The pilots of the ill-fated AirAsia flight from Surabaya to Singapore shut off power to a critical computer system that normally prevents planes from going out of control.
Two people with knowledge of the investigation into the crash on Dec 28 told Bloomberg that the action seems to have helped trigger the events that led to the crash.
The Airbus A320 climbed sharply, then lost lift and began falling even as warnings were blaring in the cockpit, the report said.
All 162 people on board were killed and 72 bodies have been found so far.
The pilots had been attempting to deal with alerts about the flight augmentation computers, which control the A320's rudder and also automatically prevent it from going too slow.
After initial attempts to address the alerts, the pilots cut power to the entire system, which is comprised of two separate computers that back up each other, the report quoted the two sources as saying.
While the information helps to show how a normally functioning A320's flight protection system could have been bypassed, it doesn't explain why the pilots pulled the plane into a steep climb, Bloomberg reported, quoting the two people who were not named.
But even with the computers shut off, the pilots should have been able to fly the plane manually, they said.
Mr John Cox, a former A320 pilot, said in an interview that Airbus discourages pilots from cutting power to systems because electronics in the highly computerised aircraft are interconnected and turning off one component can affect others.
"Particularly with an Airbus, you don't do that," said Mr Cox, chief executive officer of Washington-based industry consultant Safety Operating Systems.
All Airbus models produced since the 1980s are designed to prevent pilot errors from causing crashes.
The planes are controlled by multiple flight computers, which limit pilots from overly steep turns or going too slow.
In the event of a malfunction or loss of power, the flight protection will shut down and leave the pilots to fly the plane manually.
That appears to be what happened before flight QZ8501 entered the steep climb and stalled, the two people said.
This article was first published on January 31, 2015.
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