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

On the morning of 12 June 2025, a Virgin Australia Airlines Boeing 737‑800, registered VH‑YIL, operated a passenger transport flight from Brisbane, Queensland, to Sydney, New South Wales.

As the aircraft descended towards Sydney, air traffic control provided clearance for the crew to conduct a visual approach to runway 34 left. During the approach, the speed brake was not armed, and the final flap selection was not completed until 875 ft above the airport elevation (AFE). The operator’s procedures required that both items be completed before the aircraft descend below 1,000 ft above the airport elevation.

As the aircraft later descended through about 500 ft AFE, the captain checked the aircraft configuration and identified that the speed brake was not armed. The captain then armed the speed brake as the aircraft descended below 405 ft AFE. The approach continued and the aircraft landed without further incident.

What the ATSB found

The ATSB found that after air traffic control provided clearance for the crew to conduct a visual approach, a required autopilot altitude selection was not completed. As a result, the aircraft later deviated above the desired approach path. 

The crew immediately recognised the deviation and, in response, the captain disengaged the autopilot and auto thrust to manually re‑establish the approach descent profile, without informing the first officer. This led to an unexpected increase in flight crew workload. Then, in attempting to re‑establish the desired approach path, the crew did not fully complete the landing procedures and associated checklist before descending below the stabilisation criteria check altitude. Subsequently, the flight crew did not perform the required missed approach but instead continued the approach and landing.

Safety message

Unstable approaches continue to be a leading contributor to approach and landing accidents and runway excursions. This incident highlights how quickly a small oversight can disrupt an otherwise standard approach. If the disruption leads to a breach of the stabilised approach criteria, early recognition of the situation and prompt execution of a go‑around, rather than continuing the approach, will significantly reduce the risk of approach and landing accidents.

This incident also highlights that when crews are faced with the unexpected, effective crew resource management, with clear communication between the crewmembers, is essential. This ensures effective teamwork when responding to disruptions. Additionally, effective flight crew monitoring in a multi‑crew environment is paramount to aircraft safety. Bringing deviations to the attention of the pilot flying ensures that the aircraft remains on a safe flight path and is correctly configured for the relevant phase of flight.

The investigation

The occurrence

On the morning of 12 June 2025, a Virgin Australia Airlines Boeing 737-800, registered VH‑YIL, operated a passenger transport flight from Brisbane, Queensland, to Sydney, New South Wales.

As the aircraft descended towards Sydney in day visual meteorological conditions and with the autopilot engaged, air traffic control provided clearance for the crew to conduct a visual approach to runway 34 left. At that time, an altitude of 2,000 ft was set in the altitude window of the autopilot mode control panel (MCP). After receiving clearance for a visual approach, operational procedures required that the pilot flying[1] select an altitude equivalent to 500 ft above the airfield elevation (in this case 500 ft, as the airport was close to sea level) in the MCP altitude window (see the section titled Approach procedures). However, the captain, acting as pilot flying, inadvertently did not make this selection and the first officer, acting as pilot monitoring, did not identify that this altitude selection had not been completed.

As the aircraft descended toward 2,000 ft above mean sea level (AMSL) (Figure 1), the aircraft intercepted the final approach track. The crew expected the descent to continue, but the aircraft began automatically levelling off to capture the 2,000 ft altitude set in the MCP altitude window, taking the aircraft above the desired approach descent profile.

The crew immediately recognised the deviation and identified that the incorrect altitude was entered into the MCP altitude window; to continue the descent, but without verbalising the action, the captain entered 500 ft in the altitude window. Shortly after, the captain recognised that this selection would not re‑establish the required approach path, so, without first alerting the first officer to their intentions, disengaged the autopilot and auto thrust to manually re‑establish the approach descent profile.

Figure 1: Overview of the approach

Source: Google Earth, recorded flight data and ATSB

As the crew worked to re-establish the desired approach path while completing the pre‑landing procedures, the speed brake was unintentionally not armed, and the final flap selection (flap 40) was not made until 939 ft above the airport elevation. These items, and the associated landing checklist, were required to be completed before the aircraft passed 1,000 ft as set out in the operator’s procedures (see the section titled Stabilised approach).

