Lion Air crash. Reuters

Indonesian investigators on Wednesday said the Lion Air plane that crashed last month killing 189 people was not airworthy and should have been grounded.

The Boeing 737 Max plane crashed into the Java Sea shortly after departing from Jakarta on 29 October.

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A preliminary report has found technical problems had been reported on previous flights.

The 737 Max is a new version of Boeing’s original 737 and has become its fastest selling plane.

The preliminary report details what is known by authorities about the short time the plane was in the air, but investigators said it did not give a definitive cause for the accident.

The findings by the National Transport Safety Committee (KNKT) suggest that Lion Air put the plane back into service despite it having had problems on earlier flights.

The pilots appeared to struggle with an automated system designed to keep the plane from stalling – a new feature in the 737 Max family.

Anti-stall systems prevent a plane from pointing upwards at too high an angle, where it could lose its lift.

However, during the 29 October flight, the plane’s automatic anti-stalling system repeatedly forced the plane’s nose down, even when the plane was not stalling – possibly due to a faulty sensor, the report said.

Pilots tried to correct this by pointing the nose higher, until the system pushed it down again. This happened more than 20 times.

It is unclear why the pilots did not employ procedures to disable the automated system.