ANZ B773 at Brisbane on Nov 18th 2017, descended below minimum safe altitude on approach

Last Update: June 26, 2018 / 13:31:12 GMT/Zulu time

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Incident Facts

Date of incident
Nov 18, 2017

Classification
Incident

Flight number
NZ-135

Aircraft Registration
ZK-OKN

Aircraft Type
Boeing 777-300

ICAO Type Designator
B773

Airport ICAO Code
YBBN

An ANZ Air New Zealand Boeing 777-300, registration ZK-OKN performing flight NZ-135 from Auckland (New Zealand) to Brisbane,QL (Australia), was on the SAVER 1A Standard Arrival route to Brisbane's runway 01 and was cleared to descend to 3000 feet MSL when ATC observed the aircraft descend through 2500 feet MSL and alerted the crew, the aircraft however continued to descend to about 2000 feet MSL. The aircraft continued for a landing without further incident.

Australia's ATSB rated the occurrence an incident and opened an investigation.

On Jun 26th 2018 the ATSB released their final report concluding the probable cause of the incident was:

As a result of high workload, a loss of awareness of the aircraft’s descent profile, and apreconception with ensuring that the aircraft did not get high on the approach flight path, theflight crew initiated a descent below the cleared altitude of 3,000 ft as the aircraft waspositioning for final approach. The flight crew corrected the error and levelled the aircraft atabout 2,000 ft shortly before being alerted of the altitude breach by ATC.

The ATSB reported the captain was pilot flying, the first officer pilot monitoring. The ATSB described the sequence of events leading to the descent below cleared altitude:

Shortly after passing VETIS, at 0922:45, and while maintaining 4,000 ft, ATC cleared the aircraft to descend to 3,000 ft and for the ILS runway 01. The PF selected 3,000 ft in the MCP altitude window (see Figure 3 at 0922:52) and pressed the altitude selector7 to initiate further descent. The PF later observed that the altitude selector was probably not properly pressed. As a result, the expected flight mode change did not occur and the aircraft did not commence the descent as expected. The QAR data recorded the autoflight pitch remaining in the VNAV mode, however, the flight crew later reported that the VNAV mode changed from VNAV PTH to VNAV ALT. The PF reported that this pitch mode change was unexpected and unfamiliar. The flight crew operations manual stated the following with respect to the pitch mode entering VNAV ALT:

When a conflict occurs between the VNAV profile and the MCP altitude, the airplane levels and the pitch flight mode annunciation becomes VNAV ALT. The airplane maintains altitude. To continue the climb or descent, change the MCP altitude and push the altitude selector or change the pitch mode.

In response, and with the intent of ensuring that the aircraft did not get high on the desired descent profile, the PF selected the V/S8 mode, and then the FLCH9 mode, to initiate the descent. The aircraft commenced descending and at 0923:33 the autoflight system commenced reducing the rate of descent to capture the cleared altitude of 3,000 ft—indicated by the ALT10 mode activating. Shortly after, and as the aircraft passed through LOGAN, the autoflight system transitioned back into the VNAV mode and then the ALT mode. As the aircraft was turning towards GLENN, and maintaining the cleared altitude of 3,000 ft, the PF selected 1,000 ft in the MCP and then the FLCH mode with the intent of continuing the descent further. About 5 seconds later, at 0924:03, the aircraft commenced descent from 3,000 ft. At 0924:14, as the aircraft had passed 2,850 ft, ATC instructed the aircraft to maintain best speed, and at least 180 kt until 5 NM final. The PM acknowledged the instruction. At 0924:35 the PF raised the MCP target altitude to 2,000 ft and changed the flight mode to V/S. At 0924:50, ATC alerted the aircraft that it was cleared to 3,000 ft and that it was descending through 2,200 ft, which was acknowledged. The aircraft levelled at about 2,000 ft and maintained that altitude until it intercepted the glideslope for the ILS. The aircraft landed at 0929 without further incident.

