Virgin Australia B738 at Cairns on Oct 24th 2022, descended below minimum safe height

Last Update: August 9, 2023 / 15:03:44 GMT/Zulu time

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Incident Facts

Date of incident
Oct 24, 2022

Classification
Incident

Airline
Virgin Australia

Flight number
VA-793

Departure
Brisbane, Australia

Destination
Cairns, Australia

Aircraft Registration
VH-VUT

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Virgin Australia Boeing 737-800, registration VH-VUT performing flight VA-793 from Brisbane,QL to Cairns,QL (Australia), was on a RNAV HENDY 8A standard instrument approach BARIA transition to Cairns when the aircraft descended below the minimum safe height with air traffic control issuing a low altitude warning to the crew and instructing them to return to safe altitude. The crew climbed back to safe altitude, commenced a missed approach and landed sfely on second approach.

The ATSB released their preliminary report summarizing the sequence of events:

At 1945 local time, the aircraft was cruising in darkness at flight level (FL)2 380 about 215 NM to the south of Cairns. At that time, air traffic control (ATC) provided the crew with clearance to conduct the Cairns HENDO 8Y standard arrival (STAR) via the BARIA waypoint3 transition.

The flight crew entered the HENDO 8Y STAR into the flight management computer (FMC) and selected the BARIA transition. The HENDO 8Y STAR progressed into the required navigation performance (RNP) Y instrument approach for runway 33 at Cairns. While clearance for the approach had not been provided at that time, the crew anticipated the clearance and loaded the approach into the FMC. From HENDO, the minimum altitude for commencing the RNP Y approach was 6,800 ft above mean sea level (AMSL). The HENDO waypoint was located within the 6,500 ft minimum sector altitude (MSA)4 segment to the south of Cairns.

The approach procedure had two different initial approach fixes (IAF) (Figure 2) with associated paths to a common intermediate fix (IF) at waypoint CS540. From the BASIL IAF, the approach proceeded via CS520, CS521 and CS523, and from the HENDO IAF via CS522 and CS523. In order to load either path into the FMC, the flight crew needed to select one of the two approach transitions.

The flight crew did not recognise that an approach transition selection was required and consequently did not select one. As a transition had not been selected, the FMC presented a discontinuity in the entered flight path at the HENDO waypoint (see the section titled Flight management computer). The flight crew misidentified the approach IF, CS540, as the IAF and resolved the FMC discontinuity by connecting HENDO to CS540. This selection removed the 6,800 ft descent altitude constraint associated with HENDO in the RNP approach programming.

At 1954, when the aircraft was 136 nm south of HENDO, ATC cleared the flight to track direct to the HENDO waypoint and 6 minutes later the crew commenced descending the aircraft. At 2010:51, when the aircraft was about 11 NM southeast of HENDO, ATC provided the crew with clearance to conduct the RNP Y runway 33 approach.

One minute later, the aircraft approached HENDO descending through about 7,300 ft with the autopilot engaged and an altitude of 6,800 ft selected in the autopilot mode control panel. At about that time, the captain selected the approach’s minimum descent altitude of 800 ft, but sensed that this selection was incorrect and therefore reselected an altitude of 6,800 ft. The captain then reviewed the approach briefing, confirmed that the aircraft was tracking as intended and the vertical navigation autopilot mode was active and again selected 800 ft.

At 2011:38, about 7 NM prior to crossing HENDO, the aircraft descended below 6,800 ft and 9 seconds later descended below the 6,500 ft MSA. Six seconds later, at 2011:53, ATC observed that the aircraft had descended below 6,800 ft and contacted the crew to confirm the aircraft’s altitude. The captain then reselected 6,800 ft and manually arrested the descent. ATC then issued a low altitude alert to the crew and advised them to climb immediately. Three seconds later, at 2012:07, the aircraft stopped descending at about 5,920 ft and then commenced a climb. At 2012:28, the aircraft climbed back above 6,800 ft. No ground proximity warning system alerts were generated during the incident.

A missed approach was then commenced, and the crew conducted a second approach without further incident.

