What happened
On March 24, 2019, Scoot Tigerair flight TR-996, an Airbus A320-232 registered as 9V-TAU, was operating a scheduled passenger flight from Singapore Changi Airport to Taiwan Taoyuan International Airport. The flight was carrying 178 passengers and 6 crew members.
During the descent phase, the flight crew attempted to reduce the aircraft's landing weight by activating the Auxiliary Power Unit (APU) to increase fuel consumption. While operating at high altitude, the crew encountered an APU bleed air limitation. As the aircraft continued its descent, the first of two automatic cabin pressure control systems failed. Shortly thereafter, the second system also failed, leaving the crew to manage the cabin pressure manually.
While operating in manual mode, the pilot flying provided incorrect instructions regarding the direction of the manual pressure control switch. The pilot monitoring followed these instructions, which inadvertently opened the outflow valve further. This caused the cabin altitude to rise rapidly from approximately 4,224 feet to over 9,000 feet. At 12:54 UTC, an "EXCESS CAB ALT" warning was triggered. The crew donned oxygen masks, deployed the passenger oxygen masks, and requested a descent to 10,000 feet. The aircraft landed safely at Taoyuan International Airport at 13:11 UTC with no injuries reported.
The investigation
The Taiwan Transportation Safety Board (TTSB) examined flight data from the flight data recorder (FDR), cockpit voice recorder (CVR), and cabin pressure controller (CPC). The investigation focused on the sequence of the dual system failures and the crew's manual intervention.
Investigators analyzed the mechanical state of the cabin pressure components. Testing of the outflow valve revealed that external contamination on the valve cover had caused sticking, which led to the second system failure due to high motor current when the valve attempted to open. The first failure was attributed to an intermittent contact issue within the valve position potentiometer, which could not be replicated in a laboratory setting. The investigation also reviewed the crew's training records and the operational procedures used during the manual pressure control phase.