Aircraft preparation event involving Saab 340B, VH-OLL, Melbourne Airport, Victoria, on 19 November 2024

Investigation summary

What happened

On 19 November 2024, a Saab 340B, registered VH-OLL, was being prepared at Melbourne Airport, Victoria, for a passenger air transport flight. During the preparation of the aircraft, the first officer fitted a propeller strap to the left engine propeller, and a propeller strap extension to the propeller strap and aircraft stairs.

The normal procedures for aircraft preparation required the first officer to conduct a final external check, in which they would remove the propeller strap and strap extension, then rotate the propeller blades. The first officer conducted the final external check, however instead of removing the propeller strap, they disconnected the strap extension from the propeller strap and the aircraft stairs. 

After the first officer entered the aircraft with the strap extension and the aircraft doors were closed, an airport services officer trainee (ASO trainee) assigned to the aircraft provided the flight crew a clearance to start the engines. The captain started both engines, having not detected the propeller strap during their before start check.

After the engines were started, a passenger alerted the flight attendant that they had observed a strap on the propeller prior to the start. The flight attendant notified the flight crew, and the flight crew stopped the engines.

Post-incident inspection identified that the propeller strap remained on the propeller blade, with the pins which connected the strap to the engine cowling having separated. 

What the ATSB found

The investigation identified that actions from multiple individuals contributed to the development of this incident. The first officer, the captain and the ASO trainee were all required to attend to the propeller prior to the engine start. The first officer did not remove the propeller strap and rotate the propeller as required, whereas neither the captain nor the ASO trainee detected the strap prior to the captain starting the engines. 

The action by the first officer to remove the strap extension and not the propeller strap created a condition wherein the propeller strap remained on the aircraft, while the mechanical restraint which prevented the aircraft stairs from being retracted (the strap extension) was no longer present. By removing the strap extension, the action of the first officer allowed the flight attendant to retract the aircraft stairs. The absence of the strap extension and the retracted stairs contributed to the captain and the ASO trainee having an expectancy that the propeller strap had been removed, reducing the likelihood they would identify it had not. 

The investigation found that the Regional Express procedures and training for aircraft dispatch provided limited guidance. The dispatch procedures did not describe the form or function of the propeller or strap extension, and nor did they identify that the orientation of the propeller could be a supplementary indicator for identifying the placement of the propeller strap. Had the ASO trainee been provided better training to support monitoring for propeller straps, it is likely they would have detected the strap remained attached to VH-OLL and not provided a clearance to start the engine. 

The ASO trainee was assigned to conduct the dispatch of VH-OLL as part of practical training requirements, under the supervision of another ASO (ASO supervisor). The investigation found that whereas the ASO supervisor should have monitored the dispatch in order to assure the performance of the ASO trainee, the supervisor instead departed the apron and was not present to observe the dispatch. 

The investigation also found that while Regional Express used supervised practical training as a mechanism to develop the competency of ground handling personnel, it had not defined the roles and expectations of training supervisors. The procedures did not describe the requirements for supervisors to maintain visibility on the work of trainees, and nor did they define who had overall accountability for safety sensitive work conducted in training. 

What has been done as a result

Regional Express reviewed the circumstances of the incident and took several actions to provide greater information to ASOs to support their checking for propeller straps during aircraft dispatch, and to provide greater assurance that ASOs and flight crew were adequately checking for propeller straps during aircraft preparation. These actions were:

• Updating the receipt and dispatch training for ASOs, to provide images showing the propeller strap and strap extensions fitted to a Saab 340. The updated training also explained that the propeller position could be used as a supplementary check to indicate if the propeller strap has been removed. Regional Express also disseminated an Airport Advisory with these images and information.
• Amending the Saab 340 flight crew line training program, to include checks of the final external check and removal of the propeller straps and strap extensions, and of the ‘clear left/right’ visual checks of each engine prior to engine start.
• Amending the Regional Express internal safety audit template, to include observations of the checks ASOs conduct of propeller straps.

Regional Express also took action to clarify the roles and expectations of personnel supervising training. Regional Express updated its Airport Services Manual to include the responsibilities of training supervisors. The manual was also updated to state that staff under training are not considered competent until such time as they are assessed as competent and that trainers are responsible for the duties and tasks performed by trainees during any task for which a trainer is providing training or supervision. Regional Express also issued an Airport Advisory to its personnel identifying these requirements.

Safety message

This incident highlights the critical safety role performed by ground handling personnel, who are often the last line of defence in ensuring commercial aircraft are prepared safely for departure. Operators should seek to ensure that ground handling procedures and training are designed to effectively support the ability to check for safety critical items. Operators should consider the conspicuity of items which need to be detected and removed prior to flight, and how ground handling personnel are taught to look for these items. 

The incident also highlights the potential for on-the-job training to introduce safety risks. While it is valuable for personnel to practice and demonstrate their capability prior to being deemed competent, it is critical that competent personnel maintain oversight of trainees in this context. Operators should ensure that they clearly articulate the roles and expectations of personnel supervising practical training for safety critical tasks. 

