Chronic problems with a faulty rudder system and the way pilots tried to respond were among a string of factors contributing to the crash of an Indonesian AirAsia jet last year that killed all 162 on board, investigators said on Tuesday.
The Airbus A320 crashed into the Java Sea on Dec. 28, less than halfway into a two-hour flight from Indonesia's second-biggest city of Surabaya to Singapore.
In their first public report on the disaster, Indonesian investigators did not pinpoint a single underlying reason why flight QZ8501 disappeared from the radar, but set out a sequence spotlighting the faulty component, maintenance and crew actions.
It said stormy weather did not play a role in the accident.
The crash was part of a string of aviation disasters in Southeast Asia's biggest economy, where rapid growth in air travel has overcrowded airports and stirred safety concerns.
Investigators said a system controlling rudder movement had cracked soldering that malfunctioned repeatedly, including four times during the flight and 23 times the previous year.
Officials told reporters there were indications from the black box data recorder that crew had tried to shut off power to the computer that controls the rudder system by resetting a circuit breaker, something not usually done during flight.
But they cautioned there was no proof of this.
The interruption of power to the computer caused the autopilot to disengage and removed automated protections that prevent an upset, handing manual control to the crew, they said.
"Subsequent flight crew action resulted in inability to control the aircraft...causing the aircraft (to) depart from the normal flight envelope and entering a prolonged stall condition that was beyond the ability of the flight crew to recover," the National Transportation Safety Committee said in a statement.
The airline's maintenance system was "not optimal", investigators said, adding that the airline had carried out 51 safety measures to improve conditions since the crash.
"There is much to be learned here for AirAsia, the manufacturer and the aviation industry," AirAsia parent group <AIRA.KL> founder Tony Fernandes said on Twitter.
In Europe, Airbus declined immediate comment.
"Airbus has just received the final accident report. We are now carefully studying its content," a spokesman said by email.
The report is not intended to attribute blame but rather to make recommendations to avoid future accidents. Recommendations were addressed to the airline, Airbus and regulators.
Indonesia has seen two other major crashes in the past year, including a military cargo aircraft that went down in an urban area in northern Sumatra in July, killing more than 140 people.