Indonesia’s safety investigators are probing a pressurisation system problem onboard a Lion Air Boeing 737-900ER that resulted in the aircraft having to make a diversion during a routine domestic flight.

The jet, registered PK-LFK, already had a record in its maintenance log book that the cabin rate of descend was difficult to control during a previous day flight. The issue was not resolved, but an engineer issued a deferred maintenance item that allowed the aircraft to depart with a single cabin pressure controller.

On 1 April, the aircraft took off from Jakarta for Jambi with 206 passengers and seven crew members. During the climb past 8,000ft, the pilot noticed that the cabin vertical speed “climbed excessively” and that the cabin pressure control selector was in the manual position. This led him to manually select the outflow valve selector switch to open.

Passing 10,000ft, the cabin altitude aural warning was activated, but stopped after the pilot pressed the “horn cutout” button. During the climb to 18,000ft, the passenger oxygen masks dropped and the pilot decided to divert to Palembang.

During descent, the pilot was again unable to control the cabin pressure and had to activate the outflow valve selector to open and close positions.

The jet landed without incident at Palembang, but the pilot had to manually select the outflow value to the open position to bring cabin altitude and differential pressure to zero, before the door could be opened.

No one was injured in the incident.

Investigations are ongoing but the National Transportation Safety Committee has called on Lion Air to improve pilot training to ensure understanding of aircraft systems and to follow procedures accordingly.

Source: Cirium Dashboard