All TransAsia Airways pilots who fly ATR propeller jets are taking proficiency tests after one of the carrier's planes crashed in Taiwan, killing at least 39 people.

The airline said it had cancelled 90 flights over the next three days to accommodate the tests, which have been ordered by Taiwan's Civil Aeronautics Administration.

A total of 71 TransAsia pilots are involved in the programme.

Investigators believe the two pilots of the ATR-72 that went down in Taipei on Wednesday switched off a running engine after the other one went idle.

Experts say this was not part of the accepted procedure under such circumstances.

"It's a mistake," said John Cox, a former US Airways pilot and now head of a safety-consulting company.

"There are procedures that pilots go through - safeguards - when you're going to shut down an engine, particularly close to the ground. Why that didn't occur here, I don't know."

Local prosecutors have said they will be looking into the possibility of "professional error".

Moments before the plane banked sharply and hurtled into the Keelung River, one of the pilots reportedly told air traffic controllers: "Mayday, mayday, engine flameout."

Fifteen people survived the accident, which happened shortly after take-off and was captured on video by a passing motorist's dashboard camera.

Four more bodies were recovered on Saturday morning as grieving relatives looked on, taking the number of dead to 39.

Four of the 58 people on board the flight, bound for the island of Kinmen, are still missing.

The dead include the pilots, whose actions suggest they followed the line of the river to avoid high-rise buildings and banked sharply to bring the plane down in the water rather than crash on land.

Both have been hailed as heroes.

The crash, earlier this week, was captured on a car's dashboard camera.

Originally posted at 13.31