News: Ensuring a stablised approach

The ATSB has published “too low on approach” and “inflight decision making” as two of its safety watch priorities.

ATSB’s SafetyWatch highlights the safety concerns that arise out of investigations such as the one into a landing incident in Perth in February 2016.

The ATSB urges operators and flight crews to give heightened attention to these risk areas; in this case, understanding your aircraft systems, and adhering to cockpit monitoring and communication procedures to ensure a stabilised approach during the approach and landing phases of flight. Unexpected events can substantially increase an already-high cockpit workload. If the criteria for safe continuation of an approach are not met, flight crew should initiate a go-around.

Airbus A320 registered PK-AXY and operated by PT Indonesia AirAsia

This serious incident involving an Airbus A320 on approach to Perth Airport has illustrated ATSB’s ongoing safety concerns in relation to pilots not effectively managing their aircraft’s flight path when unexpected events arise during the approach to land.

This incident occurred on 19 February 2016, when a scheduled passenger service was arriving from Denpasar, Indonesia. During the cruise, the captain’s flight management and guidance computer (FMGC1) had failed, and the flight crew had elected to use the first officer’s duplicate systems.

The flight crew were conducting an instrument landing system (ILS) approach to Perth Airport. They made a number of flight mode changes and autopilot selections - normal for an ILS approach with all aircraft operating systems available, but some of which relied on data from the failed FMGC1.  As a result, the autothrust system commanded increased engine thrust and the crew, who had not expected this response, elected to conduct a go-around. An increased crosswind then prompted air traffic control to effect a change of runway to a runway without a precision instrument approach procedure.

The approach and landing phases of flight are amongst the highest of workload for flight crews…

The unresolved system failures, the conduct of the go-around, and the subsequent runway change all resulted in a significant increase in cockpit workload. This, combined with the crew’s unfamiliarity and preparation for the non-precision instrument approach to the new runway, hampered their management of the next descent.

During the approach to the new runway, the crew descended the aircraft earlier than prescribed, but believed that they were on the correct flight path profile. They became concerned that they could not visually identify the runway, and focused their attention outside the aircraft. This distraction meant that the crew were not effectively monitoring the descent and the captain descended the aircraft below the segment minimum safe altitude.

As the aircraft continued to descend, the air traffic controller received a “below minimum safe altitude” warning for the aircraft. The air traffic controller alerted the crew to their low altitude and instructed them to conduct a go-around. The crew then conducted another approach and landed without further incident.

ATSB Chief Commissioner Greg Hood said that the approach to land is one of the most critical phases of flight, and stressed the importance of flight crews understanding their aircraft systems and adhering to cockpit control, monitoring and communication procedures to ensure a stabilised approach during the approach and landing phases of flight.

“The approach and landing phases of flight are amongst the highest of workload for flight crews, and domestically and internationally where we see the highest accident rate” Mr Hood said. “It’s a complex operation at the best of times, but when something unexpected occurs such as a failure of an aircraft system in-flight, it can add substantially to flight crews’ workload. It is critical that flight crew fully understand their aircraft systems and how they will respond in a degraded mode, and adhere to cockpit protocols and procedures to ensure a stabilised approach resulting in a safe landing. In this case, there was considerable added complexity for the flight crew as a result of adverse weather, and an air traffic control change to a runway without a precision approach.”

“The ATSB urges all flight crew to ensure that they understand their aircraft systems, and how the aircraft will respond in a degraded mode, and to adhere to cockpit protocols and procedures to ensure a safe approach and landing. If there’s any doubt or confusion, or if the stable approach criteria is not being met, communicate it, and never hesitate to conduct a go-around.”

Read the final report, AO-2016-012: Descent below segment minimum safe altitude during a non-precision instrument approach involving Airbus A320, PK-AXY, 17 lm WSW Perth Airport, Western Australia on 19 February 2016.

Last update 16 January 2018