When United Airlines had police forcibly remove a paying customer from his plane seat, dragging him down the aisle, the social-media response was immediate and global. United said the man was ejected because it needed his seat for an employee who was required in Louisville for duty the following day. The incident raises a host of legal, regulatory and corporate-branding questions:

1. Can airlines just throw seated passengers off planes?

Yes. When you purchase an airline ticket you're also entering into a detailed contract that specifies the many rights an airline has related to the trip. One of them allows the carrier to seize your seat in return for compensation and an alternate means of transport, usually on a later flight. Airlines also can determine that when their employees must be somewhere for duty -- a status that, yes, may place them ahead of paying customers -- a passenger's seat can be taken. That's because not positioning a crew member can lead to a future flight cancellation or delay.

2. Was that United's explanation?

That was the situation on April 9 in Chicago, when United needed to get its staff to Louisville. United also says the passenger who became a social media sensation had been "disruptive" and "belligerent." U.S. law prohibits passengers from threatening or interfering with airline crews, a standard that has been aggressively enforced since the September 11 terrorist attacks. To be sure, if you are violent, belligerent, intoxicated or aggressive toward a crew member or being disruptive or disorderly toward other passengers, it is a safe bet that you will be booted from the plane. And the police may be summoned. Thatcher Stone, a lawyer who has successfully sued airlines over bumping incidents, advises that passengers should always follow instructions of airline crew, no matter how strongly they may feel.

3. What rules apply when a passenger is bumped?

The U.S. Department of Transportation permits airlines to ask passengers to give up a reservation voluntarily in exchange for some compensation and a seat on a later flight. That typically means a voucher for a future purchase. Airlines solicit volunteers and start their offers low, generally US$150 or US$200, gradually increasing the compensation to encourage enough travelers to offer a seat. (United had reportedly offered US$800 on the flight to Kentucky.) If not enough volunteers come forward, airlines then go the involuntary route, which regulators call "involuntary denied boardings." When choosing who gets the bump, airlines typically assess your value: How often do you fly, how much did you pay for the ticket, how many miles have you amassed in the loyalty program, is there a connection in your trip? Are you headed to Denver or Tahiti? How many more flights remain that day to the destination? Are there minors flying? And yes, this process can occur even after an airplane has been boarded. On April 10, the DOT said it would review the denied boarding procedures followed on the United flight.

4. What recourse do passengers have?

First, such passengers are entitled to a "written statement describing their rights." Stone said in lawsuits he's brought against airlines, they most often didn't bother to give passengers this statement. The compensation an airline must pay increases with the length of time a passenger is delayed reaching his or her destination. If the carrier can get you to the destination within an hour of the scheduled arrival, there's no compensation. If it takes longer than two hours (or four hours on an international flight), then the airline must pay four times the airfare or as much as US$1,350. You may also be able to score some meal vouchers or access to an airline's lounge, depending on the situation. These are cases where being polite generally wins out over threats or shouting.

5. Why do airlines overbook, anyway?

Overselling is an airline's response to a routine occurrence: People don't always appear for a flight they've purchased. Selling more tickets than a plane's seat count is a method to address that situation. In a consent order last summer, the Transportation Department explained its view of oversales as a way for airlines "to fill seats that would otherwise have gone empty due to 'no shows,' thereby achieving operational efficiencies including revenue enhancement for carriers, and resulting in benefits for passengers as a whole by enabling carriers to offer them lower fares."

6. How often do airlines bump passengers?

Not too much. Of about 660 million passengers last year who boarded the 12 largest U.S. airlines, about 475,000 gave up their seats or were bumped. The majority of those were volunteers - only 40,600 had to relinquish their seats unwillingly. Because airlines have amassed years of traffic data, down to days, times, seasons and specific routes, they rarely need to give large vouchers to customers. The data also help them know how to tweak their oversales for each flight, which has helped drive involuntary denied boardings to extreme lows: Out of 131 million passengers last year, American Airlines Group Inc. bumped only 8,312 involuntarily, according to federal statistics. As a result of their data, airlines' need to bump has decreased over the past decade and is likely to dip further over time.

7. What have the repercussions been for United Airlines?

United suffered a pummeling on social media immediately after the incident. One tweet joked that the airline had set aside a new seating area known as "fight club." Late-night comic Jimmy Kimmel lampooned the carrier on his show. Its stock dropped as much as 4.4 percent on Tuesday, then recovered almost half of that to US$69.85 at 2:55 p.m. The longer term impact on the company isn't clear.

8. Do all airlines do what United did?

Yes, all airlines occasionally need to swipe a seat from a paying customer. But not all airlines oversell their flights as a way to combat no-shows. JetBlue Airways Corp. never has, following the belief of its founders that every passenger should be guaranteed a seat. In its brief history, Virgin America also eschewed overselling. Its new owner, Alaska Air Group Inc., does oversell so the same practice could extend to the merged company.

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