They can start over something small, an armrest dispute or annoyance at someone stuffing more than their fair share of luggage into an overhead locker.
But fights on planes can escalate dangerously, and according to the body representing most of the world's airlines, unruly passengers have become a significant problem on flights.
READ MORE: • Anger in the air - out come the tasers
The International Air Transport Association and has been collecting data since 2007 and 48,000 cases have been reported to 2015. A decade ago there was one case in every 1600 flights but latest figures show the ratio had worsened to one incident per 1200 flights.
That's a fraction from 100,000 flights every day and while most are minor - verbal abuse or failing to follow the instructions of crew - around 10 per cent involve some physical violence or act that could endanger all those on board.
Tim Colehan leads IATA's work on dealing with unruly passengers and says there is no simple explanation for the rise in anger in the air.
"One suggestion is that its simply a reflection of societal changes where there is greater prevalence and acceptance of anti social behaviour," he told the Herald at the association's annual meeting in Cancun, Mexico.
''The problem for the airline industry is that what might be accepted as acceptable behaviour on the ground has no place in a pressurised cabin in the air where safety and security are absolutely paramount."
Frustration with a flight, such as issues with neighbouring passengers, lack of meal choice, in-flight entertainment or seat, mental health problems, job or relationship issues and being unable to smoke and now use laptops on some flights are among the triggers.
In New Zealand airlines say they have not seen any increase in problems in cabins although Civil Aviation Authority figures show a spike in incidents last year when it issued 67 infringement notices, up from 43 the year before.
Like IATA, the unions believe serious incidents are on the rise, a view also shared by a group representing passengers in the United States, the world's biggest aviation market. But there is disagreement on whether its unruly passengers or unruly airlines who are to blame.
Pent up rage
Flyer Rights spokeswoman Kendall Creighton says there have been years of pent-up rage against the airlines, particularly in the US, which is now being released.
''A lot of the incivility we've seen lately comes from stress. From the moment you arrive at the airport and have to deal with the automated self-check-in kiosks that have replaced human counter staff, to the long lines at security, to the gotcha fees at the gate, then maybe an announcement that your flight's overbooked - and if it isn't, then it's guaranteed to be 100 per cent full," she told the Herald.
The number one complaint her organisation hears from air travellers is shrinking legroom and cramped seats.
''People are tired of being packed into airplanes like sardines. What's particularly maddening is that the airlines have been raking in record profits for seven straight years while putting the squeeze on passengers," Creighton says.
''It's a real pressure cooker now. We're putting 100, 200, 300 people in an aluminium tube, usually designed for far fewer, because the airline industry is always looking to add capacity," she says.
"The result is people feel claustrophobic and flight attendants say they have to de-escalate conflict all the time."
Creighton says airlines in the US are falling behind in dealing with scraps in planes and she's watching with interest an early-stage Air New Zealand trial of a virtual reality headset that can not only feed information on passengers to flight attendants, but also read their emotions.
''Perhaps that's our solution," she says.
IATA's Colehan disagrees on who's to blame.
''We see no correlation between product and unruly passenger behaviour."
More budget carriers had brought down prices and in markets like New Zealand the full-service carriers are offering lower fares than ever before on highly competitive routes. This means more and a wider cross section of people are travelling.
Colehan says this ''democratised travel" is not fuelling the problem though.
''It affects all different kinds of airline and its in first, business and economy [classes] and all regions of the world. It really is a global problem."
And a potentially expensive one.
To divert a long-haul aircraft can cost $260,000 after it's had to dump fuel to reach its landing weight and pay airport charges and handling charges.
A booze trip
In New Zealand Etu union represents cabin crew and its head of aviation Kelvin Ellis says there's anecdotal evidence of a worsening problem.
"A hell of a lot more people are flying and they don't view it as a special event these days. People will buy a cheap ticket, jump on a flight to Fiji and it's just a booze trip," Ellis says.
''The other driver that we're starting to see is the way airlines especially budget carriers are pushing schedules that are impossible to meet and people are getting delayed and they're letting go their frustrations."
Delays and drinking in airport bars are a bad combination, Ellis says.
IATA figures based on a Canadian study show that about a quarter of cases involve alcohol.
It's not only cabin crew who can cut excessive drinking but airport bars, lounges, restaurants and duty-free retail staff who can practice the same responsible sale and service of alcohol already used by airlines, Colehan says.
Pilots in New Zealand are also worried about the risks posed by unruly passengers.
Tim Robinson, New Zealand Air Line Pilots' Association (NZALPA) president, says the safety of a perfectly normal flight may suddenly be in jeopardy because of possible violence or harassment from a passenger.
