A preliminary report from the ATSB's on-going investigation into a flight below minimum altitude incident involving a Boeing 737 south of Canberra notes the operator involved has taken a number of proactive safety actions.
On the evening of 13 June 2024, the Boeing 737-800 had departed Denpasar, Indonesia, operating Batik Air's inaugural flight to Canberra, the report details. Forecast tailwinds resulted in an estimated arrival time just prior to 0600 on 14 June, which was earlier than planned, and before Canberra Tower and Canberra Approach air traffic control commenced services for the day.
The crew elected to proceed, without delaying to wait for those air traffic services to become available, which meant arriving in Canberra using the Canberra Airport CTAF (common traffic advisory frequency - where pilots use radio calls to announce their positions and arrange separation from other aircraft).
As the aircraft descended in darkness towards Canberra, the crew prepared to conduct the AVBEG 5A standard arrival route (or STAR - which uses satellite-based positioning waypoints to transition aircraft from en route flight to, in this case, an initial approach fix waypoint for Canberra Airport's instrument landing system approach).
"While the crew intended to fly the STAR, they did not request this from the air traffic controller managing the airspace," ATSB Chief Commissioner Angus Mitchell said.
Instead, the controller expected the crew to track along the clearance previously provided direct to Canberra Airport from the AVBEG waypoint, which is to the north-west of the airport and is also the first waypoint of the AVBEG 5A STAR.
The crew proceeded with their planned standard arrival route, which meant the aircraft deviated from the cleared track direct to Canberra, and instead tracked to a series of waypoints to the south-west and south of the airport.
The controller, unaware of the flight crew's intentions, did not query the deviation, but did instruct them to maintain 10,000 ft to remain clear of a restricted area around the Deep Space Communications Complex at Tidbinbilla to the west of Canberra (this separation is built into the STAR).
"After receiving this instruction, the flight crew became uncertain as to whether the aircraft would be operating within, or outside of, controlled airspace during the standard arrival route and approach," Mr Mitchell explained.
After levelling the aircraft at 10,000 ft, the crew subsequently requested ATC clearance to conduct the instrument landing system (ILS) approach to Canberra. The controller advised them that the Canberra tower was closed and that CTAF procedures applied for the airspace.
A short time later, when tracking towards the airport from the south and having descended outside of controlled airspace, the flight crew identified they were above the desired flightpath, and the captain decided to conduct a holding pattern at the approach waypoint of MOMBI to reduce altitude.
"During this holding pattern, the aircraft levelled out at 4,700 ft, but this meant it descended below minimum holding altitude of 5,600 ft, and at one point the aircraft passed 924 ft above terrain," Mr Mitchell said.
After rejoining the approach, the aircraft commenced descending again, following the runway 35 glidepath, and it landed without further incident.
The report notes that while the aircraft was in the holding pattern, a controller in the Canberra Tower was preparing to commence the tower service for the day and they observed the aircraft below the minimum holding attitude.
"The Tower controller made multiple attempts to contact the crew on the Canberra CTAF, but did not receive a response," Mr Mitchell explained.
At about the same time, the Canberra Approach controller commenced for the day - taking over the frequency the flight crew were listening to - and issued a safety alert that the aircraft was operating below the minimum safe altitude.
The flight crew responded that they were visual with the runway and continued their approach.
The ATSB's continuing investigation will consider, among other elements, Batik Air's procedures, training and route implementation processes, as well as air traffic control procedures and training.
"A final report with analysis and findings will be released at the conclusion of the investigation, but we note the operator has already pro-actively taken safety actions," Mr Mitchell concluded.
"These include revising their Canberra Airport briefing documentation, issuing flight crew notices highlighting procedures for operating in non-controlled airspace, and rescheduling flights to Canberra to ensure they arrive during air traffic control operating hours."
You can find here the preliminary report: Flight below minimum altitude involving Boeing 737, PK-LDK, 19 km south of Canberra Airport, Australian Capital Territory, on 14 June 2024
