A man walks past a notice board informing passengers of a strike by members of the RMT union at London Bridge railway station, in London, Britain, January 8, 2018. REUTERS/Dylan Martinez 

A notice board informs passengers of a strike by members of the RMT union at London Bridge railway station, in London, Britain, January 8, 2018. REUTERS/Dylan Martinez 

A notice board informs passengers of a strike by members of the RMT union at London Bridge railway station, in London, Britain, January 8, 2018. REUTERS/Dylan Martinez 

Passengers cross a boarding bridge to enter an Airbus A320-214 airplane of Swiss airlines at Zurich airport, Switzerland, April 14, 2016. REUTERS/Arnd Wiegmann 

Passengers cross a boarding bridge to enter an Airbus A320-214 airplane of Swiss airlines at Zurich airport, Switzerland, April 14, 2016. REUTERS/Arnd Wiegmann 

Pope Francis descends the stairs of the papal plane after arriving in the United States at Joint Base Andrews outside Washington September 22, 2015. REUTERS/Tony Gentile

Pope Francis descends the air stairs from his aircraft to be welcomed by U.S. President Barack Obama (not pictured) upon his arrival at Joint Base Andrews outside Washington September 22, 2015. REUTERS/Kevin Lamarque

Pope Francis arrives for the first time in the United States at Joint Base Andrews, Maryland September 22, 2015. REUTERS/Kevin Lamarque

An Air Force personnel keeps watch as a crew member removes the passenger stairs from Air Force One after U.S. President Barack Obama's arrival at Joint Base Andrews in Washington September 14, 2015. Obama traveled to Des Moines, Iowa to hold a town hall to discuss college access and affordability. REUTERS/Carlos Barria 

U.S. President Barack Obama salutes as he disembarks from Air Force One at Joint Base Andrews, Maryland, August 25, 2015. President Barack Obama, fresh from vacation and nine rounds of golf in MarthaÕs Vineyard, launched into a busy two weeks promoting renewable energy and his nuclear deal with Iran. REUTERS/Carlos Barria 

U.S. President Barack Obama arrives aboard Air Force One at Joint Base Andrews, Maryland, August 25, 2015. President Barack Obama, fresh from vacation and nine rounds of golf in Martha's Vineyard, launched into a busy two weeks promoting renewable energy and his nuclear deal with Iran. REUTERS/Carlos Barria TPX IMAGES OF THE DAY

U.S. President Barack Obama waves as he boards Air Force One on the way back to Washington at the McCarran International Airport in Las Vegas, Nevada August 25, 2015. REUTERS/Carlos Barria 

U.S. Secretary of State John Kerry walks up the stairs as he boards his aircraft from Andrews Air Force Base for a flight to Havana, Cuba, August 14, 2015. Kerry declared a new era in relations as he celebrated restored diplomatic ties in Havana on Friday, but he also urged political change in Cuba, telling Cubans they should be free to choose their own leaders. REUTERS/Pablo Martinez Monsivais/Pool 

U.S. President Barack Obama arrives aboard Air Force One at Jomo Kenyatta International Airport in Nairobi July 24, 2015. U.S. President Barack Obama flew into Kenya late on Friday for his first presidential visit to his father's homeland, aiming to boost trade and security ties in east Africa. REUTERS/Thomas Mukoya 

U.S. President Barack Obama arrives to board Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst 

U.S. President Barack Obama boards Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst 

U.S. President Barack Obama waves as he boards Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst TPX IMAGES OF THE DAY

U.S. President Barack Obama boards Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst 

Passengers board a Ukrainian airplane at Bassel al-Assad international airport in Latakia, May 24, 2015. Syria welcomed its first commercial flight from Europe in three years as an Autolux flight from the Ukrainian capital Kiev arrived in the government-held coastal city of Latakia on Sunday. Only 10 passengers arrived in Syria on the flight with return tickets costing around $600 per person. Picture taken May 24, 2015. REUTERS/Omar Sanadiki TPX IMAGES OF THE DAY

Yemenis, who were stranded in Egypt, pray as they disembark from their plane after returning home, at Sanaa international aiport May 20, 2015. Some 200 Yemenis who have been stuck in Egypt and unable to return home arrived in the capital Sanaa on Wednesday in the first such flight from Egypt, though many more are expected in the coming days and weeks, officials say. REUTERS/Khaled Abdullah 