"Domestically, New Zealand airlines and crew have a good relationship with security and police - in any incident, we are assured that local police will meet us at the gate."
For cabin crew, their main priority is the safety of their passengers. While dealing with an incident in the air, they're not usually focused on collecting evidence for a prosecution afterwards, he says.
Fallout from 9/11
Pilots and crew have strong legal powers in planes, something Colehan says is not well understood by many passengers. But he says rules imposed after the September 11, 2001 terror attacks may have given the appearance of a weakening of their authority.
''Pre 9/11 the authority of the captain was quite clear and they would come out and deal with the situation but now they're behind locked cockpit doors. He or she with four stripes on the shoulder could intervene but that's not possible now."
While it was reported Korean Airlines issued crew with Tasers last year, crew trained in de-escalation techniques can usually defuse conflict. Nearly all planes have plastic handcuffs on board though.
Pre 9/11 the authority of the captain was quite clear and they would come out and deal with the situation but now they're behind locked cockpit doors.
Robinson says that, along with cabin crew, airline staff on the ground play a vital role in preventing violence in the air as the 'first defence'.
''As such, we would like to see all airlines committed to a programme of zero tolerance for disruptive passengers and provide adequate training and support for their employees in what is, effectively, a security role when required."
Air New Zealand and Jetstar say staff are being trained.
''The safety of our customers and our people is paramount and we continually train, refresh and support our teams regarding our processes to manage the impact of disruptive passengers," says an Air New Zealand spokeswoman.
The airline was comfortable with both its current internal measures and the CAA rules and guidelines around handling incidents.
Jetstar says it has zero tolerance for any disruptive or anti-social behaviour and its crew are trained to act quickly if a problem arises on board.
"Our cabin and ground crew teams are also trained to observe passenger behaviour prior to boarding, especially in relation to alcohol consumption. If they believe a passenger is intoxicated, which could result in potential disruptive issues on the flight, they will deny boarding," a spokesman says.
Paying the price
In the United States, disruptive passengers can face fines of US$25,000. Fines are much less here - $1000 for boarding a plane while intoxicated for example.
Etu would like to see higher penalties considered, however, IATA's Colehan says the New Zealand system of infringement notices being issued by police or aviation security much in the same way as a speeding ticket, is a good one.
Transport Minister Simon Bridges says the current regulatory settings provide sufficient powers for airlines and pilots to appropriately deal with situations when they occur.
''As such, this is a matter for airlines and their pilots."
But there is a push for more uniform international rules from IATA and pilots.
''We are concerned about international jurisdiction for our international crews, and are concerned there exists legal loopholes when we fly between countries. This isn't a new issue but likely to be one we need to face with more urgency, given the boom in air travel," says NZALPA's Robinson.
His and overseas pilots groups say rules imposed by the Tokyo, Hague and Montreal conventions do not provide sufficient legal protection.
"Jurisdiction to prosecute rests solely with the state of registration of the aircraft. National legislation to ensure full jurisdiction over all offences committed on aircraft operating to and from a state is required, as introduced by some countries such as Canada, the United States, Australia, and most recently the United Kingdom," he says.
Colehan says IATA is pushing for all countries to ratify the 2014 Montreal Protocol, which extends the jurisdiction over offence to the destination country of the flight in addition to the country of aircraft registration. This closes a loophole which allowed many serious offences to escape legal action.
Enforcement issues made the New Zealand infringement notices system attractive.
''It's OK having the authority but if an unruly passenger is delivered at three in morning (in another country) what do you do about it? Prosecutors sometimes don't want to prosecute because they don't see a criminal prosecution as being in the public interest - courts are already full of people who have committed very serious incidents."
Auckland Airport chief executive Adrian Littlewood says the form and shape of a building can help relieve stress and by making the transfer into planes smooth.
''We need to take care of our bit of it, put passengers in control and [provide] the right information ... and make the physical experience as sensible and pleasant as possible. You'll start to see that come through with the projects that are just about to complete."
An example is a ''recompose area" after security where passengers have more space to re-gather and repack their belongings.
Littlewood says the aviation industry needs to work to make travel as uniform as possible.
''Every country has its own set of rules and they can change in an instant - that's what can create anxiety for people so the more we can do as an industry to keep simplifying and making it easy for people to understand where they've got to be and their obligations as a traveller makes life easier."
Grant Amos trained as a psychologist and has run Fly Without Fear since 1981. He has helped about 9000 people deal with anxiety over flying in that time and says the lack of human contact when checking in and flying contributed to travel stress.
Amos says travellers should go to the airport assuming there would be new rules, ask questions of people who are in authority and give themselves lots of time.
They should also prepare themselves differently.
''People have unrealistic expectations about the experience of flying."