The pilots of a doomed Lion Air Boeing 737 MAX scrambled through a handbook to understand why the jet was lurching downwards in the final minutes before it hit the water killing all 189 people on board, three people with knowledge of the cockpit voice recorder contents told Reuters.
The investigation into the crash in October has taken on new relevance as countries worldwide grounded the model last week after a second deadly accident in Ethiopia killed 157 people.
Investigators examining the Indonesian crash are considering how a computer ordered the plane to dive in response to data from a faulty sensor and whether the pilots had enough training to respond appropriately to the emergency, among other factors.
Following the second fatal accident, US authorities are reviewing whether enough was done to ensure the plane was safe to fly, while attention has also focused on the training of the Lion Air crew and whether aeroplane manuals are clear enough.
It is the first time the voice recorder contents from the Lion Air flight have been made known.
The captain was at the controls of Lion Air flight JT610 when the near-new jet took off from Jakarta, and the first officer was handling the radio, according to a preliminary report issued in November.
Just two minutes into the flight, the first officer reported a "flight control problem" to air traffic control and said the pilots intended to maintain an altitude of 5,000 feet, the November report said.
The captain asked the first officer to check the quick reference handbook, which contains checklists for abnormal events, the first source said.
For the next nine minutes, the jet warned pilots it was in a stall and pushed the nose down in response, the report showed. A stall is when the airflow over a plane's wings is too weak to generate lift and keep it flying.
The captain fought to climb, but the computer, still incorrectly sensing a stall, continued to push the nose down using the plane's trim system. Normally, trim adjusts an aircraft's control surfaces to ensure it flies straight and level.
"They didn't seem to know the trim was moving down," the third source said. "They thought only about airspeed and altitude. That was the only thing they talked about."
The pilots remained calm for most of the flight, the three sources said. Near the end, the captain asked the first officer to fly while he checked the manual for a solution.
As the 31-year-old captain tried in vain to find the right procedure in the handbook, the 41-year-old first officer was unable to control the plane, two of the sources said.
"It is like a test where there are 100 questions and when the time is up you have only answered 75," the third source said. "So you panic. It is a time-out condition."
The Indian-born captain was silent at the end, all three sources said, while the Indonesian first officer said "Allahu Akbar", or "God is greatest".
The plane then hit the water, killing everyone on board.
The US Transportation Department plans to audit the Federal Aviation Administration's certification of the Boeing 737 MAX.
European and Canadian regulators say they want to make up their own minds whether a software upgrade promised by Boeing is adequate.
Lion Air and Boeing declined to comment.