A Singapore Airlines plane approaching Canberra in February twice breached minimum altitude requirements and at one point was 700ft below the lowest height at which it was safely allowed to fly.
High terrain around Canberra meant the Boeing 777-200 with 13 flight crew and 235 passengers was supposed to fly no lower than 5300ft on a sector between two waypoints known as SCBSG and SCBSI but the plane descended to 4600ft.
This was after it had previously breached a minimum altitude of 7500ft on another sector, according to a report released Thursday by the Australian Transport Safety Bureau.
The incident on February 22 this year came after air traffic control surprised the crew by instructing them to conduct a different arrival to the one they expected.
The aircraft was operating Singapore’s Capital Express service to the Australian Capital and its New Zealand counterpart, Wellington, in good visual conditions.
The new approach used ground-based VHF transmitters but the crew wanted to continue using GPS waypoints and needed to reprogram the flight management computer to accommodate this.
When they manually entered the SCBSI waypoint, they erased SCBSG and removed a 7500ft minimum sector altitude constraint. The captain manually re-entered the missing waypoint but did not notice the altitude constraint was missing.
This saw the aircraft descend to about 7000ft as it headed towards SCBSG without the crew noticing. A warning from air traffic control prompted them to disconnect the autopilot and climb back to 7500ft.
Prior to passing SCBSI, the crew elected to conduct a visual approach but did not tell air traffic control, as required by standard operating procedures.
The first officer was manually flying the aircraft as it descended below the 5300ft minimum safe altitude.
The captain noticed the approach profile was low at about same time a controller warned the plane was below the minimum safe altitude and the first officer levelled off at about 4600ft.
“The flight crew advised the controller that they had the runway and terrain in sight,’’ the report said.
“The controller then cleared the flight to conduct a visual approach. After being cleared for a visual approach, the first officer commenced a climb to about 5,000 ft and re-established the aircraft on the desired approach profile.”
The ATSB noted there had been a number of incidents in Australia involving foreign flight crew diverting from flight paths.
These included incidents involving Thai Airways, AirAsia X and Garuda Indonesia.
It said the Singapore incident highlighted the importance of preparation and communication before starting a phase of flight.
“Requesting a preferred clearance early allows ATC to ensure that a clearance can be provided, or if not available, allows the flight crew time to prepare for a different clearance,’’ it said.
The incident also underlined the importance of adhering to standard operating procedures, something from which the Singapore crew deviated when the manually entered the waypoint, investigators said.
The ATSB had identified numerous accidents worldwide that were the result of human errors in data calculation or entry with the consequences ranging from rejected take-offs to crashes.