Passengers who make trouble on international flights face swifter prosecution from next year when a new amendment to a global treaty comes into effect.

Incidents involving unruly passengers are less frequent but have become more serious, according to a study by an international airline trade group two years ago which found that 60 per cent of on-board crimes went unpunished.

The problem stems from a 1963 agreement among 186 countries, known as the Tokyo Convention, that gave jurisdiction over prosecuting an unruly passenger to the nation where the plane is registered. That means that a passenger who gets drunk and belligerent on an American Airlines flight to France can be prosecuted only in the US, where American Airlines is registered, not in France, where the plane lands.

Last week, Nigeria joined 21 other countries to ratify an amendment to the Tokyo Convention, giving the amendment the necessary support for the change to go into effect January 1. The amendment allows countries where the plane lands to prosecute a troublemaker on an international flight.

“Everybody on board is entitled to enjoy a journey free from abusive or other unacceptable behaviour,” said Alexandre de Juniac, director general and CEO of the International Air Transport Association, a trade group for the world’s airlines. “But the deterrent to unruly behaviour is weak.”

The necessary 22nd nation to ratify the amendment came on November 26 when the secretary general of the International Civil Aviation Organisation, Fang Liu, accepted the endorsement of the amendment from Nigeria.

“The protocol addresses the issue of rising incidents of unruly and disruptive behaviour on board aircraft by significantly improving the ability of [countries] to expand jurisdiction over relevant offences and acts to the [countries] of landing and the [country] of the operator,” Liu said in a statement. “The protocol will also serve to enhance global aviation security provisions by expressly extending legal recognition and protections to in-flight security officers.”

In 2017, there were 8,731 incidents of unruly passengers on flights operated by airlines that are members of IATA, the airline trade group, compared with 9,837 in the previous year. The vast majority of the incidents involve excessive drinking, according to an IATA study.

Despite this drop in the number of incidents, their severity was found to have increased. The cases in which passengers brandished weapons or threatened the lives of passengers or crew members jumped to 279 in 2017 from 66 in 2016, according to IATA.

Extremely serious incidents – defined as a breach of the flight deck, an act of sabotage or a credible threat of seizing the aircraft – rose to 50 in 2017 from 20 in 2016, IATA said.

The cost of diverting a plane because of an unruly passenger can range from US$10,000-200,000, depending on the circumstances, the trade group estimated.

In the US, the Federal Aviation Administration said it recorded 90 incidents of unruly passengers in 2017, down from 101 incidents the previous year. In 2000, the agency increased the fine for causing a disturbance on a plane from US$1,100 to as much as US$25,000.

“The safety and well-being of every traveller is and will remain the highest priority for US airlines,” said Katherine Estep, a spokeswoman for Airlines for America, a trade group for the biggest air carriers in the US “Our members take these matters seriously, and inappropriate behaviour toward crew or passengers is not tolerated.”

A man who was accused of sexually assaulting a passenger on a 2017 flight from Los Angeles to Panama may have escaped prosecution because of confusion over who has jurisdiction when punishing crimes on international flights.

The problem of unruly passengers has prompted some airlines to take unusual measures.

In 2016, a passenger began attacking other passengers and flight attendants on a Korean Air flight from Vietnam to South Korea.

In response, Korean Air began to strengthen its security measures, including improved training of flight attendants in the use of stun guns. Airlines representatives also said the carrier was considering assigning at least one male flight attendant on each flight to help subdue disorderly passengers.