UNITED Airlines Chief Executive Oscar Munoz said less than one passenger was involuntarily denied boarding per 23,000 on an average last year, responding to questions from lawmakers regarding the rough removal of a passenger from one of its flights.

The airline uses offers to encourage volunteers to deboard a plane and had 16 volunteers for each passenger who had to be involuntarily denied boarding, Munoz said in the letter to the U.S. Senate Commerce Committee late on Wednesday.

Last year, United involuntarily denied boarding to 3,765 customers across more than 86.8 million mainline passengers in the airline, Munoz said.

The airline had a Thursday deadline from the committee to answer detailed questions about the incident on United Flight 3411, from which 69-year-old Vietnamese-American doctor David Dao was dragged off by airport security officers to make space for the four crew members on the flight.

Dao was hospitalized and, according to his lawyer, lost two front teeth, broke his nose, and suffered a concussion, and will likely sue the airline.

The flight from Chicago to Louisville on April 9 saw the four Republic Airlines crew members arrive at the gate around the time of boarding and United offered increased compensation for volunteers onboard due to the last minute nature of the request, Munoz said in the letter to the Senate committee.

When the request for volunteers was unsuccessful, the United crew followed an "involuntary denied boarding process," he added.

Four passengers were then selected to deboard the plane, with fare class and domestic itineraries as the deciding factors.

Separately, United Airlines said on Thursday it would offer passengers who volunteer to forfeit their seats on overbooked flights up to $10,000 and limit the use of law enforcement to safety and security issues alone.

The company also said traveling crews are to be booked onto a flight 60 minutes before departure.