Fuel gauges are seen in the engine room of the James Newman Woolwich Ferry in London January 23, 2015. Each ferry has a Boat Engineer on board for every crossing, who monitors the engines. Now, as in the past, there are proposals for plans to replace the Woolwich Free Ferry with either a bridge or a tunnel. However, many of the ferry workers believe there will always be a ferry service at Woolwich, which has been a place for crossing the Thames since the Saxon times. The free ferry opened in 1889, with each boat licensed to carry 1000 passengers and 15-20 vehicles. Latest figures show that on average just under 80,000 vehicles and 22,000 foot passengers use the ferry every four weeks. REUTERS/Russell Boyce (BRITAIN - Tags: BUSINESS POLITICS SOCIETY MARITIME TRANSPORT)

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An Australian national flag and a New South Wales state flag (top) fly at half-mast atop the Sydney Harbour Bridge on July 19, 2014, as a sign of respect for those killed in the Malaysia Airlines passenger jetliner MH17 that crashed over eastern Ukraine. At bottom, a ferry passes the Sydney Opera House. Australian Prime Minister Tony Abbott blamed Russia on Friday for the shooting down of MH17, killing all 298 people on board, including 28 Australians. Abbott appeared to go further than other Western leaders in apportioning blame over the crash, demanding that Moscow answer questions about the "Russian-backed rebels" that he said were behind the disaster. REUTERS/David Gray (AUSTRALIA - Tags: TRANSPORT DISASTER) 

An Australian national flag and a New South Wales state flag (top) fly at half-mast atop the Sydney Harbour Bridge on July 19, 2014, as a sign of respect for those killed in the Malaysia Airlines passenger jetliner MH17 that crashed over eastern Ukraine. Australian Prime Minister Tony Abbott blamed Russia on Friday for the shooting down of MH17, killing all 298 people on board, including 28 Australians. Abbott appeared to go further than other Western leaders in apportioning blame over the crash, demanding that Moscow answer questions about the "Russian-backed rebels" that he said were behind the disaster. REUTERS/David Gray (AUSTRALIA - Tags: TRANSPORT DISASTER TPX IMAGES OF THE DAY) 

A family member of a missing passenger from the capsized passenger ship Sewol prays while waiting for news from rescue and salvage teams at the port in Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju. REUTERS/Kim Hong-Ji (SOUTH KOREA - Tags: DISASTER MARITIME RELIGION TPX IMAGES OF THE DAY) 

A policeman stands guard as a priest says prayers at the port in Jindo, where family members are waiting for bodies recovered from the capsized passenger ship Sewol April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju. REUTERS/Kim Hong-Ji (SOUTH KOREA - Tags: DISASTER MARITIME RELIGION) 

Two policemen stand guard as a priest says prayers at the port in Jindo, where family members are waiting for bodies recovered from the capsized passenger ship Sewol April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju. REUTERS/Kim Hong-Ji (SOUTH KOREA - Tags: DISASTER MARITIME RELIGION) 

Family members look at a noticeboard with descriptions of bodies recovered from the capsized passenger ship Sewol at the port in Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju. REUTERS/Kim Hong-Ji (SOUTH KOREA - Tags: DISASTER MARITIME) 

Emergency service workers place a body, recovered from the capsized passenger ship Sewol, into an ambulance at the port in Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju. REUTERS/Kim Hong-Ji (SOUTH KOREA - Tags: DISASTER MARITIME) 

Emergency service workers carry a body, recovered from the capsized passenger ship Sewol, to an ambulance at the port in Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju. REUTERS/Kim Hong-Ji (SOUTH KOREA - Tags: DISASTER MARITIME) 

South Korean rescue workers travel in boats near a giant offshore crane, which will used in the salvage operation of capsized passenger ship Sewol which sank last Wednesday, near the site of the sinking in the sea off Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju.
REUTERS/Kim Kyung-Hoon (SOUTH KOREA - Tags: DISASTER MARITIME) 

South Korean rescue workers travel in boats near a giant offshore crane, which will be used in the salvage operation of capsized passenger ship Sewol which sank last Wednesday, near the site of the sinking in the sea off Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju.
REUTERS/Kim Kyung-Hoon (SOUTH KOREA - Tags: DISASTER MARITIME) 

South Korean rescue workers operate near floats where capsized passenger ship Sewol sank last Wednesday, in the sea off Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju.
REUTERS/Kim Kyung-Hoon (SOUTH KOREA - Tags: DISASTER MARITIME) 

South Korean rescue workers travel in a boat near the site where capsized passenger ship Sewol sank last Wednesday, in the sea off Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju.
REUTERS/Kim Kyung-Hoon (SOUTH KOREA - Tags: DISASTER MARITIME) 

South Korean rescue workers operate near floats where capsized passenger ship Sewol sank last Wednesday, in the sea off Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju.
REUTERS/Kim Kyung-Hoon (SOUTH KOREA - Tags: DISASTER MARITIME) 

