination, a pre‑existing crack in the exhaust collector likely reduced the engine power output during the flight. 

Helicopter maintenance

The ATSB identified that low utilisation of the helicopter may have required special maintenance actions and/or storage procedures to be undertaken to prevent component corrosion or contamination. While there was no evidence that such maintenance was undertaken, it was not possible to determine whether its absence contributed to the crack in the exhaust system. 

Autorotation

Analysis of flight data identified that the helicopter was over inhospitable terrain at the time of the power loss. After turning towards a suitable landing site, the pilot commenced an autorotation to that location. The helicopter proceeded downwind, initially attaining a maximum airspeed of about 70 kt. The airspeed was then reduced to close to the target minimum rate of descent airspeed of 48 kt, providing good potential flare effect to both slow the helicopter prior to touchdown and increase the rotor RPM if necessary.

Despite that, the pilot reported there was minimal flare effect when they approached the landing site, likely due to low rotor RPM. As a result, the pilot was unable to prevent the helicopter colliding with terrain during the termination. Importantly however, none of the occupants were injured.

Findings

From the evidence available, the following findings are made with respect to the collision with terrain involving Kawasaki 47G, VH‑BEU, 24 km north of Katherine Tindal Airport, Northern Territory on 11 September 2024. 

Contributing factors

• The right exhaust collector was found to have developed a significant crack, which likely resulted in engine power loss during flight.
• The pilot was unable to cushion the landing during termination of the autorotation, likely due to low rotor RPM, resulting in the helicopter colliding with terrain.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the pilot and passengers of the accident flight
• the operator
• the maintainer for VH-BEU
• Civil Aviation Safety Authority
• accident witness
• OzRunways data from the pilot’s iPad
• Bureau of Meteorology
• Kawasaki Heavy Industries
• the maintenance facility that conducted the post‑accident engine examination.  

References

U.S. Department of Transportation Federal Aviation Administration Helicopter Flying Handbook, FAA‑H‑8083‑21B (2019).

Lycoming Service Instruction No. 1009BE (2020).

Reciprocating engine and exhaust vibration and temperature levels in general aviation aircraft, U.S. Department of Transportation Federal Aviation Administration (1968).

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report. 

A draft of this report was provided to the following directly involved parties:

• the pilot of the accident flight
• the operator
• the maintainer for VH‑BEU
• Civil Aviation Safety Authority
• Kawasaki Heavy Industries
• Textron Lycoming
• United States National Transportation Safety Board
• Japan Transport Safety Bureau

Submissions were received from:

• the pilot of the accident flight
• the operator
• Civil Aviation Safety Authority

The submissions were reviewed and, where considered appropriate, the text of the report was amended accordingly.