Aviation safety investigations & reports

Descent below segment minimum safe altitude during a non-precision instrument approach involving Airbus A320, PK-AXY, 17 km WSW Perth Airport, Western Australia on 19 February 2016

Investigation number:
AO-2016-012
Status: Completed
Investigation completed

Final Report

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What happened

On the evening of 19 February 2016, an Airbus A320 aircraft, registered PK-AXY and operated by PT Indonesia AirAsia was on a scheduled passenger service from Denpasar, Indonesia to Perth, Australia. During cruise, the captain’s flight management and guidance computer (FMGC1) failed. Due to the failure, the flight crew elected to use the first officer’s duplicate systems. For the aircraft’s arrival in Perth there was moderate to severe turbulence forecast below 3,000 ft with reports of windshear. The crew commenced an instrument landing system (ILS) approach to runway 21.

During the approach, the flight crew made a number of flight mode changes and autopilot selections, normal for an ILS approach with all aircraft operating systems available. However, some of those flight modes and autopilot selections relied on data from the failed FMGC1 and the autothrust system commanded increased engine thrust. The crew did not expect this engine response and elected to conduct a go-around. With an increasing crosswind on runway 21, the crew accepted a change of runway, to conduct a non-precision instrument approach to runway 06.

With the time available, the first officer programmed the new approach into his FMGC and conducted the approach briefing. During this period, the captain hand flew the aircraft and manually controlled the thrust. During the approach to runway 06, the crew descended the aircraft earlier than normal, but believed that they were on the correct flight path profile.

While descending, both flight crew became concerned that they could not visually identify the runway, and focused their attention outside the aircraft. At about that time, the approach controller received a “below minimum safe altitude” warning for the aircraft. The controller alerted the crew of their low altitude and instructed them to conduct a go-around. The crew then conducted another approach to runway 06 and landed.

What the ATSB found

The ATSB identified that the flight crew were unsuccessful in resolving the failure of the FMGC and had a limited understanding of how the failure affected the aircraft’s automation during the ILS approach. This resulted in the unexpected increase in engine thrust, which prompted a go-around.

The flight crew had a significant increase in workload due to the unresolved system failures, the conduct of a go-around and subsequent runway change. This, combined with the crew’s unfamiliarity and preparation for the runway 06 instrument approach, meant they did not effectively manage the descent during that approach.

The flight crew’s focus of attention outside the aircraft distracted them during a critical stage of flight. The crew did not detect that they had descended the aircraft below the specified segment minimum safe altitude.

The flight crew commenced their descent for the second runway 06 instrument approach later than normal, initially necessitating an increased rate of descent and at 300 ft the engine thrust reduced briefly to idle.

Safety message

Handling of approach to land is one of the ATSB’s SafetyWatch priorities. Unexpected events during the approach and landing can substantially increase what is often a high workload period. Adherence to standard operating procedures and correctly monitoring the aircraft and approach parameters provides assurance that the instrument approach can be safely completed. A go-around should be immediately carried out if the approach becomes unstable or the landing runway cannot be identified from the minimum descent altitude or missed approach point.

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The occurrence

Context

Safety analysis

Findings

Safety issues and actions

Sources and submissions

Appendices

General details
Date: 19 February 2016   Investigation status: Completed  
Time: 22:01 WST    
Location   (show map): near Perth Airport   Investigation type: Occurrence Investigation  
State: Western Australia   Occurrence type: Flight below minimum altitude  
Release date: 16 January 2018   Occurrence class: Operational  
Report status: Final   Occurrence category: Serious Incident  
  Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer Airbus  
Aircraft model A320  
Aircraft registration PK-AXY  
Serial number 5359  
Operator PT. Indonesia AirAsia  
Type of operation Air Transport High Capacity  
Sector Jet  
Damage to aircraft Nil  
Departure point Denpasar, Indonesia  
Destination Perth, WA  
Last update 14 November 2018