The occurrence

On 19 November 2024, a Saab 340B, registered VH-OLL, was being prepared for a passenger air transport flight between Melbourne Airport, Victoria and Merimbula, New South Wales. The aircraft was being operated by Regional Express. The aircraft was crewed by a captain, a first officer (FO), and a flight attendant. There were 31 passengers on board.

The aircraft had been operated on a return flight to and from Devonport, Tasmania. The aircraft landed at Melbourne Airport at 1045, with closed circuit television (CCTV) recordings showing the aircraft arrived at Melbourne Airport bay 57C at 1047. At 1048, the FO disembarked carrying a propeller strap which was connected to a strap extension.[1] The FO attached the propeller strap to the left (port side) propeller (to prevent rotation) and the strap extension to the handrail of the aircraft stairs (Figure 1). About 2 minutes later, passengers disembarked the aircraft.

Figure 1: Propeller strap and strap extension dimensions and appearance

Source: ATSB

At 1053, the FO commenced a pre-flight walkaround check of the aircraft that included checking both propellers were rotating freely and without damage to the blades. This required unfastening and refastening the strap on the left propeller. 

Passengers for the flight to Merimbula commenced boarding at 1107.

Multiple ground handling personnel worked around the aircraft during the turnaround. Among these, an airport services officer (ASO) was tasked with dispatching the aircraft, under the supervision of another ASO. The ASO tasked with dispatching the aircraft was completing practical training requirements (and is referred to as the ASO trainee), whereas the other ASO (ASO trainer) was completing a practical observation log for the dispatch.

The ASO trainee attended the aircraft at 1107 and conducted tasks including assisting with loading bags into the cargo hold. The ASO trainer attended the aircraft at 1109 and also assisted with loading bags. The ASO trainer left the aircraft at 1112 and went into a nearby office adjacent bay 57C.

At 1111, CCTV showed that the FO commenced the final external check of the aircraft. The FO rotated the right propeller into the dressed position, where the propeller blades were rotated into an ‘X’ orientation (see Figure 5).

The footage showed the FO continued the walkaround and approached the left propeller at 1113. While the CCTV did not record how the FO interacted with the propeller, the FO later recalled disconnecting the strap extension from the propeller strap and the aircraft stairs, and not disconnecting the propeller strap from the propeller. The FO then entered the aircraft and the stairs were retracted. 

The pilots recalled that the FO then entered the flight deck and briefed the captain that the doors had been closed. Both pilots recalled that the FO was holding a piece of strapping, of the type used for propeller straps and strap extensions, which the FO placed in the normal location on the flight deck floor. 

The pilots conducted the engine start checklist. As part of this checklist, the FO conducted a visual check of the right engine, and the captain checked the left engine. The captain recalled looking at the orientation of the propeller blades and perceiving the propeller was clear.

Soon after the FO entered the aircraft, the ASO trainee moved to the dispatch position approximately 10 m ahead of the nose of the aircraft. CCTV footage showed that the right engine started at 1113:30, with the left engine starting at 1114:35. The video showed the ASO trainee gave engine start signals before the start of each engine. 

Shortly after the left engine was started, a passenger on board the aircraft alerted the flight attendant that they believed a strap had been left on the left propeller during the start. The flight attendant then called the flight crew, and advised the captain of the passenger’s observation. The flight crew checked the strap on the flight deck floor and identified that only the strap extension was present. The captain shut down the aircraft and passengers disembarked at 1119.

Post-incident inspection identified that the propeller strap remained on the propeller, and the pins which connected the strap to the engine cowling had failed (Figure 2). The inspection did not identify any damage to the aircraft.

Figure 2: Post-incident photography showing separation of the propeller strap from cowling pins

Source: Regional Express

Context

Personnel information

Pilot information

Licence and pilot experience details for the captain and the first officer (FO) are shown in Table 1.

Table 1: Pilot details for crew of VH-OLL

Source: Regional Express

Both pilots reported sleeping well on the 2 nights prior to the occurrence. Neither pilot recalled feeling sleepy or fatigued. 

The FO had returned to Australia on 16 November having been on holiday in Europe. The FO reported that they had returned to work on 18 November and had slept normally in the days since returning.

Ground handling personnel information

Records showed that the airport services officer (ASO) trainee who conducted the dispatch of VH-OLL had been assessed as competent in the receipt and dispatch of Regional Express Saab 340 aircraft in April 2023. Regional Express advised that the organisation had determined it was necessary to re-certify the competency of the ASO trainee to conduct Saab 340 receipt and dispatch. This was because the ASO trainee had been assigned limited duties on the aircraft type after being initially assessed as competent, and because the organisation had transitioned to a new training record‑keeping system. 

Training records showed the ASO trainer had also been assessed as competent in Saab 340 receipt and dispatch, and had completed and been assessed as competent in On‑The‑Job Training and Assessing. Further detail on Regional Express ground handling procedures and training is provided in Ground handling training and supervision procedures.

