SkyWest Airlines AT72 at Moranbah on May 16th 2013, multiple GPWS alerts while descending to remain clear of cloud

Last Update: March 12, 2015 / 21:10:32 GMT/Zulu time

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Incident Facts

Date of incident
May 16, 2013

Classification
Incident

Airline
Skywest Airlines (Australia)

Flight number
XR-1661

Departure
Brisbane, Australia

Destination
Moranbah, Australia

Aircraft Registration
VH-FVR

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

A SkyWest Airlines (Australia) Avion de Transport Regional ATR-72-212A on behalf of Virgin Australia Regional, registration VH-FVR performing flight XR-1661 from Brisbane,QL to Moranbah,QL (Australia), was on approach to Moranbah when the crew initiated a descent to remain clear of cloud however received a number of GPWS alerts. The crew continued for a safe landing at Moranbah.

Initial reports had indicated the crew aborted the approach, diverted to Emerald,QL 90nm south of Moranbah, but then decided to return to Brisbane for a safe landing.

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

No weather data are available.

On Mar 12th 2015 the ATSB released their final report complaining that the investigation "also identified significant underreporting by VARA of ATR72 TAWS-related occurrences to the ATSB" and concluding the factors into the occurrence were:

Contributing factors

- Approaching the circuit, the captain assessed that a descent below the standard circuit height was necessary to avoid cloud, but did not communicate this to the first officer in a timely manner, thereby preventing identification of a descent limit or appropriate approach alternatives.

- Due to the crew’s focus on avoiding the cloud, the high rate of descent at a lower than normal altitude was not identified and corrected by the crew in the short time available, resulting in the terrain awareness warning system 'Terrain Ahead' and ‘Too Low Terrain’ alerts.

- Despite briefing the intent to conduct a visual approach, descent in visual conditions was not assured and the crew did not discontinue the approach. This resulted in an undesired aircraft state and subsequent terrain awareness warning system alerts.

Other factors that increased risk

- There was a significant underreporting by Virgin Australia Regional Airlines Pty Ltd of ATR72 terrain awareness warning system-related occurrences. [Safety issue]

The ATSB reported the captain (ATPL, 4,530 hours total, 1,750 hours on type, 3.5 hours in command) had completed the command upgrade just a few days before the occurrence and had accumulated 3 1/2 hours in command since. The captain was pilot flying, fatigue levels were considered low by the investigation. The first officer (ATPL, 2,880 hours total, 610 hours on type) was pilot monitoring. The ATSB annotated: "According to the operator’s requirements, the FO had sufficient experience to be paired with a captain who had recently been checked to line."

The ATSB reported that the crew was initially planning to perform an NDB-A approach to Moranbah's runway 16, however, another aircraft, a Dash 8-300, was to arrive about 2 minutes prior to them and planned for a visual approach. The ATR crew therefore decided to change their plans and also conduct a visual approach to runway 16. The ATSB reported: "The crew reported that after descending though an overcast cloud layer at about 6,000 ft, they became visual with the Moranbah township and the area surrounding the airport. They noted some low cloud and patches of fog around the runway 34 threshold."

The aircraft positioned to join the traffic pattern, the autopilot began to capture altitude, set to 2300 feet MSL and about 1500 feet AGL, at 2400 feet MSL, at the same time the captain "recognised that, if the current flight path was continued and the aircraft levelled at 1,500 ft AGL on downwind, they would enter cloud. Without discussing it with the FO, the captain decided and announced that they were disconnecting the autopilot and continuing the descent in an attempt to remain clear of the cloud." The captain believed they only needed to descend 200-400 feet to stay clear of the cloud and descended visually always with the ground and runway in sight. Descending through 562 feet AGL the Terrain Awareness Warning System (TAWS) activated indicating "Too Low, Terrain" shortly followed by "Terrain Ahead", "Too Low Terrain" and "Too Low Gear". The aircraft levelled at 440 feet AGL.

The ATSB reported: "The recorded average vertical speed during the descent from circuit height to 440 ft AGL, the lowest recorded height before the crew initiated a climb, was 1,750 ft/min."

The ATSB summarized crew testimonary: "The crew indicated that as they were ‘visual’, clear of the cloud, had the ground/runway in sight, were aware that no obstacles existed along the downwind leg of the circuit and that they knew why the TAWS alert activated, they could acknowledge the alerts and continue the approach. The FO reported that during the descent, they had an understanding of what the captain was trying to achieve and at no stage did they have any concern about the safety of the aircraft or the captain’s decision to descend. As such, the FO believed that communicating any in-flight observations to the captain was unnecessary and would only have added to their existing workload."

The crew continued the downwind, climbed the aircraft to 870 feet AGL (1500 feet MSL). The aircraft was configured for landing, during the turn onto the base leg a bank angle of 38 degrees was achieved and the TAWS announced "Don't sink". As the aircraft wasn't descending the caution was considered erroneous and was cancelled by the crew.

The aircraft was stabilized on final approach descending through 500 feet AGL and performed a safe landing.

The ATSB analysed with reference to the "Don't sink" TAWS alert: "In this case, the TAWS Mode 3 functionality was armed when the crew initiated the climb from 440 ft AGL when in a clean configuration. The ‘Don’t Sink’ alert activated when the crew commenced their descent for landing without first reaching 1,500 ft AGL (as measured by the radio altimeter)."

The ATSB reported: "During its investigation, the ATSB examined whether there was a potential issue with the TAWS system fitted to the ATR72. This included a search of the ATSB occurrence database to determine the number of TAWS/GPWS alerts reported for each aircraft model flown by similar Australian operators in the 12 months prior to the occurrence. Given the hours flown by model and operator in that period, the rate of reported TAWS/GPWS alerts was calculated for each operator/aircraft model combination. The examination found that VARA’s rate of TAWS/GPWS notifications was disproportionately lower than those of the other operator/aircraft model combinations examined. As a result, all TAWS/GPWS records in VARA’s own occurrence database were requested. A number of previously unreported TAWS/GPWS alerts were identified that were integrated with the other occurrence reports in the ATSB’s occurrence database. Consolidation of the VARA- and ATSB-held notifications of TAWS/GPWS alerts showed that the combined rate of reported occurrences involving the TAWS installation in the ATR72 was consistent with reporting rates from similar turboprop operators in Australia. Action was taken by VARA to ensure that in future all TAWS/GPWS notifications were reported to the ATSB (see the section titled Safety issues and actions)."

The ATSB analysed: "as the descent progressed, the captain remained focused on avoiding the cloud, and the FO was conducting various tasks and looking out as required to assess the cloud. During this time, the aircraft’s descent rate increased to a maximum of around 1,900 ft/min; however, due to the crew’s focus on avoiding the cloud and other tasks associated with the approach, this was not noticed in the available time. The continuation of the descent resulted in an undesired aircraft state, in that the aircraft was lower than desired without being correctly configured and with a high rate of descent."
Incident Facts

Date of incident
May 16, 2013

Classification
Incident

Airline
Skywest Airlines (Australia)

Flight number
XR-1661

Departure
Brisbane, Australia

Destination
Moranbah, Australia

Aircraft Registration
VH-FVR

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

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