The Taiwan Transportation Safety Board (TTSB) releases final report on the investigation of Mandarin Airlines aircraft flight AE7931 experienced brief loss of cabin pressure during descending.

On May 2, 2019, an ATR72-212A aircraft of Mandarin Airlines,  scheduled passenger flight number AE7391, nationality and registration number B-16851, departed from Kaohsiung International Airport to Hualien Airport with 2 flight crew, 2 cabin crew, 1 maintenance personnel and 48 passengers, totally 53 people on board. The aircraft experienced an "Excess Cabin Altitude" warning during descending. After being handled by the flight crew, the aircraft landed at Hualien Airport at 1528 without any damage or injuries.

Pursuant to the Transportation Occurrence Investigation Act of the Republic of China and, referring to Annex 13 to the Convention on International Civil Aviation, the Taiwan Transportation Safety Board (TTSB), an independent transportation occurrence investigation agency, started the investigation. The organization or agency been invited to join the investigation team included: Bureau d'Enquêtes et d'Analyses of France, Civil Aeronautics Administration, Ministry of Transportation and Communications, and Mandarin Airlines. The final report was reviewed and approved on August 7, 2020 by the14th Board Meeting of the TTSB.

Findings Related to Probable Causes:

1. Before the aircraft took off from Kaohsiung International Airport, the ground connection check valve might have been malfunctioned and could not be maintained in the fully closed position that caused the connection of the pressurized cabin and atmospheric environment. While the aircraft descending from cruising altitude, the cabin pressurization system was affected by flight altitude, reduced air intake of the air-conditioning system, and unclosed check valve, failed to provide an appropriate cabin pressure. Consequently, the excessive cabin altitude triggered the cabin altitude warning.

Findings Related to Risks:

1. Before the occurrence, Mandarin Airlines did not assess the service letter on renovation and improvement of the check valve issued by the aircraft manufacturer on February, 2018 (RIL-2018-03) and replace the new check valve with improved design.
2. Before the occurrence, there were no instructions on Mandarin Airlines work orders for preflight, transit, and daily checks that the ground connection check valve should be checked and confirmed in a fully closed position after the aircraft receiving an external air supply from a ground air-conditioning cart.
3. When the occurrence aircraft cruised at flight level 130, the air inside the cabin leaked from the unclosed check valve causing the cabin altitude increased gradually from 2,000 ft to 8,000 ft. If the flight crew had checked the exact cabin altitude value during the cruising phase, they could have found that the cabin altitude had exceeded the normal range.

Transportation Safety Recommendations:

Nil

Full final report is available for download at https://www.TTSB.gov.tw

Vivi Yang, Executive Officer

Tel: +886-2-7727-6217

E-mail: viviy2314@ttsb.gov.tw