A photograph of a man on a flight from Iceland to New York apparently taped up and gagged after alleged unruly behaviour has been circulated and published extensively. But what are the rules on restraining passengers?
The unverified photo - which surfaced on social media - shows a passenger on a flight from Reykjavik to JFK restrained in his seat with tape and plastic ties, with tape over his mouth after allegedly being drunk and disruptive.
The picture is said to have been taken by someone aboard the flight on Thursday 3 January and was posted by a friend on his Tumblr account. An Icelandair spokesman said the company could not "confirm the validity of this photo".
But he confirms there had been an incident with a passenger on the flight. "The incident involved a disruptive male passenger who was hitting, screaming and spitting at other passengers while yelling profanities," says Gudjon Arngrimsson.
"To ensure the safety of those onboard, he was restrained by passengers and crew and was monitored for his own safety for the duration of the flight. Upon arrival at JFK the flight was met by authorities who arrested the man."
So is the level of restraint shown in the photo - including gagging - allowed by law?
A number of conventions - including the Tokyo Convention (1963) and the Montreal Convention (1971) - address the issue of ensuring safety and discipline on board a plane.
The Tokyo Convention emphasises that the plane's captain is in charge of the safety of the flight and thus decides whether a passenger needs to be restrained. Before landing, the captain must notify the authorities in that country that a person on board is under restraint and of the reasons for such restraint.
The Montreal Convention governs an airline's responsibility to their passengers during an international flight, says James Healy-Pratt, head of aviation at Stewarts Law.
He says that while aviation is international by nature, there is no single over-riding convention regarding the treatment of unruly passengers.
Passengers' behaviour is subject to the laws of the country in which the plane is registered, says Healy-Pratt. And what the crew can and cannot do is also governed by the national laws of that country.
According to experts, disorderly behaviour on planes is mostly linked to excessive alcohol consumption.
Major airlines have long been aware that passengers can become disruptive after drinking too much, and staff have been given the power to deal with such incidents.
But Healy-Pratt says the events of 11 September 2001 led governments, regulators and employers to review their security arrangements, and put the emphasis on restraint training for staff.
Today, most major airlines carry restraint kits to deal with unruly passengers.
"Restraint really is the last resort," says Healy-Pratt. "Usually a verbal warning will be given by a member of the cabin crew and if that doesn't work, the captain will be approached."
Existing conventions allow for the cabin crew to request assistance from passengers if they feel the need, he says. Many cabin crew are women, and it can be difficult for crew to apply cuffs or restraints to a combative person.
"Reasonable force can be used but if the crew go beyond that and unreasonably cause injury, they could be held responsible under the Montreal Convention," says Healy-Pratt.
This offers compensation "in the event of damage caused to passengers during international air transport".
Excessive restraint or gagging has the possibility to cause injury and even death.
An airline could find a case brought against them by someone injured during restraint who felt the crew had been over-zealous, says Healy-Pratt.
"In the US, for example, this would be decided by a jury and certainly since 9/11, there would be a high burden of proof."
There is a view that when a passenger buys a ticket they are effectively signing a contract not to become disorderly on the flight. The issue of disruptive passengers and how to deal with them poses a serious problem to the industry, according to IATA, the trade association representing the industry worldwide.
Last month, it published its first edition of the Guidance on Unruly Passenger Prevention and Management.
It says: "Cabin crew are in a unique position to deal with the unruly passenger problem, as they are not able to escape the situation or to call authorities for assistance on board during flight.
"While there is no 'one-size-fits-all' approach to preventing and managing unruly passengers, focus needs to shift from reacting to unruly passenger incidents to preventing incidents before they happen."
It calls for more training and support for cabin crew.
Nowadays, many airlines and regulators are taking a tough approach to unruly behaviour by passengers, says Healy-Pratt.
And in some cases, it's passengers themselves who adopt this approach.
In September 2012, passengers aboard a flight from Chicago to Orange County reportedly tackled and restrained a fellow traveller with their belts. A newspaper report said the man had been warned by the cabin crew but the passengers took it upon themselves to restrain him.
One of the passengers was quoted as saying that next time he flies, he would be taking duct tape with him.