An AW139 rescue helicopter came within an estimated 20 ft of terrain as a result of an uncommanded descent and lateral drift during a 24 July 2020 night-time mission to retrieve bushwalkers from a national park.
The Toll Helicopters-operated AW139 had departed Shellharbour Airport, near Wollongong, NSW, with four crew onboard, comprising a pilot, aircrew officer, paramedic and doctor, tasked to recover two lost bushwalkers from the Bungonia National Park, east of Goulburn.
The flight was conducted under the night visual flight rules with the pilot, aircrew officer and paramedic using night vision goggles.
“On arrival at the search and rescue location, the helicopter tracked over high ground past the edge of an escarpment, where the terrain drops away to a valley floor,” ATSB Director Transport Safety Stuart Macleod explained.
“During the subsequent visual search phase an uncommanded, and increasing, rate of descent and lateral drift developed.”
The aircrew officer, who was in the main cabin by the right door, identified the increasing rate of descent and lateral drift towards a ridgeline and provided corrective instructions to the pilot.
It was estimated that the helicopter came within 20 ft of the ridgeline before the descent and drift were arrested.
Mr Macleod said the ATSB investigation found the pilot’s likely fixation on locating the bushwalkers resulted in them not maintaining an effective scan on the cockpit instruments and outside visual references.
“This resulted in the loss of hover reference and development of an unintended descent and lateral drift.”
In response to the loss of reference the pilot unsuccessfully attempted to engage the helicopter’s auto hover rather than commence an overshoot.
“A subsequent focus on selecting the automated mode further delayed the resumption of the scan and recognition of the increasing descent rate.”
Mr Macleod said radio communications between the paramedic and the ground party hindered communications between the pilot and aircrew officer.
“This inhibited the aircrew officer's ability to verify with the pilot whether the observed initial movement was intentional, preventing recovery from the initial drift and descent.”
In addition, the pilot subsequently did not announce losing hover reference, delaying the aircrew officer’s awareness of the developing situation and support to the pilot.
The ATSB also found the helicopter’s external white lighting (two pilot-steerable landing lights, and a steerable winch and handheld light operated by the aircrew officer) was inadequate.
“At the operating height the external white lighting was inadequate to illuminate the terrain below the helicopter, resulting in the pilot not identifying the developing rate of descent while searching for the bushwalkers.”
As a result of the incident, Toll Helicopters made several changes to their procedures and equipment aiming to prevent a reoccurrence.
These include fitting its AW139s with with a dedicated high-powered search light, amending sterile cockpit procedures, and changes to the operations manual.
The operator has also introduced additional human factors training with a focus on spatial disorientation, confirmation bias and communication techniques for all flight and medical crew.
“Operations at night in low light conditions can be challenging to even the most experienced crews,” Mr Macleod said.
“Low light conditions reduce available visual cues for maintaining aircraft position and undesired aircraft states can develop rapidly.
“To mitigate these risks, crews conducting night operations in such conditions should maintain adequate references, taking into account equipment limitations such as external lighting, and maintain an effective scan to ensure continual awareness of the position and movement of the helicopter.”Last update 08 April 2022