Aircraft information

VH-OLL was a Saab 340B, serial number 340B‑175. It was manufactured in Sweden in 1990, and was first registered in Australia in March 2000. The aircraft was powered by 2 General Electric CT7‑9B turboprop engines, equipped with Dowty Aerospace R.390/4‑123‑F/27 propellers.

Saab 340 propeller restraints

Propeller strap

The propeller strap was intended to prevent propeller movement when the aircraft was parked. The strap consisted of a loop to slide over the propeller blade, 2 cables with pins for securing to the engine cowling, and a streamer to aid visibility (Figure 3).

Figure 3: Schematic drawing of Saab 340 propeller strap

Source: Saab, annotated by the ATSB

Following the incident, the strap was found attached to a propeller blade, with the cowling pins having sheared due to torque from the propeller during the engine start. Post‑incident photography (Figure 2) showed the propeller strap fitted to the left engine’s propeller was orange, with an orange ‘remove before flight’ steamer attached at the rear of the propeller blade. 

Strap extension

Regional Express also used a propeller strap extension, which connected the propeller strap to the aircraft stairs. The Regional Express Flight Crew Operating Manual (FCOM) stated the extension was developed:

to provide a visual indication of a restricted area around the propeller… (and provide) safety mechanisms restricting the passenger door stairs from being closed without removing the extension and propeller bridle (propeller strap).

The strap extension was manufactured of bright orange webbing. It attached to the propeller strap using a metal clasp, which was released by sliding a latch backwards (Figure 4). However, the latch was missing from the strap extension attached to VH-OLL. The extension attached to the aircraft stairs with velcro.

Figure 4: Exemplar propeller strap and strap extension

 Source: ATSB

Propeller orientation

Regional Express Saab 340 procedures described the setting of the propellers at 2 orientations. A propeller was in the ‘undressed’ orientation when the blades were oriented vertically and horizontally (+ position). The propeller strap was fitted to the propeller in this orientation.

A propeller was in the ‘dressed’ orientation when the blades were rotated at 45° from vertical and horizontal (X position). The propeller strap could not be fitted in this orientation. Figure 5 shows a Regional Express Saab 340 with the propellers at dressed and undressed orientations. 

Figure 5: Regional Express Saab 340 aircraft with left engine (image right) propeller in undressed orientation, right engine (image left) propeller in dressed orientation

Source: ATSB

Regional Express advised that pilots were taught that the orientation of propellers to the dressed position was an ‘added indicator’ that the propeller strap had been removed. Pilots were also taught, however, to confirm the absence of the propeller strap. Experience within the Regional Express operations was that after a propeller strap had been removed, a propeller would sometimes rotate in wind into an upright (undressed) orientation.

Other information relevant to propeller strap conspicuity

Propeller straps were manufactured of 25 mm wide webbing, with the extension strap manufactured of 50 mm wide webbing. When viewed from a dispatch position of between 10 and 15 m from the nose of the aircraft, the propeller strap presented as only a thin piece of fabric. In daylight conditions when the sun was high in the sky, the propeller strap sat in the shadow of the engine (Figure 6).  

Figure 6: Saab 340 with strap extension installed (upper) and removed (lower)

Source: ATSB

Viewing the propeller strap from the flight deck's left (captain) seat ordinarily required a pilot to lean forward and look left. This was normally achievable within a comfortable range of movement. Figure 7 shows the appearance of the propeller and propeller strap from the left seat, in a simulated forward‑lean position. 

Figure 7: View of propeller strap from left (captain) seat

Source: ATSB

Operating procedures

The Regional Express Saab 340 FCOM described the operating procedures applicable to the crew of VH-OLL. 

Propeller strap fitment and checks

Post-flight 

The FCOM required flight crew to ensure the propeller strap and strap extension (which travelled with the aircraft in the flight deck) were fitted to the left engine propeller prior to passengers disembarking. The FCOM also required a post‑flight inspection after each flight, which included a check of each propeller for freedom of rotation.

Both pilots onboard VH-OLL recalled that it was normal practice for FOs to fit the propeller strap and strap extension to the left engine propeller during all turnarounds. 

Following the arrival of the preceding flight (from Devonport), the FO conducted the required post‑flight inspection checks and fitted the propeller strap and extension to the left engine propeller. 

Final external check

The final external check procedure was required prior to each flight, after passengers had boarded. The procedure required that the FO conduct various tasks including removal of the propeller strap from the left propeller during turnarounds,[2] and dress the propellers to 45°.

After the final external check, the FCOM required that the FO confirm the number of passengers on board the aircraft with the flight attendant, then brief the captain. The brief was to include confirming that the doors were closed and the tail strut[3] stowed.

The FO recalled conducting the final external check prior to the planned departure to Merimbula. They recalled intending to remove the propeller strap and extension, however they inadvertently removed only the extension. The FO recalled that disconnecting the strap extension had been slightly more difficult than normal, because of the missing latch on its clasp. The FO could not identify a reason they had inadvertently removed the extension and not the propeller strap.

