Last Updated Feb 6, 2015 5:34 PM EST
TAIPEI, Taiwan -- One of the two engines on TransAsia Airways Flight 235 went idle 37 seconds after takeoff, and the pilots apparently shut off the other before making a futile attempt to restart it, Taiwan's top aviation safety official said Friday.
It was unclear why the second engine was shut down, since the plane was capable of flying with one engine. Taiwan's official China News Agency said investigators were looking into the possibility of "professional error."
Wednesday's crash into a river in Taipei minutes after takeoff killed at least 35 people and left eight missing. Fifteen people were rescued with injuries after the accident, which was captured in a dramatic dashboard camera video that showed the ATR 72 propjet banking steeply and scraping a highway overpass before it hurtled into the Keelung River.
There would be no reason to have shut down the good engine, experts said.
"It's a mistake," said John M. 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."
Multi-engine planes, whether jets or turboprops like the ATR, are designed to fly on one engine. When an engine quits, one technique that pilots often use, Cox said, is to identify and tell each other which engine is still running, then for one of them to place a hand behind the throttle controlling that good engine - guarding against an accidental shutdown.
Cox said it is too early to draw certain conclusions but it's likely that the crew's failure to control the plane and shutting down the operating engine "will be part of the causal factors to this accident."
The details on the engines were presented at a news conference in Taipei by Aviation Safety Council Executive Director Thomas Wang as preliminary findings from the flight data recorder.
Wang said the plane's right engine triggered an alarm 37 seconds after takeoff. However, he said the data showed it had not shut down, or "flamed out" as the pilot told the control tower, but rather moved into idle mode, with no change in the oil pressure.
Then, 46 seconds later, the left engine was shut down, apparently by one of the pilots, so that neither engine was producing any power. A restart was attempted, but the plane crashed just 72 seconds later.
Several Internet aviation sites, including Flightradar24, questioned whether the pilots may have mistakenly turned off the wrong engine in an attempt to restart the idled one.
Anthony Brickhouse, a safety-science professor at Embry-Riddle Aeronautical University, said investigators won't really know what happened to the engines until they do a "tear-down" and actually examine them - not just rely entirely on information from the flight-data recorder - to determine whether one or both were still producing power.
TransAsia said in a statement that all 71 of its ATR pilots would retake proficiency examinations as requested by the Civil Aeronautics Administration.
The pilot had 4,900 hours of flying experience, said Lin Chih-ming of the Civil Aeronautics Administration.
Taiwanese Vice President Wu Den-yih, mindful of the island's reputation as a tourist destination and its tense relations with China where most of the flight's passengers were from, went to a Taipei funeral parlor for prayer sessions to pay respects.
At the parlor, where bodies are being stored, Wu expressed condolences and praised pilot Liao Chien-chung, who died in the crash. The pilots may have deliberately steered the plane away from buildings and into the river in the final moments.
"When it came to when it was clear his life would end, (the pilot) meticulously grasped the flight operating system and in the final moments he still wanted to control the plane to avoid harming residents in the housing communities," Wu said.
"To the plane's crew, the victims ... I here express condolences."
Divers with a local fire agency found one female and three male bodies Friday along the muddy Keelung River bottom about 50 yards from the crash site, a Taipei City Fire Department official surnamed Chen said.
The agency suspects the eight bodies that are still missing may be in equally murky areas and has sent 190 divers to look for them. Taiwan's Ministry of National Defense dispatched three S-70C rescue helicopters to search along a river system that runs into the ocean off Taiwan's northwest coast.
More than 30 relatives of victims cried wildly, prayed or were comforted by Buddhist volunteers at the riverside crash site as divers in black wetsuits brought back the four bodies. Some divers came ashore with their hands joined in prayer for the people they brought back.
The pilot's and co-pilot's bodies were found earlier with their hands still on the controls, Taiwan's ETToday online news service said.
Wang said the engines had shown no problems before the flight and repeatedly stated that the plane would have been able to take off and fly even with only one engine working.
Evidence that the TransAsia pilots may have shut down the wrong engine drew comparisons with the 1989 crash of a British Midland Airways Boeing 737 jet shortly after takeoff from London's Heathrow Airport.
In that accident, a fan blade failure in the left engine led to vibrations and smoke and fumes in the cockpit. The pilots believed that the right engine had failed and reduced power to it, which caused the vibrations to stop, convincing the crew that they had identified the troubled engine. As the pilots tried to make an emergency landing, the left engine quit, and attempts to restart the right engine failed. The plane crashed a half-mile short of the runway, killing 47 people; 79 survived.