| Section 21 (2) of the Transport Safety Investigation Act 2003 (TSI Act) empowers the ATSB to discontinue an investigation into a transport safety matter at any time. Section 21 (3) of the TSI Act requires the ATSB to publish a statement setting out the reasons for discontinuing an investigation. The statement is published as a report in accordance with section 25 of the TSI Act, capturing information from the investigation up to the time of discontinuance. |
Overview of the investigation
The occurrence[1]
Aeromedical tasking and flight preparation
In June 2020, the ATSB commenced a transport safety investigation into a main rotor blade strike involving a Leonardo Helicopters AW139, registered VH-EGK and operated by Queensland Government Air, about 16 km west‑south‑west of Caboolture airfield, Queensland.
On 20 June 2020, at about 1709 local time, the aircrew (comprising the pilot, aircrew officer (ACO) and rescue crew officer (RCO)) and medical crew (flight doctor and paramedic) were tasked with retrieving a patient from a property near Caboolture and transporting them to the Royal Brisbane Hospital. In preparation for the flight, the pilot completed an operational pre‑flight risk assessment, which indicated ‘normal operations’, permitting acceptance of the task without further discussions with the operator’s duty management pilots. The remaining crew refuelled the helicopter and configured the cabin for a night winch rescue.
Transit to the site
At about 1746, the helicopter departed Archerfield Airport. The flight was operated under the instrument flight rules,[2] with the aircrew using night vision goggles.[3]
On approach to the rescue site, the ACO contacted the ground paramedic to confirm the patient’s status and ascertain whether any areas nearby could be used for a landing, to avoid the requirement to winch. The ground paramedic advised that a suitable landing site was not available, but there was a possible suitable winching position about 20 m behind the ambulance. The ACO requested the ground paramedic keep the ambulance lights and roof top emergency beacon on, to assist in identification of the winch location.
Arrival on site and insertion winches
The helicopter arrived in the vicinity of the site at about 1800 in dark conditions, approximately 30 minutes after last light.[4] The pilot and ACO identified the site, completed the pre-landing checklist, and put the ‘Trakkabeam’[5] on. The aircrew discussed the site characteristics and planned the approach.
Recorded flight data showed that several orbits of the rescue site were conducted. During the final orbit, the pilot briefed the crew on the site. At about 1813, the pilot commenced a descent to the site and completed power checks and briefed the escape route. At about 1818, the pilot manoeuvred the helicopter to a low reference datum[6] of about 200 ft above ground level and 50 m north of the insertion winch location. From this position, the aircrew conducted the reconnaissance brief. They checked the power setting, radio altitude reading, security of harnesses, and if they had ‘good references’. The pilot then handed the conning[7] over to the ACO.
Between about 1821 and 1826, the crew completed 2 insertion winches to lower the RCO, flight paramedic and stretcher, from a height between 65–100 ft. The helicopter then departed the site to the east and orbited for about 20 minutes while the patient was prepared for extraction. During this time, the pilot and ACO commented on the rain passing through the area and prepared for the extraction winches.
The extraction winch and collision with terrain
The patient was moved from the side of the hill to a position in an open area considered suitable for winching. At about 1841, the RCO radioed the ACO to report that they were ready for the winch. The pilot verbalised to the ACO ‘same brief as before’, which was acknowledged by the ACO. The pilot and ACO both reported there was no re-identification, verbalisation or re-briefing of the critical hazards (outlying trees) associated with the revised extraction winch position. They then completed their final checks. At about 1846, the helicopter was established in a hover about 80 ft directly above the patient. At interview, the pilot approximated the helicopter’s main rotor was rotating about 1 m from the surrounding tree line.
The flight paramedic and the patient on a stretcher were winched into the helicopter. As the winch was lowered to collect the RCO, the ACO directed the pilot to move forward and right, twice. As the RCO was about to be winched, they verbalised for the ACO to hold them on the ground as they had momentarily lost their night vision goggles mount. At about 1851, the ACO confirmed to the pilot that they were holding a good position. The recorded data showed that, over the last minute, the helicopter’s height ranged between about 50–65 ft above the ground (861–876 ft above mean sea level) and the heading changed between about 111–120°.
Shortly after, the doctor called out ‘trees, trees’, at which point the helicopter’s main rotor blades contacted a tree. Immediately following the main rotor strike, the ground paramedic reported seeing a branch fall through the trees, with an estimated diameter of 10 cm and length of about 1–1.5 m.