The FO could not recall if they attempted to dress the propeller while conducting the final external check. Closed circuit television footage provided limited resolution of the orientation of the propeller blades, however indicated that the left engine propeller remained in the undressed orientation while the FO conducted the final external check. With the propeller strap installed, it would not have been possible for the FO to rotate the propeller significantly.

Removal and stowage of propeller restraints

Regional Express procedures did not specify how the strap extension should be removed from the aircraft prior to departure. That is, there was no procedural guidance concerning whether the strap extension should be disconnected from the propeller strap, and if so whether the propeller strap or the strap extension should be removed from the aircraft first.

During the final external check, pilots would commonly remove the propeller and extensions straps together, such that the propeller strap cowling pins would be removed from the engine cowling and the strap extension would be removed from the aircraft stairs. The latch connecting the propeller strap and the strap extension would not be disengaged. 

Pilots would then commonly roll up the connected straps, bring the bundled straps into the flight deck and place them on the floor behind the centre console. Figure 8 illustrates this practice.

The FCOM stated that after the last flight of the day, the captain was required to ensure the external security of the aircraft, including that the doors were closed and locked and the propeller straps were fitted to both engines. The left engine strap extension was not to be fitted at this time. 

The FO reflected that their action of removing the strap extension without the propeller strap may have resulted from them inadvertently applying the procedures for the last flight of the day during the turnaround for VH-OLL. However, the FO stated they did not believe this to be the case, as they had been aware passengers had boarded the aircraft for the flight to Merimbula. 

The FO recalled that after completing the external check, they entered the aircraft holding the propeller strap extension. The FO then entered the flight deck and briefed the captain, before placing the strap extension on the floor. The captain recalled seeing the strap extension, which they presumed was the strap extension with the propeller strap attached.

Figure 8: Exemplar Saab 340 propeller strap and strap extension stowed in typical location behind the centre console

Source: ATSB

Engine start

The FCOM included the engine start checklist, which described actions required prior to starting the aircraft engines. The FCOM stated that the captain would call this checklist after the FO returned to the cabin and briefed the captain that the doors were closed, and after the captain signalled for the ground crew to remove the wheel chocks.

The engine start checklist comprised 6 items, with the final item being ‘PROP’. The instructions for completing the ‘PROP’ check stated:

Before starting any engine, the [captain] shall check propeller area is clear, the prop tie has been removed and fuel cap is on, then call “Clear left”.

The [FO] shall check propeller area is clear, the prop tie has been removed, the fuel cap is on and the fuel door is closed, then call “Clear right”.

Perform this check in addition to the clearance obtained from ground staff.

[FO to call] Engine Start Checklist Complete.

The FCOM required that following the completion of the engine start checklist, the captain commence engine start for each engine.

The captain recalled calling for the engine start checklist and conducting each of the required steps including visually checking the left propeller area. The captain recalled observing the left propeller in an orientation which was not horizontal and perpendicular, indicating to the captain it had been dressed in a manner consistent with the completion of the external check. Reflecting on the incident, the captain identified that their check of the left propeller area had not been as thorough as it could have been, and that this may have been influenced by their expectation the propeller strap had been removed having sighted a strap in the flight deck.

Ground handling procedures

Saab 340 dispatch procedures

The Regional Express Airport Services Manual (ASM) described dispatch procedures for Regional Express Saab 340 aircraft. These procedures were also described and illustrated in the Regional Express Saab 340 receipt and dispatch training material. The duties of an ASO conducting a dispatch (dispatcher) included:

• When instructed by the flight crew, removing tail strut and stowing it in the cargo hold before closing the cargo door.
• Moving to the front of the aircraft, whilst observing the side of the aircraft and wing for anything abnormal including open panels, structural damage or spills.
• Waiting at the front of the aircraft for the captain to signal to remove the chocks from the aircraft wheels, then removing the chocks and displaying them to the captain.
• Waiting at the front of the aircraft in the dispatch position for the captain to signal engine start. The procedures stated that while waiting in this position, the dispatcher was to ‘ensure all bungs, covers and straps have been removed from engines and pitot tubes.’
• After receiving the engine start signal from the captain, perform the engine start clearances using hand signals, then observe the started engines for any abnormal indications.

After the engines had been started, the dispatcher was required to disconnect the ground power unit and drive the unit away from the aircraft. After the unit had been removed, the dispatcher was to provide a final thumbs up signal to the captain. 

Neither the ASM nor the training material made specific mention of the propeller strap. Neither document described where the dispatcher should attend while checking the aircraft prior to issuing the engine start clearances, nor what to look for to identify problems with aircraft preparation. Neither document described the expected orientation of the propeller blades when the propeller strap was removed and fitted (dressed and undressed position), nor did the documents include any images of the propeller strap or strap extensions.

