It's a hard time to be in airline public relations, maybe particularly United Airlines' (UAL) PR department. First, the great leggings controversy — when two teenaged girls were not allowed to board a flight because they wore leggings — and, caught on video, a passenger was physically dragged through the aisle by two police officers after refusing to leave an overbooked flight. People weren't especially impressed by the airline's handling of the situation, and the airline has now changed its policy.

Airlines deal with a huge number of people every day and it is perhaps fair to admit that employees are only human. But sometimes incidents occur, whether due to interpretation of rules customers don’t agree with or simply bad judgment. What’s important is how that unravels and how the company reacts.

With social media, the challenge is greater for airline companies; individual events that capture the world's attention can go viral and create a public relations nightmare for the airline.

Here are a few instances in the recent past where unhappy passengers have taken on big airline companies. We're leaving out the times when companies have faced customer ire for mishandling technical glitches or inclement weather delays. These you couldn't predict on a crisis-prevention spreadsheet.

A Disinvited Passenger's Road to Stardom: United Airlines

In April, 2017 a United Airlines flight from Chicago to Louisville was overbooked, and four people were asked to leave the aircraft. Three out of four complied, the fourth did not, and United wasn't having it. The airline contacted the police department, and David Dao — a 69 year old doctor from Kentucky — was forcibly removed from his seat, physically dragged through the aisle by his arms and ended up in the hospital. The man ended up with a concussion, a broken nose, and lost two teeth.

Fellow passengers — many clearly distraught over Dao's treatment — documented the incident on their phones. Within 24 hours the videos went viral and was trending for days after the event, and United (UAL) shares tumbled — though a quick recovery followed.

United initially apologized for the "overbook situation" in a statement, but referred questions about the handling of the man to the police department. Even though one of the officers involved was placed on leave, United's response was not received well by the public. Social media sites were flooded with "anti-United" memes and slogans. After some time, Oscar Munoz — United's chief executive — issued a second statement taking full responsibility for the incident, but it seems the damage was already done.

United eventually settled a suit filed by Dao for an undisclosed amount, and the airline now has a policy which says not to use law enforcement to remove passengers from planes unless it's necessary for safety.

Legging It Go: United Airlines

Shannon Watts, according to CNBC, is the founder of Moms Demand Action, a gun-control group born from the aftermath of the Sandy Hook shooting. Her Twitter handle has over 34,000 followers. As Watts’ tweets went viral, the airline tried to come up with a coherent answer. In a statement made later, it responded that the passengers who were turned away or forced to change were "pass riders" required to follow certain rules. Pass riders are employees and family/friends of employees enjoying flight benefits, and the airline said they had to adhere to a dress code as they represented the airline. "To our regular customers, your leggings are welcome," United concluded.

Airlines do have the right to refuse to board anyone whom they deem not dressed up to standard. And this isn't the first time airline corporate standards of decent attire has clashed with what customers regard as acceptable. Among other wardrobe-related airline incidents: Green Day’s lead singer Billie Joe Armstrong famously tweeted in 2011 that he was deplaned by Southwest Airlines (LUV) for saggy pants. ABC reported that company apologized and issued a statement that the matter had been resolved.

A Cat’s Terminal Tale: American Airlines (AAL)

Also in 2011 Jack the Cat was lost at New York’s JFK terminal just before he was due to board a flight to San Francisco. It seems that the kennel Jack was flying in toppled and the cat ran. No amount of searching, including enlisting a pet detective, according to a report by CNN, was able to locate the missing feline. His owner, Karen Pascoe, turned to social media launching a Facebook page – Jack The Cat is Lost in AA Baggage at JFK that got over 24,000 likes.

Nearly 12 days later, Jack was found when he fell out of the ceiling of one of the customs offices at the airport. The airline jumped to salvage the situation and offered to pay for the vet bills. What looked like a happy ending turned sour when the cat eventually died a little later of the aftereffects of severe malnutrition and dehydration.

Fretting About a Broken Guitar: United Airlines

United Airlines took another hit back in 2009 when, unable to get the airline to process a claim for his damaged guitar, Sons of Maxwell singer-songwriter Dave Carroll, wrote a song about it – United Breaks Guitars.

Carroll was on a flight from Canada to Nebraska in 2008 when he heard a passenger scream about someone throwing guitars as they landed for their layover at Chicago. When Carroll’s band mate peered outside the airplane window, he saw his bass being manhandled. Unable to register a complaint with the airline then, the duo flew to their final destination before realizing Carroll’s guitar had been damaged.

His website provides a blow-by-blow description of what transpired over the next nine months, including when United rejected his offer to cover by flight vouchers the $1,200 he spent on getting the guitar repaired. As the incident grew, the song got two sequels – and all of them went viral on YouTube.

The airline ended up donating $3,000 to a charity of Carroll’s choice to call a truce, reported the Denver Post.

Too Fat to Fly: Southwest Airlines

Filmmaker Kevin Smith was buckled into his seat on a Southwest flight in February 2010 when he was ejected from the flight – for taking up too much space. Smith shot off a series of tweets starting his boycott of the airlines. Smith who, according to CNN, had more than 1.6 million followers at that time, began what quickly blew into a PR storm for the airlines.

Southwest’s official Twitter handler responded, but the outrage had already begun. Smith vowed never to fly Southwest again. His story got some happier media attention six years later when he ended his boycott and flew with the airlines to get to a convention.

That’s not the only instance where physical characteristics have caused passengers to be thrown off the flight. NBC News reported that in 2011, a man was asked to get off a Horizon Air plane for being too tall. At 6’9" the man, according to the report, had reached out to the airline in advance to request a seat with more legroom. But not only did the airline not heed his request, he had to miss his flight for not being able to cram his legs into a small space.

Differential Existence: Aeromexico/American Airlines

Waris Ahluwalia, a Sikh-American model, designer and actor wears a turban as a part of his faith. Last year, he was denied boarding on an Aeromexico plane and asked to buy the ticket for another airline because he refused to remove his turban for a security check, reported CNN. The actor channeled his outrage through his Instagram account, posting pictures with his boarding pass and that immediately got attention in social media, traditional media followed suit. The CNN report said that the airline later issued an apology.

Another incident last year caused a Syracuse-bound American Airlines flight from Philadelphia to be delayed by over two hours. A passenger passed a note to the flight attendant claiming she was ill, but later disclosed that her foreign-looking co-passenger scribbling strange things on paper made her uncomfortable said an article in the Washington Post. While the informant opted to fly on another American plane, the subject of her discomfort was questioned by ‘some sort of agent.’ The man as it turns out was a highly decorated Italian mathematician solving a differential equation.

While airlines figure out how to rewrite the response book, customers are learning that social media is a carryon no passenger should be without.

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