What happened
During an instrument landing system (ILS) approach, the pilot flying descended the aircraft well below the standard 3° glideslope. This descent resulted in the aircraft deviating more than half scale deflection from the glideslope, violating AIP requirements. The deviation was severe enough to trigger a minimum safe altitude warning (MSAW) from air traffic control, prompting controllers to notify the crew of their low altitude. Despite this warning, the aircraft continued to fly more than half scale deflection below the glideslope for a distance of 5 NM.
As the aircraft transitioned into visual conditions, the crew switched to a visual approach without obtaining the necessary clearance from ATC. During this period, the pilot monitoring ceased monitoring the glideslope, which meant the deviation was neither detected nor challenged by the second crew member.
The investigation
The investigation focused on the crew's lack of a shared mental model and inconsistencies in the operator's standard operating procedures (SOPs). It was found that while the pilot flying believed they were following the ILS approach, the pilot monitoring believed they were flying a visual approach. Furthermore, the crew failed to conduct the required briefing for the transition to a visual approach.
The ATSB identified that the operator's SOPs regarding approach procedures in visual conditions were inconsistent. Specifically, the procedures allowed for the discontinuation of an instrument approach upon becoming visual but lacked a requirement to brief such a change. This practice also contradicted CASR Part 121 guidance, which advises against discontinuing instrument approaches to continue visually to prevent unstable approaches. Additionally, the investigation noted that the requirement for glideslope deviation calls was removed during visual approaches, which reduced the pilot monitoring's ability to manage approach stability.
Findings
- The pilot flying descended significantly below the 3° glideslope despite being cleared for an ILS approach.
- The pilot monitoring did not monitor or challenge the glideslope deviation.
- The crew lacked a shared mental model regarding whether they were performing an instrument or visual approach.
- The operator's SOPs contained inconsistencies regarding the transition from instrument to visual approaches.
- The pilot monitoring's effectiveness was reduced because the requirement to make glideslope deviation calls was absent during visual procedures.