Though it was not documented, the ASO trainee recalled that the procedures included a requirement to observe the aircraft prior to and during the engine start, including checking that the propeller straps had been removed. The ASO trainee reflected that they would normally be able to see the propeller straps from the dispatch position, and that it was possible they did not detect the straps on VH-OLL due to being affected by sun glare. The ASO trainee stated that the propeller straps were normally removed by first officers while the dispatcher was conducting other duties, and that they had never previously identified a propeller strap inadvertently left on an engine prior to start.

The ASO trainer recalled that the requirements for aircraft dispatch included that the dispatching ASO was to observe the aircraft prior to the start, to identify anything abnormal. The ASO trainer did not recall any specific requirement for the dispatcher to look for the propeller strap. They identified that during the engine start the ASO trainee would likely have an expectancy that the crew had removed the propeller strap, consistent with operating procedures. 

Neither the ASO trainee nor the trainer understood the implication of the propellers being rotated to the dressed position as an indicator that the propeller strap had been removed. 

Ground handling training and supervision procedures

The ASM stated that Regional Express management personnel were to ensure that ASOs were provided with ‘100% supervision’ until an ASO was trained and assessed as competent. The ASM stated that training and assessment could comprise various components including classroom, computer-based and practical training. For aircraft receipt and dispatch, the ASM required that each ASO meet a minimum practical competency of 5 receipts and 5 dispatches. The ASM stated these could be tracked using a practical observation log checklist.

On the day of the incident, the ASO trainee was completing receipt and dispatch duties under supervision to meet the practical performance requirements described by the ASM. The ASO trainer was completing a practical observation log recording this performance. The log showed that the trainee had completed 3 aircraft receipts and 2 dispatches.  

The procedures stated that, where possible, trainers should ensure that each learner physically inspects each item during inspection tasks, and performs each performance task. The ASM did not describe the roles or specify a requirement for trainers to observe trainee performance of safety critical tasks, including those recorded using a practical observation log.

As described in Ground handling personnel information, the ASO trainer had completed and been assessed as competent in On-the-Job Training and Assessment. Training materials from this course stated:

• On-the-job trainers were to ensure they were in control of training sessions.
• A trainee was to be considered as still in training until they had been confirmed as competent.
• During on-the-job training, trainers should monitor trainees and only allow them to learn by mistakes if it was safe to do so.
• If an on-the-job trainer needed to ‘step in and correct critical mistakes’ they must stop the assessment. 

In a post-incident assessment of the ASO trainer involved in the dispatch of VH‑OLL, Regional Express identified that the responsibility of a trainer and assessor included ‘maintaining 100% supervision’ and ‘ensuring all safety protocols are followed, including the removal of equipment such as prop straps’. The assessment further indicated that the supervisor was expected to maintain direct supervision of trainees during critical tasks.

The ASO trainer recalled that they had decided to depart the apron after the cargo door for VH‑OLL was closed, to prioritise documenting the dispatch on the observation log. The ASO trainer further recalled the engine start occurred sooner than they expected, and implied that they had anticipated having sufficient time to return to the apron to observe the engine start. The ASO trainer identified that they did not observe the dispatch or the engine start, and stated that there was no requirement for the training supervisor to maintain eyes on the aircraft. However, the ASO trainer reflected that they should have maintained watch over these events in their role as supervisor. 

Environmental conditions and aircraft position

Conditions on the morning of 19 November were fine. At 1100, the Bureau of Meteorology weather station at Melbourne Airport recorded a temperature of 16°C and winds at 9 kt. There was broken cloud at about 34,000 ft.

Records from Geoscience Australia showed that the sun was high in the sky at 1115, with an estimated elevation of about 59°. The sun was in the north‑east, at an azimuth of about 62°.

Recorded video showed that VH‑OLL was parked at bay 57C at a heading of about 229°, such that the nose‑to‑tail heading was about 51°. From the estimated position of the ASO trainee at the dispatch position at the nose of the aircraft, the line of sight to engine 1 was at a heading of about 63° (Figure 9). The ASO trainee recalled experiencing sun glare while looking towards the aircraft during the dispatch, and the recorded video appeared to show the ASO trainee moving their hands to their face as if to shield from the sun. The ASO trainee was not wearing a hat or sunglasses. 

Figure 9: Approximate lines of sight to engine 1 and flight deck from dispatch position, and position of the sun

Note: image represents approximate position of VH-OLL derived from video footage of the incident, annotated by the ATSB.

Previous occurrences

Aircraft preparation event involving Link Airways Saab 340

On 10 November 2022, a Link Airways Saab 340 was prepared for a commercial air transport flight from Canberra, Australian Capital Territory. During the preparation for that flight, the first officer did not attach the strap extension to the left engine propeller strap and aircraft stairs. The first officer did not remove the propeller strap, and neither the captain nor the dispatcher noticed the propeller strap prior to the engine start. 

As the aircraft departed from Canberra, the propeller strap was thrown from the blade and penetrated the fuselage, striking a passenger on the leg.

