Indonesian investigators have said the Lion Air Boeing 737 jet that plunged into the sea, killing 189 people in October, was not airworthy on a flight the day before it crashed.

They further found that Lion Air must improve its safety culture and better document repair work on its planes.

The flight from Bali to Jakarta on 28 October had experienced similar technical issues to the doomed flight the next day from Jakarta to Pangkal Pinang, said Nurcahyo Utomo, head of Indonesia’s national transport safety committee (KNKT).

The pilot of the 28 October flight chose to press on to Jakarta after shutting down the plane’s anti-stall system, Utomo said.

“This is the basis of our recommendation to Lion Air. In our view, the plane was not airworthy,” he told a news conference in Jakarta.

The Boeing 737 MAX vanished from radar about 13 minutes after taking off from Jakarta on October 29, slamming into the Java Sea moments after it had asked to return to the capital.

The transport safety agency did not pinpoint a definitive cause of the accident, with a final crash report not likely to be filed until next year.

But its investigators said that Lion Air kept putting the plane back into service despite repeatedly failing to fix a problem with the airspeed indicator in the days leading up to the fatal flight.

The report also suggested the pilots struggled with the plane’s anti-stall system as they radioed in a request to return to Jakarta’s main airport.

The findings will heighten concerns there were problems with key systems in one of the world’s newest and most advanced commercial passenger planes.

Investigators have previously said the doomed aircraft had problems with its airspeed indicator and angle of attack (AOA) sensors, prompting Boeing to issue a special bulletin telling operators what to do when they face the same situation.

An AOA sensor provides data about the angle at which air is passing over the wings and tells pilots how much lift a plane is getting. The information can be critical in preventing an aircraft from stalling.

Despite a dubious safety record and an avalanche of complaints over shoddy service, the budget carrier’s parent Lion Air Group, which also operates Batik Air and Wings Air, has captured half the domestic market in less than 20 years of operation to become Southeast Asia’s biggest airline.

Indonesia’s aviation safety record has improved since its airlines, including national carrier Garuda, were subject to years-long bans from US and European airspace for safety violations, although the country has still recorded 40 fatal accidents over the past 15 years.

But Boeing has also come under fire for possible glitches on the 737 MAX - which entered service just last year.

The APA, a US airline pilots union, said that carriers and pilots had not been informed by Boeing of certain changes in the aircraft control system installed on the new MAX variants of the 737.

Several relatives of the crash victims have already filed lawsuits against Boeing, including the family of a young doctor who was to have married his high school sweetheart this month.