Synopsis
On 13 December 2021, a Boeing 737-900ER registered PK-LQR operated by PT Lion Mentari Airlines (Lion Air) was conducting a scheduled passenger flight from Minangkabau International Airport (Padang) to Hang Nadim International Airport (Batam). The aircraft carried two pilots, six flight attendants, and 196 passengers. During climb, the crew encountered an unusual cabin altitude indication and subsequently returned to Padang. The aircraft landed without injuries. The Komite Nasional Keselamatan Transportasi (KNKT) is investigating the incident.
History of the Flight
The aircraft departed from runway 15 at 0651 UTC (1351 LT) with the Pilot in Command (PIC) as Pilot Flying and the Second in Command (SIC) as Pilot Monitoring. Prior to departure, the aircraft had a Deferred Maintenance Item (DMI) for the Right Pack 3 being inoperative, and the crew performed the No Engine Bleed Takeoff and Landing supplementary procedure.
After passing 10,000 feet, the SIC noticed an unusual cabin altitude of about 4,000 feet while the aircraft was at approximately 10,000 feet. The PIC directed the SIC to switch the pressurization system from automatic to alternate mode, then to manual mode, and to select the outflow valve to the fully closed position. The cabin rate of climb reduced from about 1,000 feet per minute to about 300 feet per minute.
Believing the situation abnormal, the crew decided to return to Padang. The PIC handed control to the SIC, who turned back and descended to 9,000 feet as instructed by Air Traffic Control. During the return, the PIC manually opened the outflow valve, causing the cabin altitude and rate of climb to increase rapidly and triggering the cabin altitude warning. The crew executed the Cabin Altitude Warning or Rapid Depressurization procedure.
After descending below 10,000 feet, the pilot announced "mask off" and coordinated with the cabin crew, who reported no injuries. The aircraft landed safely on runway 15 at 0734 UTC and taxied to the gate, where passengers disembarked normally.
Aircraft Information
The aircraft was a Boeing 737-900ER, registration PK-LQR. It was dispatched with the Right Pack 3 inoperative as a Deferred Maintenance Item, in accordance with Minimum Equipment List (MEL) item 21-02-01-03. The crew performed the corresponding supplementary procedure for No Engine Bleed Takeoff and Landing.
Investigation
The KNKT has issued safety recommendations to the aircraft operator. The investigation is ongoing, and further safety issues may be addressed as they emerge.