The ATSB investigation identified a safety issue with the guidance provided by Link Airways for training its contracted dispatchers.[4] The investigation found that:

The training material that Link provided to Swissport for dispatcher training did not include any specific requirements for dispatchers to check for propeller straps or the strap extension. Link’s ground handling manual only noted ‘prop straps’ as an item the flight crew fitted to the cabin door prior to disembarkation. The material did not include any further description of the propeller strap or its function and usage by Link. … This would have reduced the effectiveness of dispatchers to notice departures from procedure such as a propeller strap not being removed, or a strap extension not being fitted before boarding.

Regional Express routinely reviewed safety incidents and accidents involving other operators, such as those involving Saab 340 aircraft, to identify potential learnings for its own safety systems. Regional Express selected safety events for detailed review based on an assessment of the level of safety risk indicated by the incident or accident circumstances. 

Regional Express had determined that the risk profile associated with the Link Saab 340 aircraft preparation event was very low, with no records of other events of a similar nature. As a result, Regional Express did not seek to examine the safety issues identified in the ATSB investigation for implications to the Regional Express Saab operations. 

Other events involving Saab 340 propeller straps

There were no records in the Regional Express safety management system of previous incidents of propeller straps remaining on aircraft engines during engine start. However, during the investigation, 2 pilots advised of 2 separate incidents in which an FO had not removed the propeller strap during the final external check. These pilots recalled rectifying the situation by instructing the FO to remove the strap.

Saab advised that other than the Link Airways event, they had records of only one other incident involving a Saab 340 aircraft being started with a propeller strap installed. This event occurred in 2004 and there was very limited information available about it.

Safety analysis

Non-removal of propeller strap

During the final external walkaround check the first officer (FO) removed the strap extension from the propeller strap and the aircraft stairs. The first officer inadvertently left the propeller strap attached to the left engine cowling and propeller, and did not rotate the propeller to the required (dressed) orientation. The FO then returned to the flight deck carrying the strap extension, placing it in the normal location on the floor. 

The FO reflected that they could not understand why they removed the strap extension but not the propeller strap. The FO had difficulty recalling parts of the incident, and reflected that conducting the walkaround was a straightforward process which does not require much attentional effort. The implication is that the FO removed the strap extension unintentionally and without conscious awareness, in the context of a highly practiced procedural task. Unintentional action slips are known to sometimes occur in the context of such tasks (Reason, 1990). 

The unintentional removal of the strap extension by the FO thus provides an example of the inherent variability of human performance, and hence the vulnerability of procedural controls.

The investigation did not identify any factors which increased the risk of this omission. The FO had placed the propeller strap and the strap extension on the left engine propeller after landing from the preceding flight. The FO knew the strap was on the propeller, and understood the requirement to remove it prior to departure. There was no evidence that the FO was distracted during the walkaround, or that they were rushed when completing this task. While the FO had recently returned to Australia from overseas, the FO reported that they had slept well in the days before the incident and felt alert at the time.

With the propeller strap attached, it would not have been possible for the FO to rotate the propeller to the required (dressed) orientation. Had the first officer attempted to rotate the propeller, it is highly likely that they would have identified the strap had not been removed. 

When the FO removed the strap extension from the propeller strap and aircraft stairs, the stairs were retracted and the door was closed. This would not have been possible had the extension been in place. Furthermore, having returned to the flight deck with the strap extension in hand, this provided a perceptual cue to the FO that the external walkaround had been completed and that the straps (extension and propeller straps) had been removed. There were thus limited opportunities for the FO to identify that they had missed the propeller strap during their walkaround, and trapping this error relied upon the captain and the ASO trainee.

Engine start checklist

After the FO had completed the final external walkaround check and the aircraft door had been closed, the flight crew soon commenced the engine start checklist. Although the checklist required the captain to check that the left engine was clear, they did not identify that the propeller strap remained attached and proceeded to start the engine. 

The captain recalled having a very high expectancy that the propeller strap had been removed at the time they conducted the engine start checklist. This expectancy is understandable, as the captain had observed the FO enter the cabin holding a strap. The captain assumed this was the propeller strap and the strap extension. The captain had no reason to closely inspect the recovered strap to check both the extension and the propeller strap were present, and this was not normal practice.

Research has shown that expectancy affects what people attend to and how they perceive information (Wickens and others, 2013). People have limited attentional capacity, and are less likely to allocate their attention towards objects or areas they do not expect to contain relevant information. Moreover, when individuals have a very low expectancy for something, it may be missed even if directly looked upon, reflecting so‑called ‘inattentional blindness’, or the ‘looked‑but‑failed‑to‑see-effects (Wickens and McCarley, 2008). 

The captain identified that they did not check the left engine propeller thoroughly, which they attributed to their expectancy that the propeller strap had already been removed. 

The propeller strap appeared from the left seat position as a thin strap of fabric against the propeller blade, which sat in the shadow of the aircraft engine. Sighting the propeller blade required the pilot to lean forward in their seat. The implication is that while the strap was visible, it was not particularly conspicuous and required a degree of focused attention to detect. 

