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 said Wednesday.
At a news conference, 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.
The board issued a preliminary report that stopped short of placing blame for 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 sensors that measure whether the nose of the plane is pointing up or down.
An "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.
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 prioritized?" Wibowo said.
In a statement following release of the report, U.S.-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.
Several experts said, however, that Boeing likely will have to consider changes in the new anti-stall system, perhaps developing an algorithm to disregard sensor readings that appear off-base.
The report offered new details on persistent problems with sensors on the Lion Air jet and the airline's efforts to fix them.
John Cox, a safety consultant and former airline pilot, said Lion Air should have taken the troubled plane on a maintenance test flight.
"I don't think the airplane was ready for passenger service because they had not validated that they had fixed the problem," he said.
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 did not draw conclusions about why the crew lost control of the plane, but it repeated earlier recommendations 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, ensures that pilots follow proper procedures "to improve the safety culture."
Mary Schiavo, a former inspector general of the U.S. Department of Transportation, said the preliminary report offered a road map of final recommendations that are likely to emerge from the investigation.
"They will be looking for more precise reporting of problems (by pilots), and certainly a better maintenance response," she said.
Peter Lemme, an expert in aviation and satellite communications and a former Boeing engineer who wrote an analysis of the data on his blog, compared the scene in the cockpit to "a deadly game of tag" in which the plane pointed down, the pilots countered by manually aiming the nose higher, only for the sequence to repeat about five seconds later.
That happened 26 times during the 11-minute flight, but pilots failed to recognize what was happening and follow the known procedure for countering incorrect activation of the automated safety system, Lemme told The Associated Press.
Lemme said he was troubled that there weren't easy checks to see if sensor information was correct, that the crew of the fatal flight apparently wasn't warned about the problems on previous flights and that the Lion Air jet wasn't fully repaired after those flights.
"Had they fixed the airplane, we would not have had the accident," he said. "Every accident is a combination of events, so there is disappointment all around here," he said.
The U.S. National Transportation Safety Board and Boeing experts are helping the Indonesian investigators.
Boeing has a great deal at stake in defending its plane.
More than 200 MAX jets have been delivered to airlines around the world. Pilots at American Airlines and Southwest Airlines complained this month that they had not been given all information about the new automated anti-stall safety system on the MAX.
Boeing shares fell 14 percent in the last three weeks through Tuesday, as investigators focused on the possible role of faulty sensor readings on the new plane. They rallied on Wednesday, however, to close up $15.47, or 4.9 percent, at $333.50, the stock's best day in 10 months.
Koenig reported from Dallas, Texas.