A still from a video shot by a motorist shows the TransAsia Airways plane cartwheeling over a motorway just before it crashed.

The captain of a TransAsia Airways ATR mistakenly switched off the plane's only working engine seconds before it crashed in February, killing 43 people, Taiwan's Aviation Safety Council (ASC) said in its latest report on Thursday.

The ASC's report also showed that Captain Liao Jian-zong, who was at the controls, had failed simulator training in May 2014, in part because he had insufficient knowledge of how to deal with an engine flame-out on take-off.

"Wow, pulled back the wrong side throttle," Liao, 41, was heard to say on voice recordings seconds before the crash.

There appeared to be confusion in the cockpit as the two captains tried to regain control of the plane after the other engine lost power about three minutes into the doomed flight.

Liao reduced the throttle on the working engine but did not appear to realise his mistake until it was too late.

He tried to restart the engine before a junior first officer, who was also in the cockpit as part of his training, said: "Impact, impact, brace for impact."

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Those chilling words were the last heard on the data recordings, according to the latest report of the ASC's investigation into the February 4 crash in clear weather.

Seconds later the ATR 72-600, which had 58 people on board, crashed upside down into a shallow river in downtown Taipei after it lurched between buildings, clipping an overpass and a taxi. Fifteen people survived.

A source with direct knowledge of the report told Reuters on Wednesday the working engine had been shut off. Data readings showed the almost-new turboprop ATR 72-600 stalled and crashed shortly after it was switched off.

TransAsia had no immediate comment about the latest findings but was due to hold a media briefing later on Thursday.

The council's 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.

Sources familiar with the investigation have previously told Reuters the crash was probably caused by human error.

The investigation so far has shown that Liao, a former air force pilot, began to fly commercial aircraft in 2009 and joined TransAsia the following year. He was promoted to captain in August 2014 and joined the ATR 72-600 fleet in November.

He had a total of 4914 flight hours at the time of the crash, including 3151 in the ATR 72-500 and 250 hours in the ATR 72-600.

However, the report showed that Liao failed the simulator check in May 2014 when he was being evaluated for promotion. Assessors found he had a tendency not to complete procedures and checks, and his "cockpit management and flight planning" were also found wanting.

However, he passed after a second simulator check on June 29 and 30 and was promoted to captain, although similar problems were detected during training from July 2 to 10 last year.

Instructors commented at the time that he was "prone to be nervous and may make oral errors during the engine start procedure", displayed a "lack of confidence" and was "nervous", the report shows.

Issues cropped up again during training for the ATR 72-600 in November, when an instructor said Liao "may need extra training" when dealing with an engine failure after take-off.

Since the crash, Taiwan's Civil Aeronautics Administration 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.

A draft of the final report will be issued in November with the final report to be completed in April 2016, the council said. The cause of the crash and recommendations for the future will be included in the final report.