The captain did not recognise that the checklist was not complete and believed that the stabilised approach criteria had been met. The first officer, acting as pilot monitoring, did not identify that the speed brake was not armed, but did identify that the required final flap selection and the landing checklist had not been completed in time. However, the first officer noted that the approach path, speed and descent rate were within the criteria and announced that the approach was stable.

As the aircraft descended through about 500 ft AMSL, the captain checked the aircraft configuration and identified that the speed brake was not armed. The captain then armed the speed brake as the aircraft descended below 426 ft AMSL (405 ft above the airport elevation). The approach continued without further incident and the aircraft landed at 0905 local time.

After landing, the captain discussed the incident with the first officer and assessed that a missed approach should have been conducted.

Context

Pilot details

The captain held an air transport pilot licence (aeroplane) and class 1 aviation medical certificate. The captain had 14,975 hours of flying experience, of which 10,081 hours were on the Boeing 737 aircraft type, with 127 hours accrued in the previous 90 days.

The captain held additional non‑flying duties in the organisation with 50% of their time spent in normal flying duties. The captain also stated that in their experience of regular flying operations, visual approach clearances were unusual.

The first officer held an air transport pilot licence (aeroplane) and class 1 aviation medical certificate. The first officer had about 28,000 hours of flying experience, of which about 14,000 hours were on the Boeing 737 aircraft type, with 117 hours accrued in the previous 90 days.

The ATSB found no indicators that the flight crew were experiencing a level of fatigue known to adversely affect performance.

Approach procedures

Visual approach

The operator’s flight crew operations manual (FCOM) for the Boeing 737‑800 aircraft included the following visual approach procedure:

When cleared for a visual approach, the MCP altitude should be selected to 500 ft above field elevation, however this does not preclude setting an intermediate level‑off altitude if desired.

Stabilised approach

The operator’s policy and procedure manual provided the following stabilised approach policy that included:

All approaches must be stabilised by 1000 ft above field elevation.

An approach is stabilised when the following criteria are met:

 - Briefings and normal checklists are completed

 - Aircraft is in the correct landing configuration

 - Aircraft is on the correct lateral and vertical flight path

 - Sink rate, no greater than 1,000 feet per minute.

The policy also noted that if the stabilisation criteria were exceeded for other than momentary periods at any time below the stabilisation height, the pilot monitoring must call "NOT STABLE”, and the pilot flying must initiate a missed approach.

The FCOM also stated that a missed approach shall be executed whenever required visual reference is not obtained or maintained or when an approach is not stabilised at 1000 ft above the airport elevation.

Meteorology

The approach was conducted in visual meteorological conditions.

At 0900, 3 minutes before the incident, the Bureau of Meteorology automatic weather station at Sydney Airport recorded the temperature as 12°C and the wind as 13 kt from 247° magnetic. Cloud cover was recorded as few[2] at 3,521 ft above mean sea level (AMSL). Visibility was recorded as greater than 10 km with no recorded precipitation.

Recorded data

Virgin Australia provided the ATSB with the aircraft’s quick access recorder data which captured the incident approach.

The recorded data (Figure 2) showed that the landing flap selection (flap 40) was completed 3 seconds after the aircraft descended below 1,000 ft above the airport elevation (AFE), at 939 ft AFE (960 ft AMSL). The flaps then completed moving to that extension as the aircraft descended below 875 ft AFE (896 ft AMSL). The speed brake lever was moved to the armed position as the aircraft descended below 405 ft AFE (426 ft AMSL).

Figure 2: Graphical representation of the recorded quick access data

All times are coordinated universal time (UTC). Local time was Australian Eastern Standard Time (EST), which was UTC +10 hours. Source: Quick access recorder from VH-YIL, annotated by the ATSB

After descending below 1,000 ft, the aircraft maintained an appropriate speed and flightpath. The rate of descent exceeded the 1,000 ft per minute stabilised approach criteria limit for a 9 second period between 0904:02 and 0904:11. During this period, the aircraft descended from 747 ft AFE to 587 ft AFE, and the maximum recorded descent rate was 1,136 ft per minute at 0904:07.