The ATSB summarized the pilot comments: "The PF later reported that, as a result of not properly pressing the altitude select push button to commence the descent from 4,000 ft, the workload experienced increased significantly. This resulted in what the PF described as a loss of situational awareness. After being alerted by ATC that the aircraft was below the cleared altitude of 3,000 ft, and at that time having the runway in sight and being able to maintain visual conditions until landing, the PF decided to maintain 2,000 ft until the aircraft re-joined the approach profile to reduce the workload. The PM reported that the PF’s actions in descending below the cleared level were not challenged, as this was the first time that the PM had flown this approach."

The ATSB analysed:

The approach and landing are phases of flight known to have high workload. That workload can increase exponentially when errors and the unexpected occur. As a result of not pressing the altitude selector properly, and then checking that the required flight mode change occurred on the FMA, the PF’s workload increased substantially. This resulted in the PF losing awareness of the aircraft’s descent profile, and in particular the aircraft’s altitude versus the distance to landing, a component of what is commonly referred to as situational awareness. This loss of descent profile awareness, combined with a preconception that the aircraft should not get high on profile during the positioning for final approach, resulted in the PF initiating a descent below the altitude limit of 3,000 ft that was required to be maintained until the aircraft was established on final approach. The PF identified the error and began levelling the aircraft just before the flight crew were notified by ATC that the aircraft had breached the descent clearance limit.

The PM role requires an awareness of the PF actions as well as an awareness of the aircraft’s flight profile relative to the clearance limit and any limitations associated with the approach procedure. While the PM had not flown this approach before, the monitoring role during the approach was not effectively executed.

Metars:
YBBN 180100Z 36014KT 9999 -SHRA FEW012 SCT038 BKN050 20/18 Q1016 INTER 0100/0400 3000 SHRA BKN012 BKN025=
YBBN 180030Z 01014KT 9999 VCSH FEW012 SCT038 BKN050 21/17 Q1017 INTER 0030/0330 3000 SHRA BKN012 BKN025=
YBBN 180000Z 01011KT 300V010 9999 -SHRA FEW012 SCT032 BKN050 20/18 Q1017 RESHRA INTER 0000/0300 3000 SHRA BKN012 BKN025=
YBBN 172330Z 03011KT 9999 SHRA FEW011 SCT028 OVC105 22/15 Q1016 INTER 2330/0230 3000 SHRA BKN012 BKN025=
YBBN 172300Z 03011KT 9999 -SHRA FEW011 SCT015 BKN034 20/18 Q1017 INTER 2300/0200 3000 SHRA BKN012 BKN025=
YBBN 172230Z 04005KT 9999 VCSH FEW005 SCT015 BKN034 22/17 Q1017 INTER 2230/0130 3000 SHRA BKN012 BKN025=
YBBN 172200Z 33007KT 9999 -SHRA FEW005 SCT033 BKN045 21/17 Q1017 FM2220 05010KT 9999 -SHRA SCT025 BKN050 INTER 2200/0100 3000 SHRA BKN012 BKN025=
YBBN 172130Z 36013KT 9999 VCSH FEW008 SCT015 BKN033 21/17 Q1017 FM2200 05010KT 9999 -SHRA SCT025 BKN050 INTER 2130/0030 3000 SHRA BKN012 BKN025=
YBBN 172100Z 05009KT 9999 VCSH FEW008 SCT015 BKN024 22/15 Q1017 INTER 2100/0000 3000 SHRA BKN012 BKN025=
YBBN 172030Z 05009KT 9999 VCSH FEW012 SCT024 BKN100 22/15 Q1016 INTER 2030/2330 3000 SHRA BKN012 BKN025=
YBBN 172000Z 06008KT 9999 FEW012 SCT024 BKN100 21/15 Q1016 INTER 2000/2300 3000 SHRA BKN012 BKN025=
Incident Facts

Date of incident
Nov 18, 2017

Classification
Incident

Flight number
NZ-135

Aircraft Registration
ZK-OKN

Aircraft Type
Boeing 777-300

ICAO Type Designator
B773

Airport ICAO Code
YBBN

This article is published under license from Avherald.com. © of text by Avherald.com.
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