Deviations from procedure and the manual entry of waypoints saw a Singapore Airlines Boeing 777-200 twice dip below safe altitude while landing at Canberra on 22 February.
The incident took place on 22 February, and involved an aircraft registered 9V-SRP, which was operating a flight from Singapore to Canberra, the Australian Transport Safety Bureau says in its final report.
Prior to descent, the crew prepared to conduct a ‘Polli Four Papa’ standard arrival route with an associated RNAV-Z approach.
As it descended, however, air traffic control instructed the flight crew to use a ‘Polli Four Bravo’ arrival, which led to a VOR approach. The crew entered the new approach into the aircraft’s flight management computer while keeping the RNAV-Z approach, which created a “discontinuity in the programmed FMC flight path.”
To resolve that, the captain put a new waypoint into the FMC, erasing the ‘SCBSG’ waypoint required for the RNAV-Z approach. The approach at that waypoint must also be commenced at or above an altitude of 7,500ft, a constraint that is usually programmed into the FMC.
“The captain manually re-entered SCBSG into the FMC without detecting that the 7,500 ft constraint was now missing. The captain then manually connected SCBSG to SCBSI for the continuation of the approach,” the ATSB says.
As it tracked to the waypoint, the aircraft descended to around 7,000ft – 500ft below the minimum altitude – at which point the crew disconnected the autopilot and climbed back to 7,500ft.
“Prior to passing SCBSI, the flight crew elected to conduct a visual approach without advising air traffic control, the flight crew then descended the aircraft below the 5,300 ft segment minimum safe altitude,” the ATSB adds.
After advising air traffic control that they had the runway in sight, the controller cleared it to continue on a visual approach. The first officer then climbed the jet back to 5,000ft and re-established the desired approach profile before landing safely without further incident.
Singapore Airlines reported that the crew became “fixated” on flying the RNAV-Z approach, and violated its standard operating procedure by failing to advise air traffic control that they were flying a visual approach. It also pointed to other lapses of its manuals and training during the approach.
The ATSB also pointed to three other descent below minimum altitude incidents involving foreign aircrew in its report, and commented that they “highlight the importance of familiarity with [VOR approaches]. However, this familiarity may be reduced for foreign flight crews operating into Australia.”
In its safety message, the Bureau says that the incident highlights the importance of preparation and communication prior to commencing a stage of flight, and also requesting a preferred clearance early so that air traffic control can provide a clearance, or there is enough time to prepare for an alternative clearance.