The latest investigation report into the Taipei crash, to be released on Thursday, will say data readings showed the almost-new turboprop ATR 72-600 stalled and crashed shortly after the functioning engine was switched off, said the source.

The findings of the report, by the Aviation Safety Council, will also focus on flight operations, air traffic control, weather, the air worthiness of the plane and other factors, added the source, who could not be identified because the report has not yet been made public.

TransAsia declined to comment on the latest findings.

The council report, which neither assigns responsibility nor suggests recommendations for improvement, paints a more detailed picture of the evidence than a preliminary report released days after the crash.

In the preliminary report released in February, the council said one of the plane`s two engines failed but the pilot, for reasons unknown, shut the other functional engine, causing the plane to stall and crash.

The plane, which can fly on one engine, was carrying 58 passengers and crew when it lurched nose-up between buildings, clipped an overpass and a taxi with one of its wings and then crashed upside down into a shallow river in Taipei. Fifteen people survived.

"UNBELIEVABLE, UNFORTUNATE"

While the reason for second engine being shut down remains unclear, other sources familiar with the investigation told Reuters human error was probably behind it.

"The pilots made a mistake here. What makes this even more unbelievable, and unfortunate, was that the mistakes took place even though there were three pilots in the cockpit," said one of the sources, who declined to be identified because of the sensitive nature of the topic.

"That is why the focus of the investigation has been narrowed to the pilots and pilot training at TransAsia," the source added.

Since the crash, Taiwan`s Civil Aeronautics Administration (CAA) has put all 55 of TransAsia`s ATR pilots through oral proficiency tests on how to handle an aircraft during engine failure.

All but one of the pilots passed the tests, although some needed more than one attempt. The lone failure was demoted in rank to vice captain from captain.

"It was difficult to believe the captain turned off the wrong engine, but it happened anyway. That`s why tests were demanded of TransAsia`s ATR pilots," said another source, who also requested anonymity.

A third source said TransAsia, in addition to cancelling some flights, had made a number of reforms since the crash to boost safety measures.

These included inviting Jon Beatty, chief executive of the U.S.-based non-profit Flight Safety Foundation, to bring his team to Taiwan shortly after the crash.

TransAsia also set up an internal flight safety panel, which included Beatty, to report to its board, the source said.

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