The Taiwan Transportation Safety Board (TTSB) releases the Final Report on the investigation of Scoot Tigerair flight TR-996, the aircraft excessive cabin altitude during decent.

On March 24^th, 2019, Scoot Tigerair flight TR-996, an Airbus B320-232 aircraft, registration 9V-TAU, took off from Singapore Changi International Airport to Taiwan Taoyuan International Airport for a schedule passenger flight. There were 2 flight crew members, 4 cabin crewmembers, 178 passengers, a total of 184 persons on board. During the approach to RCTP, about altitude 14,000 ft, 28.8 nautical miles southwest from RCTP, an excessive cabin altitude warning was shown in the cockpit. Pilots donned their oxygen masks immediately and manually deployed the cabin oxygen masks. Aircraft continued the descent and safely landed at RCTP without any further event.

According to the Transportation Occurrence Investigation Act of the Republic of China, and the content of Annex 13 to the Convention on International Civil Aviation, the Taiwan Transportation Safety Board (TTSB), an independent transportation occurrence investigation agency, was responsible for conducting the investigation. The investigation team also included members from Singapore Transport Safety Investigation Bureau (TSIB), France Bureau d'Enquêtes et d'Analyses (BEA), Airbus Company, Germany Bundesstelle für Flugunfalluntersuchung (BFU), Nord-Micro GmbH & Co., and Scoot Tigerair Pte Ltd.

Findings Related to Probable Causes:

1. The two automatic cabin pressurization systems had failed one after another during descent. This resulted in the aircraft losing the automatic cabin pressurization function.
2. After the failure of both automatic cabin pressurization systems, the flight crew controlled the cabin pressure manually, as required by the ECAM checklist. The captain misunderstood the direction of operation for the MAN V/S CTL toggle switch and the corresponding effect on the position of outflow valve, and provided the wrong instruction to the first officer. The first officer did not recognize the incorrect instruction to operate the outflow valve in the direction of opening, causing the cabin altitude to rise rapidly and eventually exceeding the cabin altitude limitation.

Findings Related to Risk：

Nil

Transportation Safety Recommendations

To Singapore Scoot Tigerair Pte Ltd:

1. Enhance Crew Resource Management training especially on the situations when mismatched mental models are formed and/or incorrect instruction are given between pilots, and an open discussion should be employed to clarify the situations.

To Civil Aviation Authority of Singapore:

1. Supervise and ensure that Scoot Tigerair enhance Crew Resource Management training especially on the situations when mismatched mental models are formed and/or incorrect instruction are given between pilots, and an open discussion should be employed to clarify the situations.

Safety Actions Taken:

During the investigation TTSB noted that the Scoot Tigerair had worked with the CAAS on the following safety actions:

1. The flight crew recurrent training was revised and implemented in July 2019 to incorporate demonstration sessions to refresh the crew on the procedures required to operate the manual pressurization control system correctly and safely.
2. The two flight crew members involved with the occurrence completed a series of retraining that included crew resource management, simulator and line training. They also completed simulator and line checks satisfactorily before they returned to operational duties.
3. Scoot Tigerair also took the opportunity to reinforce aircraft system knowledge (beyond cabin pressurization) for all its pilots. ‘Aircraft System Refresher Modules’ were implemented in July 2019 covering all aircraft systems over a period of 12 months.

Full Final Report is in Chinese only and available for download at https://www.ttsb.gov.tw

Sherry Liu, Safety Investigator

Tel: 89127388-6286

Email:sherry@ttsb.gov.tw

Website: https://www.ttsb.gov.tw