What happened
Two separate incidents involving Boeing 737-800 aircraft occurred in Queensland, Australia, within two days of each other. On the night of 24 October 2022, a Virgin Australia Airlines flight, VH-VUT, was operating a night transport service from Brisbane to Cairns. On the morning of 26 October 20 and 2022, a Qantas Airways flight, VH-VZA, operated the same route during daylight hours.
During both operations, the flight crews failed to identify that an approach transition needed to be selected when entering data into the flight management computer (FMC). Because the transition was not selected, a gap was created in the programmed flight path between the HENDO waypoint and waypoint CS540. To resolve this discontinuity, the crews manually linked the waypoints. This action inadvertently removed critical altitude constraints, specifically a 6,800 ft descent restriction. Consequently, both aircraft descended below the minimum safe altitude and the minimum sector altitude, deviating from air traffic control clearances.
The investigation
The investigation examined the interface of the FMC and the complexity of the HENDO 8Y arrival procedure. It was found that the arrival and approach transitions were displayed on the same FMC page, which led the crews to overlook the requirement for an approach transition selection. For VH-VUT, the error was exacerbated by the pilot misidentifying waypoint CS540 as the initial approach fix. For VH-VZA, the crew expected a specific transition to be named in their clearance, which was not required for this specific airway clearance.
Furthermore, the Jeppesen approach charts used by both crews lacked certain waypoints (HENDO, CS522, and CS523) on their vertical profile depictions. This omission made it difficult for the pilots to notice that altitude protections were missing from their FMC programming. While the crews' briefing procedures required them to verify all operational constraints in the FMC, the errors were not detected during their pre-approach briefings.
Findings
- The primary cause of the altitude deviations was the failure to select the required approach transition in the FMC.
- Manual attempts to fix the resulting flight path discontinuity resulted in the removal of essential altitude constraints.
- The Jeppesen approach chart design, which omitted specific waypoints from the vertical profile, hindered the crews' ability to identify the error.
- Air traffic controllers intervened promptly in both instances to mitigate the risks posed by the descents.