South Korean rescue workers operate near floats where capsized passenger ship Sewol sank on Wednesday, in the sea off Jindo April 22, 2014. The crew of the South Korean ferry that sank with hundreds of people on board repeatedly asked officers on the bridge whether or not to give the order to abandon ship, but there was no response, a crew member has said. The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the sub-tropical holiday island of Jeju. REUTERS/Kim Kyung-Hoon (SOUTH KOREA - Tags: DISASTER MARITIME) 

A staircase for U.S. Republican presidential nominee and former Massachusetts Governor Mitt Romney to be used to step off his campaign plane sits on the tarmac prior to his arrival at the site of a campaign rally in Dubuque, Iowa, November 3, 2012. REUTERS/Jim Young (UNITED STATES - Tags: POLITICS ELECTIONS USA PRESIDENTIAL ELECTION) 

An employee dusts off the red carpet on the tarmac of Berlin's Tegel airport, September 22, 2011. Pope Benedict XVI will have a four-day visit to Germany beginning September 22. REUTERS/Kai Pfaffenbach (GERMANY - Tags: POLITICS RELIGION)

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U.S. President Barack Obama boards Air Force One before he departs Joint Base Andrews outside Washington for Michigan, August 11, 2011. REUTERS/Larry Downing (UNITED STATES - Tags: POLITICS TRANSPORT) 

U.S. President Barack Obama prepares to board Air Force One before he departs Joint Base Andrews outside Washington for Michigan, August 11, 2011. REUTERS/Larry Downing (UNITED STATES - Tags: POLITICS ENVIRONMENT TRANSPORT IMAGES OF THE DAY) 

U.S. singer Lady Gaga steps off her private jet at Sydney Airport July 9, 2011. Lady Gaga is in Sydney for a one-off show at Sydney's Town Hall on Wednesday. REUTERS/Dean Lewins/Pool (AUSTRALIA - Tags: ENTERTAINMENT IMAGES OF THE DAY) 

An aircraft belonging to Austral with ash on it from Chile's Puyehue-Cordon Caulle volcano chain remains standred on the tarmac of the sky resort San Carlos de Bariloche in Argentina's Patagonia June 6, 2011. A volcano dormant for decades erupted in south-central Chile on Saturday, belching an ash cloud more than 6 miles (10 km) high that blew over the Andes and carpeted the resort in neighboring Argentina. The eruption in the Puyehue-Cordon Caulle volcanic chain, about 575 miles (920 km) south of the capital, Santiago, in Patagonia also prompted Chilean authorities to shut a heavily traveled border crossing into Argentina. Chile's government said it was evacuating 3,500 people from the surrounding area as a precaution. Officials said the volcano was spitting molten rock, but there was no visible lava flow. REUTERS/Chiwi Giambirtone (ARGENTINA - Tags: DISASTER ENVIRONMENT TRANSPORT) 

Malia Obama, elder daughter of U.S. President Barack Obama, steps aboard Air Force One in Rio de Janeiro, March 21, 2011. The Obamas are travelling to Santiago, Chile, for the second stop on his Latin American visit. REUTERS/Jason Reed (BRAZIL - Tags: POLITICS) 

U.S. President Barack Obama boards Air Force One at Andrews Air Force Base, near Washington, November 23, 2010. REUTERS/Jim Young (UNITED STATES - Tags: POLITICS) 

Singapore's Prime Minister Lee Hsien Loong looks out from a window of an aircraft upon his arrival at Noi Bai airport in Hanoi October 27, 2010. Lee is in Hanoi to attend the 17th ASEAN summit running from October 28 to 30. REUTERS/Kham (VIETNAM - Tags: POLITICS) 

Bo, the pet dog of U.S. President Barack Obama and his family, steps off Air Force One at Andrews Air Force Base near Washington, August 15, 2010. The first family and Bo returned to Washington after spending the weekend in Panama City Beach, Florida. REUTERS/Jason Reed (UNITED STATES - Tags: POLITICS ANIMALS IMAGES OF THE DAY) 

Pope Benedict XVI waves from the platform at the top of the stairs before boarding his plane to Portugal at Fiumicino international airport in Rome May 11, 2010. REUTERS/Alessandro Bianchi (ITALY - Tags: RELIGION TRANSPORT) 

Passengers are reflected in the mirror of the departure terminal as they board a Turkish Airlines aircraft to fly to Turkey at the Ercan airport in Nicosia, in the Turkish-administered northern part of Cyprus, April 19, 2010. REUTERS/Murad Sezer (CYPRUS - Tags: TRANSPORT BUSINESS) 

