What happened
While approaching Darwin Airport for an ILS-Z runway 29 approach, the aircraft's auto-flight system experienced a sudden mode reversion. As the aircraft passed the initial approach fix, the localiser mode engaged briefly before the system reverted to lateral roll and vertical flight path angle modes. This change was likely caused by a synchronization issue between the navigation display and the flight control system, or by the pilot flying inadvertently pressing the approach pushbutton during the capture.
Following this reversion, the aircraft maintained a 20-degree right bank and a level flight path. The pilot flying did not re-engage the approach mode or disconnect the autopilot until approximately 200 feet above the aerodrome elevation. During this period, the aircraft deviated from the localiser course and descended below the minimum safe altitude while operating in instrument meteorological conditions. The crew attempted to re-establish the approach, but the process involved significant variations in vertical speed and altitude changes.
As the aircraft descended, the pilot monitoring was focused on correcting the deviations and did not call out the course errors. The approach became increasingly unstable, with the aircraft passing 1,500 feet well above the glideslope. Despite failing to meet several stabilization criteria at the 1,000-foot threshold, the crew elected to continue the approach, incorrectly assessing that they could stabilize the aircraft once in visual conditions.
The investigation
Investigators examined the automation behavior and the crew's response to the mode reversion. Analysis of the recorded data by Embraer suggested that the ILS navigation display source had not engaged at the moment the localiser became active, which likely triggered the system to invalidate the capture. However, Honeywell noted that limitations in data update frequency prevented a definitive conclusion regarding a synchronization error.
The investigation also looked into the crew's workload and decision-making. The pilot monitoring reported a high workload, which contributed to a failure to read back ATC clearances and a delay in identifying course deviations. Additionally, the investigation found that a late flap selection by the pilot monitoring reduced the time available to stabilize the aircraft's speed for landing.
Findings
- The auto-flight system reverted to basic modes due to either a system synchronization error or an inadvertent pilot input.
- The pilot flying failed to re-engage approach mode or disconnect the autopilot following the mode reversion, leading to lateral and vertical deviations.
- The aircraft descended below the minimum safe altitude while in instrument meteorological conditions.
- The crew did not initiate a missed approach despite the aircraft being unstable at the 1,000-foot stabilization height.
- The pilot monitoring's late flap selection hindered the ability to stabilize the approach speed.
- Alliance Airlines' standard operating procedures lacked clarity regarding the criteria for continuing an unstable approach once visual conditions are reached.