System Component Failure Or Malfunction (Non-Powerplant), Lion Air (Boeing 737-900 PK-LQR); Minangkabau International Airport, KNKT.21.12.01.04PRELIMINARY Report

On 13 December 2021, Boeing 737-900ER aircraft registered PK-LQR was being operated by PT. Lion Mentari Airlines for scheduled flight from Minangkabau International Airport of Padang (WIPT) to Hang Nadim International Airport of Batam (WIDD).

At 0651 UTC (1351 LT) the aircraft departed using runway 15. On board the aircraft was two pilots, six flight attendants and 196 passengers. The Pilot in Command (PIC) acted as Pilot Flying (PF) and the Second in Command (SIC) acted as Pilot Monitoring (PM).

After the aircraft passed altitude of 10,000 feet, the SIC noticed and informed the PIC an unusual cabin altitude indication. The cabin altitude indicated about 4,000 feet when the aircraft passing altitude about 10,000 feet. The aircraft continued climbing toward the intended altitude. The PIC requested to the SIC to select the pressurization system from automatic to alternate mode. The pilots did not see any effect after the changing of the pressurization mode selection and the PIC requested to the SIC to change the pressurization system to manual mode and to select the outflow valve to fully closed position. After selecting the outflow valve to fully close, the cabin rate of climb indication was reduced from about 1,000 feet/minute to about 300 feet/minute.

Both pilots considered that this situation was not normal and agreed to returned to Padang. The PIC handed over the aircraft control to the SIC. The SIC started turning the aircraft to Padang and descended to 9,000 feet as instructed by Air Traffic Controller (ATC).

The PIC evaluating the actionsthat had been taken by SIC on the pressurization panel thereafter, the PIC selected the outflow valve selector manually to open position. Immediately, the cabin altitude and cabin rate of climb increased rapidly and the Cabin Altitude Warning sound activated. The pilots conducted the procedure of “Cabin Altitude Warning or Rapid Depressurization”.

During the descent, the PIC requested to hold at holding point BINIB to complete the procedures. The ATC offered assistance to the pilots and responded that there was no assistance required after landing.

After the aircraft passed altitude of 10,000 feet, the pilot announced “mask off” which means that flight attendant and passengers may take off the oxygen mask as the aircraft was on safe altitude without mask. Thereafter, the pilot coordinated with the flight attendant regarding the condition in the cabin which was informed by the flight attendant that there was no injury.

After all procedures were completed, the pilot commenced the approach landing runway 15.

At 0734 UTC, the aircraft landed on runway 15 and taxied to the apron. After completing the park at the gate, passengers disembarked normally with no injury reported on this occurrence.

The KNKT issued safety recommendations addressed to the aircraft operator. The investigation is ongoing, should further safety issues emerge during the course of the investigation, KNKT will bring the issues to the attention of the relevant parties and issue safety recommendation(s) as required.