Safety analysis

As the aircraft descended towards Sydney, the crew were provided with a visual approach clearance which the captain reported was unusual. After receiving the clearance, the captain unintentionally did not make the required 500 ft selection in the altitude window of the mode control panel. The first officer, as the pilot monitoring, did not identify that this omission had occurred. Consequently, as the aircraft descended to 2,000 ft the autopilot began to level off rather than continuing the descent to 500 ft, which took the aircraft above the desired descent profile. The captain responded with an unplanned manual intervention without alerting the first officer to their intention while the flight crew were also attempting to complete the final landing procedures. This led to an unexpected increase in flight crew workload and reduced the first officer’s situation awareness.

Workload has been defined as ‘reflecting the interaction between a specific individual and the demands imposed by a particular task. Workload represents the cost incurred by the human operator in achieving a particular level of performance’ (Orlady and Orlady, 1999). A discussion of the effect of workload on the completion of a task requires an understanding of an individual’s strategies for managing tasks.

An individual has a finite set of mental resources they can assign to a set of tasks (for example, performing an approach and landing). These resources can change given the individual’s experience and training and the level of stress being experienced at the time. An individual will seek to perform at an optimum workload by balancing the demands of their tasks. When workload is low, the individual will seek to take on tasks. When workload becomes excessive the individual must, as a result of their finite mental resources, shed tasks.

An individual can shed tasks in an efficient manner by eliminating performance on low priority tasks. Alternatively, they can shed tasks in an inefficient fashion by abandoning tasks that should be performed. Tasks make demands on an individual’s resources through the mental and physical requirements of the task, temporal demands and the wish to achieve performance goals (Hart and Staveland, 1988, and Lee and Liu, 2003).

In this case, likely in response to increased workload and the absence of crew coordination, they missed required checklist items (the final flap and speed brake selections). The stabilised approach criteria required that the aircraft be in the final landing configuration by 1,000 ft above the airport elevation. The landing flap selection was made 3 seconds after descending below this height, although the captain believed that the flap selection had been made in time to meet the stabilised approach criteria requirements. However, the flaps did not reach the required position until the aircraft descended through 875 ft above the airport elevation. 

The first officer identified that the flap selection was made late and that, therefore, the stabilised approach criteria had not been met. However, as the descent rate, speed and profile were within the criteria, they announced that the approach was ‘stable’ instead of making the required ‘not stable’ announcement. Consequently, the required missed approach was not commenced, and the approach was continued. The first officer did not identify that the speed brake landing procedure action was not completed.

As the approach continued, the descent rate exceeded the stabilised approach criteria for a period of 9 seconds. This exceedance was momentary and not excessive and therefore it did not require the commencement of a missed approach.

When the unarmed speed brake was later identified by the captain, this should have acted as a further trigger for the commencement of a missed approach. Instead, this missed action was quickly rectified by the captain and the approach continued.

Findings

From the evidence available, the following findings are made with respect to the unstable approach involving Boeing 737, VH-YIL, near Sydney Airport, New South Wales, on 12 June 2025.

Contributing factors

• After air traffic control provided clearance for the crew to conduct a visual approach, a required autopilot altitude selection was not completed. As a result, the aircraft later deviated above the desired approach path.
• While re-establishing the approach path, the crew did not complete required landing procedures until after the aircraft descended below the stabilisation criteria check altitude. Subsequently the flight crew did not perform the required missed approach, instead continuing the approach and landing.

Sources and submissions

Sources of information

The sources of information during the investigation included:

• Airservices Australia
• Bureau of Meteorology
• recorded data from the quick access recorder from VH-YIL
• the flight crew
• Virgin Australia Airlines.

References 

Orlady, HW & Orlady, LM 1999, Human factors in multi-crew flight operations. Ashgate, Aldershot, p. 203. 

Hart, SG & Staveland, LE 1988, ‘Development of NASA-TLX (Task Load Index): Results of empirical and theoretical research’, In PA Hancock & N Meshkati (Eds.), Human Mental Workload. North Holland Press, Amsterdam.

Lee, YH & Liu, BS 2003, ‘Inflight workload assessment: Comparison of subjective and physiological measurements’, Aviation, Space, and Environmental Medicine, vol.74, pp. 1078- 1084.

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 flight crew
• Virgin Australia Airlines.

No submissions were received.

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