TransAsia pilots have begun taking proficiency tests days after one of its ATR 72 jets crashed into a river in Taiwan.

Aviation officials said 71 pilots are being tested on how they handle engine failure and respond to other emergencies.

Initial probe into the crash revealed one of the plane's engine went idle and lost power shortly after take off from Taipei. But the pilot decided to shut off the other running engine and attempted a restart --- a move aviation experts said was a grave mistake.

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Aviation officials said commercial aircrafts can fly with just one working engine. But authorities have not released any information that would explain why the pilots decided to shut down one working engine. 

The Civil Aviation Authority (CAA) has not ruled out "pilot error" as the cause of the crash. 

"There must have been something wrong with what the crew did," said an official from the CAA, who refused to be identified.

The CAA said only TransAsia pilots flying the ATR jets are required to take the proficiency exams. Pilots from other major airlines were not tested.

The retraining of pilots was questioned by a pilots' union, saying crashes are caused  by multiple factors.

"The CAA and the ASC can't just jump to a conclusion like that," said Lee Ping-chung, Union's Secretary General.  "It could be mechanical, the weather, airline's management of pilots and how tired pilots are."

The results of the tests are expected to be released on Wednesday.

TransAsia had canceled 52 more flights set for Monday and Tuesday, on top of the 90 flights it had already canceled. The flight cancelations were made so pilots can take the tests.

The death toll still stands at 40, 15 people survived, while three people remain missing. 

TagsTransAsia, Taiwan, plane crash, proficiency tests, TransAsia pilots