The captain recalled that rather than seeking to sight the propeller strap directly, they checked the angle of the propeller blades, perceiving the propeller blades were not in the undressed position. This indicated to the captain that the propeller strap had been removed, and the captain was thus satisfied that the propeller was clear.

The reliance on the propeller blade angle, however, was a less reliable means to check the engine. The propeller strap was long enough that the propeller blades could rotate slightly out of the vertical position in wind with the strap in place. This meant that the orientation of the propellor blades at the time of a check may provide a misleading indication as to the fitment of a propeller strap. In addition, the assessment of whether a propeller is at a dressed or undressed orientation is inherently more ambiguous than visually checking if a propeller strap is present or absent. 

Aircraft dispatch and supervision

While the FO returned to the flight deck and the crew prepared for the engine start checklist, the airport services officer (ASO) trainee who was tasked with dispatching the aircraft moved to the normal position at the nose of the aircraft to observe the engine start. 

From the dispatch position approximately 10 m from the nose of the aircraft, the propeller strap provided a smaller visual target compared to when viewed by the flight crew during their procedures. With the sun behind the aircraft tail, the propeller and the strap were in the shadow of the left wing and engine. The airport services officer (ASO) trainee was also affected by sun glare when looking towards the flight crew and the aircraft, reducing the likelihood they would detect the propeller strap.

The strap extension would have provided a significantly more conspicuous cue for the ASO trainee to identify the propeller was not clear to start. Not only was the strap extension made of broader webbing, if it had been attached to the propeller strap it would likely have also been attached to the aircraft stairs, preventing them from being retracted.

It is likely that, like the captain, the ASO trainee had a high expectancy that the engines were clear to start when they observed the start. The ASO trainee had observed the FO conduct their walkaround, and would have expected the propeller strap to have been removed. The ASO trainee did not identify the propeller strap attached to the left propeller, and signalled to the captain that they were clear to start the left engine.

Trainees who are being checked for the purpose of determining competency are by inference not yet competent in conducting those tasks. In a mature safety system, where a trainee is being assessed, the performance of safety sensitive tasks must be independently assured by someone who has been determined competent. 

There was some complexity regarding the competency status of the ASO trainee, as they had previously been assessed as competent to conduct dispatch duties. However, both the ASO trainee and the trainer understood the ASO was completing receipt and dispatch duties on the day of the incident to demonstrate their competency, and that the ASO trainer was observing this. 

In the case of the dispatch of VH-OLL, the ASO trainer was not present to observe the visual check conducted by the trainee of the engine start, having left the apron to complete paperwork. This was contrary to the expectations of supervision for a safety sensitive task, and prevented the trainer from assuring the trainee’s performance and oversighting the engine start process. When the ASO trainee did not detect the propeller strap, the trainer was not there to guide the trainee or identify that the propellor strap was missed.  

Dispatch procedures and training

The Regional Express safety system for assuring aircraft were correctly prepared for flight included visual inspection by trained ASOs prior to engine start. This provided an additional opportunity to identify any straps and covers inadvertently left in place by the flight crew. 

The safety system, however, did not effectively support the role of the dispatcher, as procedures and training provided limited guidance on how ASOs should check aircraft prior to giving clearance for engine start. The procedures contained only an instruction for dispatchers to ‘ensure covers and straps had been removed from engines and pitot tubes’, which was repeated in training material. Neither the procedures nor the training described the method by which the dispatcher should check the engine and pitot tubes, or what the dispatcher should look for. Neither described the appearance, location or function of the propeller strap. 

Procedures which require personnel to generally look over an aircraft, or to watch out for hazards, are unlikely to be effective. Without clear procedural guidance it is difficult for a person to determine whether a check has been fully completed, and an incomplete check will only be apparent in hindsight. In the context of a typically reliable operation, it is likely that the check will be conducted with an expectancy that the flight crew have conducted the actions required to prepare the aircraft, with the result being a higher threshold for detecting problems (Dismukes and Berman, 2010). There are limited procedural controls to provide assurance that cues which may indicate a problem with aircraft preparation will be attended to.

From the dispatch position, the orientation of the propeller blades in the undressed position is significantly easier to identify than the presence of the propeller strap. Flight crew procedures required pilots to rotate the propeller to the dressed position after removing the propeller strap. The orientation of propellers, therefore, had potential to be a useful supplementary indicator for identifying the placement of the propeller strap. Such an indicator, however, would have application only as a prompt to check the propeller strap had been removed. As noted in the analysis of the captain’s check of the propeller, orientation of the propeller blades is an imperfect indicator of the presence of the propeller strap.

For the dispatcher of VH-OLL, the orientation of the propeller in the undressed position was a larger and more conspicuous cue than the propeller strap against the blade. However, neither the ASO trainee nor the trainer understood that propellers were routinely rotated during different stages of aircraft preparation, including that the propeller should have been in the dressed position prior to engine start. The Regional Express procedures and training provided no information about the orientation of propeller blades during aircraft preparation.