On Aug 9th 2023 the ATSB released their combined final report into both this and the occurrence two days later, see Incident: Qantas B738 at Cairns on Oct 26th 2022, descended below minimum safe altitude concluding the probable causes of the incidents were:

Contributing factors

- Both flight crews did not recognise that an approach transition selection was required in the aircraft’s flight management computer. Not selecting the approach transition resulted in a programmed flight path discontinuity between the waypoints HENDO and CS540.

- The flight crews of both aircraft resolved the discontinuity by manually linking the HENDO waypoint associated with the arrival programming to waypoint CS540. This selection removed the 6,800 ft descent altitude constraint associated with the HENDO waypoint in the approach programming. With the altitude constraint removed, the programmed flightpaths led to both aircraft descending below the minimum safe altitude.

- The vertical profile depiction on the Jeppesen RNP Y runway 33 approach chart did not include the waypoints HENDO, CS522 and CS523 and the map presented the information associated with those waypoints over dense topographical information. This likely limited the ability of both crews to identify the descent restrictions associated with those waypoints.

- Both operator’s instrument approach briefing procedure included a requirement to ensure all necessary waypoints and operational constraints were included in the procedures loaded into the flight management computer. However, the briefing conducted by the flight crews did not identify that data entry errors had removed some altitude protections.

Other findings

- On each occasion, air traffic control intervened quickly and appropriately to resolve the risks of each descent below the minimum safe altitude.

The ATSB analysed:

Introduction

On the evening of 24 October 2022, a Boeing 737-800 operated by Virgin Australia Airlines operated a night air transport flight from Brisbane to Cairns, Queensland. On the morning of 26 October 2022, a Boeing 737-800 operated by Qantas Airways operated the same route in daylight. During both flights, the crews did not recognise that an approach transition had to be selected when entering approach data into the flight management computer (FMC) and no selection was made.

This analysis focuses on the reasons for the crews not making the approach transition selection or identifying the data entry error prior to it resulting in both aircraft descending below the minimum safe altitude.

Transition selection

The Cairns HENDO 8Y arrival procedure had 7 arrival transitions and was named after the last waypoint in the procedure (the first common waypoint for the transitions), which was relatively uncommon. The HENDO waypoint was also one of 2 initial approach fixes (IAF) and was therefore also the name of a transition for the approach. The 2 approach transition options were presented on the right-hand side of the same FMC page as the arrival transitions, with 4 arrival transitions on the left-hand side of the same page and the remaining 3 on the next page. The crews of both aircraft did not recognise that the transitions on the right related to the approach and consequently, the requirement to select an approach transition was not immediately apparent to them.

For the crew of VH-VUT, this perception was compounded by the misidentification of the waypoint CS540 as the IAF (see the section below titled Approach chart). Separately, the crew of VH-VZA were influenced by an expectation that the wording of the approach clearance would include the nomination of a transition. However, as an airways clearance to Cairns via the HENDO 8Y arrival and RNP Y approach had been provided, no separate nomination of the approach transition was required or provided.

Ultimately, these factors resulted in both flight crews not recognising that an approach transition selection was required and consequently, none was selected. Not selecting the approach transition resulted in a discontinuity between the waypoints HENDO and CS540 in the programmed flight path.

Discontinuity resolution

Having misidentified CS540 as the IAF, the captain of VH-VUT resolved the discontinuity by manually linking HENDO from the arrival to CS540 in the approach. This presented an approach track that closely aligned with the crew’s expectations, but with the omission of the altitude constraints associated with the approach waypoints HENDO, CS522 and CS523.

On board VH-VZA, after entering the approach without selecting a transition, the crew identified that waypoints were missing from the approach. As the programmed flight path continued along the same track, the crew elected to continue the approach without the entering the waypoints into the FMC and linked the waypoint HENDO from the arrival to CS540 in the approach. This decision was possibly influenced by the fact that the aircraft was operating in visual conditions and the crew were not reliant on the FMC programming for terrain clearance. However, the crew did not identify that altitude constraints associated with the unprogrammed waypoints were omitted from the programmed flight path.

