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Dao, a physician who claimed he had appointments with patients the next day, is now undergoing treatment for his injuries at a Chicago hospital.
The backlash from the incident resonated around the world, with social media users in the United States, China and Vietnam calling to boycott the No. 3 USA carrier by passenger traffic. When that did not work, they offered $800 per seat.
United Airline's CEO Oscar Munoz apologised to Dao and the other passengers, describing the incident as a "system failure" and said the airline would reassess its procedures for seeking volunteers to give up their seats when a flight is full.
"I think a key lesson for United is to make sure their communications both internally and externally are consistent and considered", Mr Holdsworth said.
Video of that incident, shot by other passengers, instantly went viral, prompting Munoz's clueless statement, in which he also promised to reach out to the passenger. That man later said he quit his job because Dao "pursued him aggressively" and arranged to provide him with prescription drugs in exchange for sex. Usually, passengers go voluntarily, lured by cash or voucher incentives offered by the airline.
The announcement Wednesday from the city's Aviation Department comes two days after another officer involved in the Sunday night confrontation was put on leave. Much of the social media uproar stemmed from Dao's status as a paying passenger who was being removed to make room for additional crew members on the overbooked flight.
Dao's relatives are focused only on his medical care, attorney Stephen L. Golan said. Sara Nelson, global president of the flight attendants union representing United, said the incident was the most severe customer backlash she'd seen in 20 years on the job and was "completely unacceptable". United has not said precisely how the four people asked to leave Flight 3411 were selected.
"It's always hard for people working at an airport if you have to go on and get people off an airplane", he said. Two more officers were suspended Wednesday.
There stood the passenger who had been dragged, now identified as Dao, on his back to the front of the plane, appearing dazed as he spoke through bloody lips and blood that had spilled onto his chin. United was trying to find seats for four employees, meaning four passengers had to deplane.
Three people got off the plane, but the man in the video refused, reportedly telling officials he was a doctor and needed to treat patients the next day. According to United's Airlines policies, all flights are subject to overbooking.
That eventually led to the video everybody has seen - a 69-year-old man being dragged off the plane by security officers after refusing to give up his seat.
Munoz, who along with his fellow legacy airlines, believe the Middle East carriers are government-subsidized, had trolled the Gulf airlines last month when he was quoted as saying "Those airlines aren't airlines".