Regional Express procedures included a stated expectation that ASOs would be provided comprehensive supervision until they were assessed as competent. The training procedures required that prior to being assessed as competent to conduct aircraft dispatch, ASOs must dispatch a minimum of 5 aircraft under supervision. The procedures also prescribed minimum requirements for supervised performance of other ground handling tasks.  

While practical training in safety sensitive tasks such as aircraft dispatch was a documented component of the Regional Express training system, the procedures and other documents did not define the roles or expectations for the personnel supervising these activities. There was no defined requirement for supervisors to observe the inspection or performance activities, and the procedures did not identify who had ultimate responsibility for assuring that safety sensitive checks or other tasks were completed, and how this assurance should be performed.

The instructional material for on-the-job training conveyed values and implications that trainees conducting practical training were at the direction of the trainer, and that the trainer must maintain overall oversight and responsibility for the training task. These values were however not explicitly codified in Regional Express procedures.

There is considerable scope for safety risks to develop when frontline personnel conduct safety sensitive tasks prior to being assessed as competent, and the primary means to mitigate these risks is with independent oversight by personnel who have been assessed as competent. It is likely that in many circumstances, the expectation that a training supervisor will maintain supervision during practical training will be intuitively understood. However, as illustrated in the dispatch of VH‑OLL, frontline personnel can be faced with competing priorities and pressures. Had the Regional Express safety system provided procedures or other risk controls which clarified this expectation of supervisors or ensure that assurance took place, it is likely the ASO supervisor would have remained present for the dispatch.  

Findings

From the evidence available, the following findings are made with respect to the aircraft preparation event involving Saab 340B, VH-OLL, at Melbourne Airport, Victoria, on 19 November 2024. 

Contributing factors

• The first officer inadvertently disconnected and removed the strap extension from the propeller strap, leaving the propeller strap on the blade. The first officer then did not rotate the propeller to the required orientation.
• The captain did not detect the propeller strap when they conducted the engine start checklist. The captain proceeded to start the left engine with the propeller strap fitted.
• The trainee airport services officer did not detect the propeller strap when dispatching the aircraft. The trainee provided a signal to the flight crew to start the left engine with the propeller strap fitted.
• The airport services officer responsible for supervising the aircraft dispatch departed the apron and did not remain with the trainee to observe the engine start. As a result, the supervisor could not assure the safety of the dispatch, and did not identify that the propeller strap had not been removed.
• The removal of the strap extension reduced the likelihood that the propeller strap would be detected because the strap extension provided an additional visual cue, and because its removal allowed for the aircraft stairs to be retracted.
• The Regional Express dispatch procedures and training did not explain the appearance, function and importance of the propeller straps or orientation of propeller blades. This provided airport services officers limited guidance on how the propellor strap was used and how to identify it had been removed. (Safety issue)
• Regional Express did not define the roles and expectations for trainers and trainees during practical ground handling training. During practical training for personnel who had not yet been assessed as competent, there was insufficient clarity for who had responsibility for assuring that safety sensitive checks and other tasks had been conducted. (Safety issue)

Safety issues and actions

Regional Express dispatch procedures and training

Safety issue number: AO-2024-059-SI-01

Safety issue description: The Regional Express dispatch procedures and training did not explain the appearance, function and importance of the propeller straps or orientation of propeller blades. This provided airport services officers limited guidance on how the propellor strap was used and how to identify it had been removed. 

Regional Express training roles and expectations

Safety issue number: AO-2024-059-SI-02

Safety issue description: Regional Express did not define the roles and expectations for trainers and trainees during practical ground handling training. During practical training for personnel who had not yet been assessed as competent, there was insufficient clarity for who had responsibility for assuring that safety sensitive checks and other tasks had been conducted.

Safety action not associated with an identified safety issue

Additional safety action by Regional Express

In addition to actions taken associated with the identified safety issues, Regional Express has advised of the following safety actions following the incident involving VH-OLL:

• Amendment to the Regional Express internal safety audit template, to include observations of the checks ASOs conduct of propeller straps
• Amendment to the Saab 340 flight crew line training program, to include checks of the final external check and removal of the propeller straps and strap extensions, and of the ‘clear left/right’ visual checks of each engine prior to engine start. 

Glossary

Sources and submissions

Sources of information

The sources of information during the investigation included:

• the flight crew of VH-OLL
• the ASO trainee and the ASO supervisor
• Regional Express
• Melbourne Airport
• Airservices Australia
• Bureau of Meteorology
• Geoscience Australia 

References

Dismukes, R., & Berman, B. (2010). Checklists and monitoring in the cockpit: Why crucial defenses sometimes fail.

Wickens, C.D., Hollands, J.G., Banbury. S., & Parasuraman R. (2013). Engineering psychology and human performance. Pearson Boston, MA

Wickens, C.D. & McCarley, J.S. (2008). Applied attention theory. CRC Press.

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 flight crew
• the ASOs
• Regional Express
• CASA
• Saab
• Swedish Accident Investigation Authority

A submission was received from Regional Express. The submission was reviewed and, where considered appropriate, the text of the report was amended accordingly.

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