United Airlines, which once encouraged passengers to fly its "friendly skies," is now in danger of becoming the air carrier that says: "Do what we say or we'll beat you up."

Talk about your unusual corporate message.

Nevertheless, that's a signal United is sending to the world in the aftermath of a huge customer service debacle. On an overbooked United Express Chicago-to-Louisville flight Sunday, a passenger who seemed to be minding his own business was ordered to leave the plane. After refusing, he was yanked out of his seat, roughed up and then literally dragged - yes, dragged on his back - from the plane by security personnel.

There's video shot by his fellow passengers all over the internet.

The flight was operated by Republic Airways with a Republic crew that was following United's rules and procedures.

Keeping in mind that other facts could emerge, this sure looks like airline personnel goofed up big time by escalating an already difficult and testy situation. The airline's top brass owes the flying public a quick and candid explanation of why this occurred, what part of its overbooking protocol opens the way for physical man-handling, and how such ugly incidents can be avoided in the future.

If United won't come clean, then aviation regulators, law enforcement or Congress should delve into the matter.

Hours after the incident, United wasn't saying very much. On Twitter, CEO Oscar Munoz said it was an "upsetting event to all of us" and he apologised for having to "re-accommodate these customers."

OK, Mr Munoz, there's no shortage of issues to delve into.

Most commercial flyers concede that airlines have a legal right (it's in the fine print that none of us ever read) to overbook flights and ask passengers to leave if there's too many on the plane. Typically, airlines sweeten that process by giving flying vouchers, sometimes hotel and transportation credits, to entice someone to get up and reschedule.

United says this flight was overbooked by four people and it had to get four airline crew members aboard and to their destination, which apparently was crucial to the system's flight operations. Monetary incentives were unsuccessful, so the airline selected four flyers to deplane.

Three of the passengers left when asked, but the carrier couldn't get a fourth person, who said he was a doctor and had to see patients in Louisville, to disembark. He insistently refused to deplane for reportedly up to 10 minutes or so. The onboard ruckus soon broke out.

While it's an unusual set of circumstances, there are some commonsense questions that beg to be addressed:

• Why didn't the airline raise its offer? If the $US800 that was finally offered wasn't enough, then up the ante. It seems inconceivable that every passenger would pass on a more lucrative deal. If it was essential to get all four airline workers on that flight, then wasn't it worth more to make a deal?

• Isn't part of the process knowing how to count the number of seats versus the number of passengers, including crew? Why weren't the overbooked passengers stopped at the gate?

• How was this passenger picked? The airline says by computer. Hate to say it, but would they have rousted someone from first class this way? After all, the airlines are making a pretty big deal about how much they charge you for sitting up front, by an aisle or window, or in back of the plane.

• Why did the security people react so strongly? What's the protocol for removing a passenger by force? Who makes that decision? Is it security team, pilot, airline boarding staff? Don't you treat an obstinate passenger differently from one who is acting out and becoming a danger to others or themselves?

• What rights does this injured passenger have? In an era when flying is increasingly tight, stressful and potentially confrontational, it would be good to know how far airlines can go and if passengers have the right of appeal. Did anyone say a congressional Passenger Bill of Rights?

Admittedly, this can get pretty granular but if United is going to investigate, it needs to get into these gritty details and make its findings available online to its many, many customers.

One more thing: Some airline executives are publicly saying that the stubborn customer acted immaturely, intimating that this was reason enough to take such drastic enforcement steps.

In a perfect travelling world, the guy would have gotten off the flight and everybody would have moved on.

Yet I'd counter that it was also the airline and security staff who acted impetuously by failing to take the extra measures needed to de-escalate a tough situation that turned violent and is becoming, at the very least, a public relations catastrophe for United.

Let's face it: Having a passenger mugged for not giving up an airline seat isn't anyone's idea of flying the friendly skies.

Chicago Tribune