Recovery and return to Archerfield
In response, the pilot immediately manoeuvred the helicopter away from the tree line. The RCO was subsequently raised off the ground, contacted the trees and went into a spin. Shortly after, the RCO was safely secured inside the aircraft. The pilot reported that feedback through the flight controls remained normal, with no abnormal vibrations. However, as a precaution, the pilot elected to return to the helicopter’s base at Archerfield Airport.
Helicopter damage assessment
An engineering inspection of the main rotor blades identified damage to one of the 5 tip caps, which was missing about 50 g of honeycomb and skin material (Figure 1). Two other main rotor blades had minor abrasions to the paint layer. No other significant damage was observed.
The helicopter manufacturer considered a loss of 40–50 g on a main rotor blade tip was generally not enough to generate a perceivable vibration in the aircrew or passenger compartments.
Figure 1: Damage to the main rotor blade tip
Source: ATSB
Environmental information
Meteorological conditions
The Bureau of Meteorology graphical area forecast applicable for the flight and occurrence location indicated the conditions were:
- visibility was greater than 10 km, with broken[8] stratus cloud between 2,000–3,000 ft above mean sea level, and from 1900 local time, broken cumulus and stratocumulus cloud between 3,000–6,000 ft
- visibility reducing to 3,000 m in isolated showers of rain, with broken stratus cloud between 1,000–3,000 ft and broken cumulus cloud between 3,000–8,000ft.
Significant rainfall had been recorded in the vicinity of the site over the preceding days. Immediately prior to the occurrence, the Bureau of Meteorology weather radar indicated light rain in proximity. The pilot reported there was more weather to the west of their site and that it was clear to the east. The cloud base had reduced as they were operating, and they had passing showers, with some rain on the windscreen. Recorded audio from the helicopter captured the aircrew discussing rain while orbiting to the east before returning for the extraction winch. Mobile phone footage taken from the ground showed moderate rain during the extraction sequence.
Lighting conditions
It was reported that there was limited ambient lighting generated from surrounding residential and farming properties. The astronomical conditions indicated very low celestial illumination in the vicinity of the site, with 1.7% moon illumination. Despite this, both the pilot and ACO reported they had good visibility, and they did not notice any scintillation[9] of their night vision goggles due to the low light.
Investigation activities
During the investigation, the ATSB:
- interviewed the aircrew, medical crew and relevant safety personnel
- examined data from the helicopter’s multi-purpose flight recorder and mobile phone footage of the winching operation
- reviewed the environmental conditions
- reviewed the operator’s standard operating procedures
- consulted the helicopter manufacturer
- considered similar occurrences.
ATSB observations
Helicopter drift and visual cueing environment
Unbeknown to the pilot, the helicopter drifted during the night extraction of the RCO, resulting in the main rotor blades impacting a tree. Further, the aircrew were potentially operating in a degraded visual cueing environment due to the meteorological and lighting conditions at the time.
Pre-flight operational risk assessment
The operator’s pre-flight operational risk assessment tool was not accurately completed, resulting in a low-risk score that did not reflect the accurate flight conditions. Further, the tool did not consider the relative humidity when evaluating the environmental conditions, nor did it include a risk level that equated to a mandatory no-go decision.
Obstacle clearance limits
During the extraction, the helicopter was hovered closer to the surrounding tree line. It was noted that there were no minimum obstacle clearance limits prescribed by the operator, particularly for when operating in confined areas.
In-flight risk assessment
The aircrew did not fully complete the reconnaissance brief for the insertion winch sequence and therefore had not identified the closest obstacles or external hover references.
Actions following a suspected or actual obstacle collision
As no apparent issues were identified with the helicopter following the main rotor blade strike, it was returned to the operator's base. This involved flying over a populated area, although the damage severity was unknown. The operator did not have a published procedure regarding the actions required by aircrew following a suspected or actual collision with an obstacle such as conducting a precautionary out-landing.
Operator safety action
In response to this occurrence, the operator has implemented a range of safety actions, such as:
- Introduced additional requirements to strengthen the operational risk assessment process. Such as, the inclusion of relative humidity (temperature dewpoint spread) in the pre-flight risk assessment operational tool, the requirement to consult with a management pilot when the risk was assessed as ‘orange – extreme caution’ or above, increased coordination with the tasking agency regarding the acceptance of the rescue task, and the recording and auditing of risk assessment scores. Similarly, an instruction was issued to aircrew emphasising that the risks associated with winching operations should be assessed at tasking, dispatch, arrival on site, and throughout the operation.
