What happened
On May 2, 2019, an ATR72-212A, registration B-16851, operated by China Airlines (Mandarin Airlines) was performing a domestic flight from Kaohsiung International Airport to Hualien Airport. The aircraft was carrying 53 people, including 48 passengers and 5 crew members.
During the descent phase of the flight, the flight crew received a "CAB ALT" (cabin altitude) warning, followed by an "EXCESS CAB ALT" warning. The cabin altitude had risen significantly, reaching approximately 10,241 feet. In response to the warning, the crew immediately initiated an emergency descent, donned oxygen masks, and declared a MAYDAY to Taipei Approach. The crew successfully stabilized the cabin pressure and landed safely at Hualuallyen Airport at 15:28, with no injuries or fatalities reported.
The investigation
Following the landing, maintenance personnel inspected the aircraft and discovered that the air conditioning ground connection check valve (part number 41125A01) was unable to maintain a fully closed position. The investigation revealed that the valve spring had fractured, likely prior to departure from Kaohsiung. This failure allowed the cabin environment to remain connected to the external atmosphere through the valve.
The investigation established that while the engine thrust during the climb and cruise phases provided sufficient airflow to maintain a stable cabin altitude, the reduction in thrust to idle during descent caused the air conditioning supply to drop. This reduction in airflow was insufficient to compensate for the leakage through the faulty valve, leading to the rapid rise in cabin altitude.
Furthermore, the investigation found that the airline had not yet implemented the manufacturer's retrofit information letter (RIL-2018-03), which proposed an improved valve design to prevent such failures. Additionally, existing maintenance checklists did not specifically require a check of the valve position after the use of ground air conditioning carts.
Findings
- The primary cause of the cabin altitude warning was a faulty air conditioning ground connection check valve that failed to remain closed due to a broken spring.
- The failure of the valve allowed cabin air to leak externally, which became critical when engine thrust was reduced during descent.
- The airline had not yet evaluated or implemented the manufacturer's recommended upgraded valve design.
- Maintenance procedures lacked specific instructions to verify the closed position of the ground connection check valve after using ground air conditioning units.
- The flight crew's monitoring of the exact cabin altitude during the cruise phase could have potentially identified the gradual rise in altitude earlier.