Pilots fought against an automated system that pitched a Boeing jetliner's nose down repeatedly because of a faulty sensor until they finally lost control and plunged into the Java Sea last month, Indonesian investigators say.
At a news conference on Wednesday, safety officials said they were still struggling to understand why the plane crashed, killing all 189 people on board.
The National Transportation Safety Commission's Nurcahyo Utomo said investigators were trying to figure out from interviews with engineers why they certified that the Boeing 737 MAX 8 was airworthy and whether they had followed required maintenance procedures.
Pilots of previous flights had reported problems with control systems on the brand-new jet.
A piece of Lion Air flight JT610 recovered from the water. Source: Getty Images
The board issued a preliminary report that stopped short of declaring a probable cause of the crash - the investigation is continuing - but it provided new details about the pilots' struggle to fly the highly automated jet and Lion Air's inability to fix problems with sensors on the plane.
Sensors that measure speed were flushed and checked, and an electrical plug was cleaned before the fatal flight.
Mechanics, however, did not check a sensor that measures whether the nose of the plane is pointing up or down.
That "angle of attack" sensor gave faulty readings throughout the short flight, triggering a system that automatically pointed the plane's nose down more than two dozen times, with pilots responding by manually fighting to correct the pitch.
Pilots even asked air traffic controllers to tell them how fast and high they were flying.
Indonesian rescue team members have retrieved body parts from the water where a plane crashed. Source: AAP
The malfunctions and warnings from the plane's control system appeared to overwhelm the pilots almost as soon as the jet became airborne, said another investigator, Ony Suryo Wibowo.
"The problem is if multiple malfunctions occur all at once, which one should be prioritised?" Wibowo said.
Boeing points finger at pilots
In a statement following release of the report, US-based Boeing declared that the MAX, its newest plane, is safe.
The manufacturer played up the possibility of pilot error.
Boeing noted that the crew of the plane's previous flight one day earlier had responded correctly to the automatic nose-down pitch and flew the plane manually. They also ran safety checklists.
The preliminary report does not say whether pilots on the deadly flight took those steps, Boeing pointed out.
Boeing has said that the procedure to correct an automatic nose-down pitch is in the plane's operating manual and pilots should have known about it.
Searchers have not yet recovered the plane's cockpit voice recorder, which could tell investigators what the pilots were doing - or failing to do - to regain control of the plane during the brief, erratic flight.
The report by Indonesia's safety commission repeated earlier recommendations made just after the disaster that pilots be better versed in emergency procedures and aware of past aircraft problems.
They recommended that Lion Air, a fast-growing low-cost airline based in Jakarta, ensure that pilots follow proper procedures "to improve the safety culture."