- Released a standards directive and updated the aircrew operations manual with additional obstacle clearance requirements. This included mandatory minimum distances from obstacles when operating in, or within the vicinity of a confined area, or manoeuvring near obstacles.
- Released a standards directive and updated the aircrew operations manual requiring the pilot in command to land as soon as possible when either ‘suspecting or observing’ helicopter damage.
Safety message
The flight planning and task preparation stage provides an important opportunity to assess the operational environment prior to task acceptance and departure. Risk management strategies such as a pre-flight risk assessment tool can help pilots apply a systematic process to decision‑making and assist with mitigating other hazards that could adversely affect the safety of flight.
However, as a pre-flight risk assessment tool cannot anticipate all hazards and corresponding risks that may emerge during the flight, aircrew should continually reassess the operational risk to determine if the applied risk controls are appropriate or require adjustment to ensure flight safety is maintained. In-flight briefings are an effective mechanism for achieving this. Likewise, such briefings aid an aircrew’s shared understanding of how the tasking is to be completed. This understanding is then reinforced through positive communications between the aircrew throughout the tasking.
Finally, this occurrence illustrates the importance of an appropriate response if a helicopter is suspected of colliding with terrain, regardless of the presumed severity. Unknown damage to a helicopter’s main or tail rotor system could quickly develop into a much more serious situation, particularly when operating over a densely populated area.
Reasons for the discontinuation
Based on a review of the available evidence, the ATSB considered it was unlikely that additional investigation would identify further important safety lessons. Queensland Government Air has been briefed about the ATSB’s observations and potential learnings. While the broader communication of this information is useful to other similar operators, in this case, appropriate safety action has been taken by Queensland Government Air.
The evidence collected during this investigation remains available to be used in future investigations or safety studies. The ATSB will also monitor for any similar occurrences that may indicate a need to undertake a further safety investigation. Consequently, the ATSB has discontinued this investigation.
[1] The ATSB’s preliminary report into the occurrence was published on 12 August 2020 based on an initial review of the evidence available at that time. Discrepancies with some of the details contained within that report have since been identified. The preliminary report remains on file but has been removed from the website. The discontinuation notice addresses the relevant factual information for the occurrence, verified up to the time of discontinuation.
[2] Instrument flight rules (IFR): a set of regulations that permit the pilot to operate an aircraft in instrument meteorological conditions (IMC), which have much lower weather minimums than visual flight rules (VFR). Procedures and training are significantly more complex as a pilot must demonstrate competency in IMC conditions while controlling the aircraft solely by reference to instruments. IFR-capable aircraft have greater equipment and maintenance requirements.
[3] Night vision goggles: a self-contained binocular night vision enhancement device that is helmet-mounted or otherwise worn by a person; and can detect and amplify light in both the visual and near infra-red bands of the electromagnetic spectrum.
[4] Last light: the time when the centre of the sun is at an angle of 6° below the horizon following sunset. At this time, large objects are not definable but may be seen and the brightest stars are visible under clear atmospheric conditions. Last light can also be referred to as the end of evening civil twilight.
[5] Trakkabeam: high intensity searchlight, used for airborne, ground, and maritime law enforcement, security, search and rescue and other agencies.
[6] Reference datum: a position above and adjacent to a confined area (winch position) in which the aircrew conduct a detailed reconnaissance and briefing of the intended operating area to identify the position of critical obstacles, hover references and engine failure escape routes etc. The operator specified the position of the ‘low’ datum to a ‘standard distance’ of about 3 main rotor diameters (42 m or 140 ft) from the confined area at about 200 ft above ground level.
[7] Conning or cueing (‘guidance patter’): process by which the pilot is verbally guided by the winch operator (the ACO) during precision hovering operations.
[8] Cloud cover: in aviation, cloud cover is reported using words that denote the extent of the cover – ‘broken’ indicates that more than half to almost all the sky is covered.
[9] Scintillation: A faint, random, sparkling effect throughout the image area creating a grainy image. It is a normal characteristic of night vision goggles and is more pronounced under low-light-level conditions.