An Indonesian officer displays wreckage from Adam Air Flight 574 found floating in the Makassar Strait in January 2007. ((Irwin Fedriansyiah/Associated Press))

The pilots of a doomed Indonesia airliner were so engrossed in troubleshooting a balky navigation system they didn't notice the automatic pilot was off and the plane was entering a fatal spiral, investigators have concluded.

Indonesia's National Transportation Safety Committee reported Tuesday that Adam Air Flight 574, a Boeing 737 with 102 people aboard, crashed into the sea on New Year's Day 2007 because of "a combination of factors including the failure of the pilots to adequately monitor the flight instruments, particularly during the final two minutes of the flight."

There were no survivors on the flight that had taken off from the main island of Java and was headed to an airport in the east of Indonesia. Flight data recorders were recovered nearly eight months later from wreckage at a depth of 2,000 metres.

"There was no evidence that the pilots were appropriately controlling the aircraft, even after the bank alert sounded as the aircraft's roll exceeded 35 degrees right bank," the safety agency said in a statement.

The plane banked 100 degrees to the right with its nose down, reaching speeds of more than nine-10ths the speed of sound and stress levels 3.5 times normal gravity before starting to break up, the agency said. Sound travels at about 1,225 km/h at sea level.

The navigation problem "diverted both pilots' attention from the flight instruments and allowed the increasing descent and bank angle to go unnoticed," the agency said. "The pilots did not detect and appropriately arrest the descent soon enough to prevent loss of control."

There is plenty of evidence the plane's navigation system was troublesome. Pilots and mechanics had logged 154 defects related to it between October and December 2006, the agency said.

The plane crashed after the pilots "became engrossed with troubleshooting" the system and paid "minimal regard to other flight requirements," it said.

In a decision not explicitly linked to the crash, Indonesia last week grounded all planes operated by Adam Air, a discount carrier, because of safety concerns.