What happened
On 1 April 2018, a Boeing 737-900 ER, registration PK-LFK, operated by Lion Air, was performing a scheduled flight from Jakarta to Jambi with 213 people on board. The aircraft was operating under a deferred maintenance item (DMI) due to a faulty Cabin Pressure Controller (CPC) identified during a previous flight. To comply with the Minimum Equipment List (MEL), the aircraft was prepared to fly using a single functional controller.
As the aircraft climbed through 8,000 feet, the crew observed an excessive cabin vertical speed. Upon inspection, the pilot found the cabin pressure control selector had been left in the manual position. While attempting to manage the pressure, the cabin altitude warning activated near 10,000 feet. As the climb continued toward 18,000 feet, the situation escalated when passenger oxygen masks deployed throughout the cabin.
Recognizing the pressurization instability, the pilot requested a diversion to Palembang. During the descent, the crew struggled to regulate the cabin pressure, manually cycling the outflow valve. Upon landing at Sultan Mahmud Badaruddin II Airport, the aircraft exhibited a significant cabin differential pressure of 5.8 psi, which initially prevented the cabin doors from being opened. The crew eventually equalized the pressure by manually opening the outflow valve.
The investigation
The KNKT investigation focused on the transition from the pre-flight maintenance procedures to the in-flight malfunction. Investigators examined the Aircraft Flight Maintenance Log, which noted that the previous day's flight had experienced uncontrollable cabin descent rates. The investigation also reviewed the Dispatch Deviation Guide (DDG) procedures and the pilot's execution of the pre-flight checks. Data from the Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) were analyzed to reconstruct the sequence of the pressurization failure and the crew's response to the oxygen mask deployment.
Findings
- The aircraft was operating with a single functional Cabin Pressure Controller under a deferred maintenance item.
- During pre-flight checks, the crew was required to follow specific DDG instructions, which included setting the Pressure Mode Selector to the alternate (ALTN) position.
- The cabin pressure control selector was found in the manual position during the climb, leading to uncontrolled cabin vertical speed.
- The inability to maintain stable pressurization resulted in the deployment of passenger oxygen masks at higher altitudes.
- A high differential pressure of 5.8 psi remained at the aircraft upon landing, necessitating manual intervention to allow passenger disembarkation.
Safety action
Following the incident, Lion Air issued a safety notice to flight crews and engineers. The notice emphasized the need to review pressurization system principles, monitor cabin rate of climb indicators closely, and strictly adhere to maintenance rectification steps to prevent similar pressurization failures.