U.S. President Barack Obama arrives at Prague's Ruzyne Airport April 8, 2010. Obama is expected to sign the new Strategic Arms Reduction Treaty (START) with Russian President Dmitry Medvedev on Thursday. REUTERS/David W Cerny (CZECH REPUBLIC - Tags: POLITICS) 

France's President Nicolas Sarkozy (R) shakes hands with Haitian President Rene Preval during a welcome ceremony at the international airport of Port-au-Prince, Haiti February 17, 2010. Sarkozy is the first French president to visit Haiti. REUTERS/Carlos Barria (HAITI - Tags: DISASTER ENVIRONMENT POLITICS) 

Personnel stand near a crashed Las Palmas bound Air Berlin Boeing 737-800 at Dortmund airport January 3, 2010. Some 165 passengers and six crew escaped unhurt from a plane that skidded off the runway at Dortmund airport early on Sunday after the pilot aborted take off, a spokeswoman for airline Air Berlin said. REUTERS/Kirsten Neumann (GERMANY - Tags: DISASTER TRANSPORT IMAGES OF THE DAY) 

Belgium's Prime Minister Yves Leterme (L), on his way to the United Nations Climate Change Conference 2009 in Copenhagen disembark late on December 17, 2009 a commercial flight at Brussels Zaventem airport after emergency services decided to close the tarmac following bad weather conditions in Belgium. Leterme who was supposed to take part in a state dinner at Christiansborg Palace in Copenhagen on Thursday decided to attempt to drive in the hope to join other world leaders working to try and beat a Friday deadline for a deal on cutting emissions and helping poor countries cope with the costly impact of global warming. REUTERS/Yves Herman (BELGIUM - Tags: POLITICS ENVIRONMENT) 

Workers push a mobile staircase for Pamir airlines in Herat airport December 17, 2009. REUTERS/Morteza Nikoubazl (AFGHANISTAN - Tags: TRANSPORT SOCIETY EMPLOYMENT BUSINESS) 

U.S. President Barack Obama boards Air Force One at Naval Air Station Jacksonville, in Jacksonville, Florida, October 26, 2009. REUTERS/Jim Young (UNITED STATES POLITICS MILITARY) 

Transmilenio passengers queue on a pedestrian bridge before boarding their bus in central Bogota September 22, 2009. Construction of Bogota's Transmilenio mass transport system was initiated in 1998 by then Mayor Enrique Penalosa. The busses started running two years later. Currently 1,46 million citizens (Bogotanos) use the system every day and the network is scheduled to be completed in 2016 by then it is hoped that 80 percent of Bogota's public transport riders will be using the Transmilenio. Picture taken on September 22, 2009. REUTERS/Fredy Builes (COLOMBIA BUSINESS POLITICS TRANSPORT) 

US President Barack Obama aboards Air Force One to travel to Chicago for a speech on healthcare, at Andrews AFB in Maryland June 15, 2009. REUTERS/Jonathan Ernst (UNITED STATES POLITICS) 

The cruise ship Pacific Dawn passes under Sydney Harbour Bridge as it heads to a wharf after a swine flu scare June 1, 2009. Health authorities are expected to allow all passengers to disembark in Sydney on Monday after three crew members on board tested positive for the H1N1 strain of flu. REUTERS/Tim Wimborne (AUSTRALIA HEALTH) 

A man watches the cruise ship Pacific Dawn pass under the Sydney Harbour Bridge as it heads to a wharf in Sydney after a swine flu scare June 1, 2009. Health authorities are expected to allow all passengers to disembark in Sydney on Monday after three crew members on board tested positive for the H1N1 strain of flu. REUTERS/Tim Wimborne (AUSTRALIA HEALTH) 

U.S. President Barack Obama boards Air Force One at Andrews Air Force Base in Maryland enroute to Arizona May 13, 2009. Obama will speak at the the commencement at Arizona State University tonight. REUTERS/Kevin Lamarque (UNITED STATES POLITICS) 

Kuwait City airport employees wipe the sand from the red carpet during a sand storm before the arrival of France's President Nicolas Sarkozy in Kuwait February 11, 2009. After an unannounced visit to Iraq, French President Sarkozy is visiting Oman, Bahrain and Kuwait. REUTERS/Remy de la Mauviniere/Pool (KUWAIT) 

Passengers board a plane at Madrid's Barajas airport as the airport resumes normal activity January 11, 2009. Thousands of passengers have suffered flight delays or cancellations at Barajas after one of the heaviest snowfalls in decades caused the international airport to shut down for five hours on Friday. REUTERS/Susana Vera (SPAIN)