The two pilots were trying to fix a problem with the plane's navigation instruments when they disconnected the device and lost control, government investigators found.

'Without the autopilot, the plane went out of control, listing to the right and pitching down,' investigator Santoso Sayogo told a press conference Tuesday.

The Boeing 737-400 belonging to the budget carrier, which was barred from flying this month over safety worries, was carrying 96 passengers and six crew when it plunged into the sea on January 1, 2007.

Transport Minister Jusman Syafei Djamal said Adam Air had registered 154 defects in the Boeing 737-400's navigation system in the three months before the crash, showing the planes were poorly maintained.

'The accident happened because of a combination of several factors, including the failure of both pilots to intensively monitor flight instruments, especially in the last two minutes of the flight,' Indonesia's transport safety chief Tatang Kurniadi added.

The investigation was based on information from the plane's 'black box' flight recorders, recovered from the sea last August.

Another investigator said the plane was travelling at 10 times the normal landing speed when it hit the water and would have broken up on impact. No bodies were ever recovered.

Indonesia imposed a three-month flying ban on Adam Air after uncovering 'violations that could put passengers' safety at risk'.

The move followed a series of incidents that raised doubts over the airline's safety record, most recently when an Adam Air Boeing 737-400 with 175 people on board skidded off the runway in foul weather this month.

Last year, all Boeing 737-300 aircraft operated by the airline were grounded temporarily after the fuselage of one plane cracked on landing, and in 2006 an Adam Air plane became lost for several hours, eventually landing many miles from its intended destination.

The three-month ban is part of a push by Indonesia's government to improve the country's air safety record following a series of fatal accidents blamed on lax enforcement of safety regulations, poor maintenance and a lack of investment in transport infrastructure.

Last March, 21 people were killed when a jet from national carrier Garuda skidded off a runway.

Shortly after that, the European Union banned all 51 Indonesian airlines from its airspace over security concerns and the United States advised its citizens not to use them.

Transport minister Djamal said the government had stepped up its monitoring of the country's airlines and was now conducting quarterly inspections as part of efforts to overturn the EU ban.

'We will provide feedback to the airlines to enable them to improve their safety procedures,' he said.

'We will also impose an immediate operating ban if there is any indication that an airliner may be putting passengers' lives at risk.' -- AFP