Under what situations can a passenger be restrained by flight crew?

The BBC actually has an article related to this entitled "Who, What, Why: Is it legal to restrain air passengers?". It explains:

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.

However...

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.

In addition, the IATA published a "Guidance on Unruly Passenger Prevention and Management"(1.29 MB), which states:

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 advises that there should be more training and support for cabin crew.

I do not think there is a rule written somewhere that gives the cabin crew the right to restrain a passenger, and I do not think there is a training for that to cabin crew (how to restrain a passenger). The rules vary from an airlines to another, from one civil aviation authority to another, but the guide lines will always be the three steps way of dealing with unruly passengers:

1. Warn the passenger verbally.
2. Still unruly? Give him/her a written warning, there is a form to be filled with witness's signatures. Mainly this is to freak the unruly passenger out and to have some documents to back the airlines up in case things goes wrong.
3. Still unruly? Let the captain call the security and upon arrival passengers will be told to be seated after landing, security will come and take the unruly passenger in front of everyone. What a humiliating situation! Authorities do not tolerate with unruly passengers onboard.

Now those were the guidelines, but sometimes these aren't enough and unruly passenger might compromise the safety of the passenger/aircraft. If that's the case and the unruly passenger is way out of control and starts to freak other passengers, then no specific rules will apply! Just common sense! The cabin crew have to act according to common sense in any emergency case, the emergency procedures and policies are to be followed literally in many cases but sometimes common sense has to take over especially when the book does not cover that scenario.

What would you do if you make a party in your house, and one of the guests starts to make problems and break things and harm other guests? you will call the police, kick him out or restrain him. In aircrafts you are only left with the latter, I think that's what lead the cabin crew in that flight to restrain the passenger - common sense.

Legally, if the cabin crew followed the guidelines mentioned above, with the existence of many witnesses, then they are clear! Some people might think this is inappropriate and against human rights and blah blah, but they have saved many passengers and the aircraft from a bigger problem!

One more thing to mention, after 9/11 things are much different, authorities are super sensitive to any kind of unruliness in aviation. Airports security, carry-on luggage rules, customs and passport control. So I think it is normal if the cabin crew got some of this sensitivity. just guessing.

Regarding the methods to restrain unruly passengers, I never heard of an airlines that provide restraining tools for the cabin crew. Icelandair said they do and I am skeptical about this, why would they provide duct tape? If they would provide tools to restrain people then they should provide hand cuffs or tie wraps!

Anyway, in my 10 years as cabin crew, I haven't faced a single case of restraining a passenger by cabin crew. Although I have seen many unruly passenger but the three steps above worked in all the cases.

Sky Marshals

Some airlines, in certain routes, have sky marshals onboard. They are usually government agents who are positioned in aircrafts undercover, either as passengers or as cabin crew. They are allowed to restrain people and even put hand cuffs on them. As I said they are government officers and they have their rules.