(CNN) -- When it comes to air travel, just about everyone has a complaint, no matter which security line they use.
Passengers are tired of long lines, baggage fees and last-minute delays. Airline employees and flight attendants could do without the cranky travelers who refuse to wait patiently, turn off cell phones or stay in their seats.
Sometimes that frustration escalates into "air rage" incidents that still disturb the friendly skies post-September 11. Reported instances of unruly passengers rose internationally about 29% between 2009 and 2010, following an estimated 27% rise between 2008 and 2009, according to the International Air Transport Association, which represents about 240 airlines worldwide.
The numbers are a small part of the picture because they only include reported instances. They don't count all the times a member of the flight crew manages to calm an anxious flier or successfully mediates disputes over seats or armrests.
In the United States, the Federal Aviation Administration keeps numbers on unruly passengers who interfere with the duties of a crew member, but reporting is left to the discretion of the crew. Numbers related to security violations are not included because those cases are handled by the Transportation Security Administration, which keeps its own data.
But even those statistics may or may not include incidents in which the crew requested that local law enforcement meet the plane and take control of a passenger, an FAA spokeswoman said.
"Air rage has remained a problem post-September 11, although it is much more subtle than before the attacks," said University of Akron assistant professor Andrew R. Thomas, who keeps track of incidents on his site, AirRage.org.
In his book "Soft Landing: Airline Industry Strategy, Service and Safety," Thomas wrote that in the 24 months before September 11, there were 30 documented cases of unruly passengers partially or completely entering the cockpit.
"Today, because of heightened security awareness and the reinforced cockpit door, air rage is likely to involve a confrontation between a disruptive passenger and a member of the flight crew or another traveler," he said.
Yet, apart from the occasional Alec Baldwin-size disturbance, aviation experts said the impact of disruptive passengers on day-to-day operations is barely perceptible. Since 2006, when 1,417 unruly passenger incidents were reported to the TSA, the number has fluctuated on a mostly downward trend, hitting 1,218 in 2011. As of May 1, 385 unruly passenger incidents had been reported to the TSA.
Those figures include passengers being disruptive, intoxicated or confrontational with security screeners, law enforcement officers, airport employees or passengers in airports and on aircraft. It also includes times when a pilot requested law enforcement to meet the aircraft because of an unruly passenger.
A similar trend is reflected in the FAA's unruly passenger statistics between 1995 and 2011, with the number of incidents peaking at 330 in 2004 before gradually dwindling to 131 last year.
"More than 28,000 flights take off each day in the United States, so if you look at the statistics, they're pretty small in relation to the big picture," said Kees Rietsema, department chair of business administration at Embry-Riddle Aeronautical University.
"It's not something the crew stresses about on a day-to-day basis. Their concerns are basically the same as in the past: staying on schedule, weather, congestion at airports."
From security pat-downs to the behavior of other travelers, a number of factors can provoke anxiety. But usually, the addition of alcohol to the mix is what tips the scales into rage territory, said Ronald Carr, assistant professor of aviation physiology at Embry-Riddle.
A report released in the United Kingdom in 2008 found that incidents of air rage had soared from 696 in 2003-2004 to 2,702 in 2007-2008. The figures from the Civil Aviation Authority and the Department for Transport revealed that alcohol consumption and smoking were the main factors in nearly 63% of reported incidents, and 78% of cases involved male passengers.
Americans of a certain age might recall a time when airlines were the gold standard of customer service, when champagne flowed freely and every passenger received a hot meal. It was also the era when air travel was strictly the domain of the wealthy and business passengers, and family travel was reserved for the most special of occasions.
Service began to deteriorate with the deregulation of the industry in 1978 and continued into the 1990s with the arrival of bargain airlines, airline industry expert Joyce Hunter wrote in her book, "Anger in the Air." Airlines across the board were forced to cut prices to keep up, leading to cutbacks in service and amenities that had defined the golden era of air travel.
The term "air rage" entered the public consciousness in the 1990s, spurred by a rash of incidents, culminating in the death of Jonathan Burton in 2000 at the hands of fellow passengers attempting to restrain him as he rushed the cockpit door.
Between 1994 and 1997, the number of air rage incidents reported around the world had more than quadrupled from 1,132 to 5,416. By 2000, the number of air rage incidents in the United States topped 10,000, Hunter said in her book.
"(September 11) has compounded the growing epidemic of passenger rage, while making it more urgent than ever that airlines around the world find ways to prevent it," Hunter said.
"Airlines used to brag of 'something special in the air.' On September 11, that 'something' turned into a toxic mix of fear and anger that would slowly seep into the hearts and minds of airline travelers and personnel alike, further eroding the civility of travel that was once an integral part of the industry's culture," she wrote.
The number of incidents appeared to dwindle in the years following September 11, only to return to an upswing in the mid-2000s. In 2010, the International Air Transport Association asked the U.N. body representing the world's aviation regulators to take a look at management of unruly passengers in light of evidence that most rarely face official action.
"It's a matter of getting the contracting states around the table and acknowledging this is a serious issue: that the number of unruly passengers is on the rise and it needs to be decided how countries address this," Steve Lott, former International Air Transport Association head of corporate communications in North America, told Orient Aviation magazine in 2010.
Flight attendant Heather Poole has witnessed firsthand growing frustration among passengers in the past decade, and she said it's worse than ever.
"Passengers are far less patient today than ever before. This makes them quicker to snap," said Poole, author of "Cruising Attitude."
"Before September 11, if we asked passengers to remain seated to let those with tight connections to get off first, people would stay seated. Now no one cares. People practically push each other over to get off and on the plane. It's sad to see."
Cell phones and alcohol tend to be the most common triggers of confrontations with passengers. In the past decade, training has focused on the cabin crew's role in defusing a situation to avoid the need for drastic action, such as diverting a plane or calling law enforcement to meet the aircraft, she said.
Flight attendants are compelled to respond more quickly to a potential threat but also to do whatever it takes to keep the situation from escalating.
"We have to be more patient than ever before. It's not easy. We're human, too," she said. "When you're trapped in a tube at 30,000 feet, we can't call the cops or the fire department if need be. It's just you and me.
"That's why it's always best to remove a potential problem before the flight takes off. If you really must freak out, do so when we're safe and sound on the ground. No one wants to divert a flight."