What happened
On 13 December 2021, a Boeing 737-900ER, registration PK-LQR, operated by PT Lion Mentari Airlines, was performing a scheduled flight from Padang to Batam. The aircraft was carrying 196 passengers and eight crew members. The flight had departed from Minangkabau International Airport using runway 15.
As the aircraft climbed through 10,000 feet, the second in command noticed an abnormal cabin altitude indication, which showed approximately 4,000 feet despite the aircraft being at 10,000 feet. In an attempt to stabilize the cabin, the pilots transitioned the pressurization system from automatic to alternate mode, and eventually to manual mode, while closing the outflow valve. While this reduced the cabin rate of climb, the crew determined the situation was abnormal and decided to return to Padang.
During the descent, while the pilot in command was evaluating the pressurization panel, they manually moved the outflow valve selector to the open position. This action triggered a rapid increase in both cabin altitude and the rate of climb, activating the cabin altitude warning. The crew promptly executed the required procedures for rapid depressurization. After descending below 10,000 feet, the crew instructed passengers to remove their oxygen masks. The aircraft landed safely at Padang with no injuries reported.
The investigation
The KNKT investigation focused on the aircraft's technical status and the crew's response to the pressurization anomaly. The aircraft had been dispatched with a deferred maintenance item regarding an inoperative right pack. Previous flight data indicated that a 'PACK' caution light had been illuminated on the preceding leg from Batam to Padang, which had been logged in the aircraft maintenance records.
Investigators examined the flight data recorder and cockpit voice recorder to reconstruct the sequence of events. The investigation also reviewed the maintenance actions taken by engineers in Padang, which included BITE tests on the Pack/Zone Temperature Controller and the Digital Cabin Pressure Controller. The investigation is ongoing to determine the underlying cause of the initial altitude discrepancy.
Findings
- The aircraft was operating with a deferred maintenance item (DMI) for an inoperative right pack.
- During the climb, the crew identified an unusual cabin altitude indication.
- The manual opening of the outflow valve by the pilot in command caused a rapid, uncontrolled increase in cabin altitude and rate of climb.
- The crew successfully managed the rapid depressurization procedure and landed the aircraft without injury to passengers or crew.