In each case, this led to the approaches continuing with the altitude constraints removed and when an altitude below the minimum safe altitude was selected, the aircraft commenced automatically descending to that altitude. This resulted in both aircraft descending below the approach altitude constraints and then the minimum sector altitude. This was contrary to air traffic control clearances and, for VH-VUT, also reduced obstacle clearance assurance as the flight was conducted in darkness.

Approach chart

The HENDO 8Y arrival and Cairns RNP Y runway 33 approach were a complex procedure pairing with similarly complex charts.

In addition, the Jeppesen approach chart used by both crews was designed and published in accordance with ICAO guidance, but did not include the waypoints HENDO, CS522 and CS523 on the vertical profile depiction. This information was included on a comparable chart produced by Airservices Australia. Had those waypoints been included on the vertical profile of the Jeppesen chart, the likelihood of the descent restrictions being identified would have been increased.

The missing waypoints on the vertical profile and thicker track line from CS540 on the approach chart likely contributed to the captain of VH-VUT misidentifying that waypoint as the IAF. These missing waypoints were also the waypoints omitted from the FMC programming if an approach transition was not selected. Therefore, their omission was not an immediate indicator of a data entry error.

Furthermore, the Jeppesen chart presented the information (including the altitude constraints) associated with the missing waypoints and segments over topographical information on the map and separated from the relevant waypoints and segments. This reduced the readability of the information and may have contributed to the crews not associating the relevant information with their respective waypoints and segments thereby further reducing the crews’ ability to identify the associated altitude constraints.

Approach Briefing

Both operators required the approach briefing to ensure that the data entered into the flight management computer included all relevant waypoints and altitude constraints from the procedure chart.

In each case, the pilot flying read from the procedure chart while the pilot monitoring reviewed the data in the FMC. This method was not fully independent and relied on the pilot flying to identify all applicable details. The missed altitude constraints (and waypoints in the case of VH-VUT) were not read out by the pilot flying and therefore, their omission was not identified by the pilot monitoring’s check. The pilot monitoring’s assessments of the correct data input was also supported by the navigation display map view tracks closely aligning with expectations.

During the approach briefing conducted by the crew of VH-VUT an opportunity was presented to pause the brief and identify the data entry error. This occurred when the pilot monitoring became confused as to the sequence of the data being read by the pilot flying. The pilot monitoring sought clarification from the pilot flying. This likely occurred as the data entered was not complete and should have acted as a trigger for reassessment of that data.

In the case of VH-VZA, the crew did identify the missing waypoints but did not fully consider the altitude constraint implication associated with these waypoints. A comprehensive brief would probably have identified these missing constraints. While the terrain was visible to the crew and they were not reliant on FMC programming for terrain protection, the airways clearance did require adherence to the limiting altitudes.

In summary, the approach briefs conducted by the crews did not ensure that the charts, and the programmed approach flight path, were fully and independently assessed. Therefore, the data entry errors leading to the removal of the altitude protections were not detected.

Air traffic control intervention

On each occasion, air traffic control quickly identified the aircraft descending below the 6,800 ft altitude constraint and immediately contacted the crews. As the VH-VUT incident was at night and not in visual conditions, the controller issued a low altitude alert to the crew and advised them to climb immediately.

The conditions were clear and during daylight for the crew of VH-VZA and therefore air traffic control was able to provide a clearance for a visual approach.

Air traffic control acts to coordinate the flow of aircraft arriving at an airport, but also plays an important role in identifying risks to aircraft. On these occasions, air traffic control intervened quickly and appropriately to resolve the risks of each descent below the minimum safe altitude.

Aircraft Registration Data

Registration mark

VH-VUT

Country of Registration

Australia

Date of Registration

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Airworthyness Category

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TCDS Ident. No.

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Manufacturer

THE BOEING COMPANY

Aircraft Model / Type

737-8FE

ICAO Aircraft Type

B738

Year of Manufacture

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Serial Number

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Maximum Take off Mass (MTOM) [kg]

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Engine Count

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Engine

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Main Owner

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Main Operator

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