Incident Summary
On March 24, 2019, Scoot Tigerair Pte Ltd. flight TR-996, an Airbus A320-232 with registration 9V-TAU, departed Singapore Changi International Airport at 0848 local time (Taipei time) for Taiwan Taoyuan International Airport. The aircraft carried 2 pilots, 4 cabin crew, and 178 passengers, totaling 184 occupants. During the descent, at approximately 1254 at an altitude of about 14,000 feet and 28.8 nautical miles southwest of Taoyuan Airport, the cabin altitude exceeded safe limits. The flight crew donned oxygen masks, deployed passenger oxygen masks, and continued the descent, landing safely on runway 05L at 1311. No injuries or damage to the aircraft were reported.
Sequence of Events
According to flight data recorder (FDR), cockpit voice recorder (CVR), and cabin pressure controller (CPC) data, and crew interviews, the flight crew noted that the estimated landing fuel weight was between 5.9 and 6.0 tons, raising the risk of a heavy landing. To increase fuel consumption, they decided to start the auxiliary power unit (APU) in flight. At 1234:15 at 27,000 feet, the first officer started the APU and pressed the APU Bleed button. Twelve seconds later, an APU Bleed advisory message appeared. The crew realized the APU bleed limitation is below 20,000 feet, so the first officer turned off the APU Bleed and reactivated it after descending below 20,000 feet.
At 1242:06 at 19,732 feet, a CAB PR SYS 1 FAULT message appeared; the crew cleared it. Later, after entering clouds, they activated engine anti-ice. At 1251:20, the captain instructed to turn on engine anti-ice and turn off APU bleed. At 1252:32, the first officer deactivated APU Bleed, switching air conditioning to both engines. At 1252:42 at 14,748 feet while holding at Jammy, a CAB PR SYS 1+2 FAULT warning appeared; cabin altitude was -192 feet. Following procedure, the crew selected manual cabin pressure control mode and used the MAN V/S CTL switch.
At 1253:39 at 14,296 feet, cabin altitude rose to about 4,224 feet. The first officer noted the increase; the captain instructed to open the outflow valve (OFV) and then only slightly. The first officer pushed the manual control switch down for 4 seconds, then the captain said push up. At 1253:59, cabin altitude reached about 6,000 feet; the captain asked why it was increasing so fast. Eleven seconds later, he instructed to continue opening the valve; the first officer acknowledged. At 1254:20 at 13,956 feet, cabin altitude reached 9,680 feet; one second later, an EXCESS CAB ALT warning activated. The crew donned oxygen masks and deployed passenger oxygen masks. Since the aircraft was already descending at 1,600 ft/min at 13,000 feet, they did not initiate an emergency descent but requested descent to 10,000 feet. At about 1301:49 at 6,680 feet, the captain made a cabin announcement. Air traffic control vectored the aircraft for an ILS approach to runway 05L, and the aircraft landed safely at 1311.
Investigation Findings
The Taiwan Transportation Safety Board (TTSB) investigation found the following:
- Both automatic cabin pressure control systems failed sequentially, losing automatic regulation.
- After the failures, the crew selected manual mode, but the pilot flying (captain) incorrectly judged the relationship between the manual control switch direction and cabin altitude change, giving erroneous commands. The pilot monitoring (first officer) did not identify the error, leading to the outflow valve opening and a rapid cabin altitude increase.
- The crew was unfamiliar with the maximum altitude limitation for APU bleed operation.
- There was no evidence of medical, drug, or alcohol factors affecting crew performance. Both pilots held valid licenses and were fit for duty.
- Post-incident testing of the cabin pressure controller and outflow valve found no abnormalities affecting manual mode operation.
- The probable causes of the automatic system failures were: a potentiometer sensing the outflow valve position had intermittent internal contact, and contamination on the outflow valve flap caused sticking, leading to high current when opening. These were unrelated and did not affect manual mode.
Probable Cause
Both automatic cabin pressure control systems failed; after selecting manual mode, the pilot flying incorrectly judged the manual control switch direction and gave erroneous commands, and the pilot monitoring did not identify the error, causing the outflow valve to open and the cabin altitude to rise rapidly, triggering the excess cabin altitude warning.