A general view of an aerobridge at the newly built airport in Islamabad, Pakistan May 6, 2017. REUTERS/Caren Firouz 

A general view of aerobridges at the newly built airport in Islamabad, Pakistan May 6, 2017. REUTERS/Caren Firouz 

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 

The left wing of a Singapore Airlines passenger plane touches an aerobridge at the Ninoy Aquino International Airport at the onslaught of Typhoon Rammasun, (locally named Glenda) in Paranaque, Metro Manila July 16, 2014. The typhoon killed at least 10 people as it churned across the Philippines and shut down the capital, cutting power and prompting the evacuation of almost more than 370,000 people, rescue officials said on Wednesday. Nearly 400 flights were grounded during a four-hour closure of the Manila airport. Two airliners suffered minor damage when gusts blew them into nearby obstacles, airport officials said. REUTERS/Stringer (PHILIPPINES - Tags: DISASTER ENVIRONMENT TRANSPORT) 

Guests walk inside an aerobridge at the newly constructed Terminal 2 at Chhatrapati Shivaji International Airport in Mumbai January 10, 2014. Spread over an area a little over 439,000 square meters, the four level terminal has been designed to cater to an estimated 40 million passengers annually, according to the press release. REUTERS/Danish Siddiqui (INDIA - Tags: BUSINESS TRANSPORT) 

Passengers walk through an aerobridge to embark an Air France plane on the tarmac of Charles de Gaulles International Airport in Roissy near Paris, January 8, 2013. Investors in Alitalia who helped rescue the airline four years ago are considering selling their shares, with some pushing for a deal with long-time stakeholder Air France-KLM, sources with knowledge of the situation said on Monday, January 7, 2013. REUTERS/Charles Platiau (FRANCE - Tags: TRANSPORT BUSINESS) 

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) 

Georgian Airways charter flight A9-1930 sits at the aerobridge before its flight to Moscow at the Tbilisi international airport, January 8, 2010. The charter flight left from Tbilisi for Moscow on Friday in the first direct flight between Georgia and Russia since the two countries fought a brief war in 2008. REUTERS/David Mdzinarishvili (GEORGIA - Tags: TRANSPORT BUSINESS) 

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) 

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) 

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) 

A passenger walks through an aerobridge to embark a plane in Ljublajna's Brnik airport, May 11, 2009. REUTERS/Srdjan Zivulovic (SLOVENIA TRANSPORT SOCIETY) 

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) 

U.S. President-elect Barack Obama steps aboard a military plane at Chicago Midway Airport January 4, 2009 before flying to Andrews Air Force Base near Washington. Obama is moving to Washington 16 days before being sworn in as the 44th President of the United States on January 20. REUTERS/Jason Reed (UNITED STATES) 

A man looks out of a window while exiting a Thai Airways flight from Phuket, the first to land at Bangkok's Suvarnabhumi Airport December 3, 2008, after a week-long anti-government protest paralyzed air travel. Anti-government protesters lifted their crippling, eight-day blockade of Thailand's main airport on Wednesday, raising the hopes of 230,000 stranded tourists even though there is no end in sight to the wider political crisis. REUTERS/Chaiwat Subprasom (THAILAND) 

China's President Hu Jintao and his wife Liu Yongqing arrive at the airport in Lima November 19, 2008. Jintao is in Peru to attend the Asia Pacific Economic Cooperation (APEC) forum. The United States, China, Japan and some emerging economy heavyweights will push for quick action on free trade deals at a summit meeting this weekend as an antidote to a looming recession in much of the world. REUTERS/Pilar Olivares (PERU) 

Air Force One sits on the tarmac at Andrews Air Force Base near Washington after U.S. President George W. Bush returned from a day trip to New York, November 11, 2008. REUTERS/Jim Young (UNITED STATES) 

U.S. Democratic presidential nominee Barack Obama steps aboard his campaign plane at Raleigh-Durham Airport following a nearby campaign rally in North Carolina, October 29, 2008. Obama is campaigning in North Carolina and Florida on Wednesday before the November 4 presidential election. REUTERS/Jason Reed (UNITED STATES) US PRESIDENTIAL ELECTION CAMPAIGN 2008 (USA) 

U.S. Democratic presidential nominee Senator Barack Obama (D-IL) steps off his campaign plane at Shenandoah Regional Airport in Staunton, Virginia, October 28, 2008. Obama is campaigning in Pennsylvania and Virginia on Tuesday before the November 4 election. REUTERS/Jason Reed (UNITED STATES) US PRESIDENTIAL ELECTION CAMPAIGN 2008 (USA) 

U.S. Republican presidential nominee Senator John McCain (R-AZ) (R) and his wife Cindy walk off their campaign plane in Fayetteville, North Carolina October 28, 2008. REUTERS/Brian Snyder (UNITED STATES) US PRESIDENTIAL ELECTION CAMPAIGN 2008 (USA) 

Secret Service agents step off U.S. Democratic Presidential nominee Barack Obama's campaign plane in Pittsburgh, Pennsylvania, October 27, 2008. Federal agents have broken up a plot to assassinate Obama and shoot or decapitate 102 black people in a Tennessee murder spree, the ATF said on Monday. REUTERS/Jason Reed (UNITED STATES) US PRESIDENTIAL ELECTION CAMPAIGN 2008 (USA) 

Liberal leader Stephane Dion walks off his campaign plane in Montreal October 9, 2008. Canadians will head to the polls in a federal election on October 14. REUTERS/Shaun Best (CANADA) 

Laureen Harper laughs as she boards the campaign plane with her husband Conservative leader and Canada's Prime Minister Stephen Harper in Regina, Saskatchewan September 8, 2008. Canadians will head to the polls in a federal election October 14. REUTERS/Chris Wattie (CANADA) 

Britain's Olympic rowing team pose for photographs with British Airways staff at Heathrow airport in west London before flying to Beijing for the Olympic Games July 30, 2008. REUTERS/Luke MacGregor (BRITAIN) (BEIJING OLYMPICS 2008 PREVIEW) 

Airplanes of German Airline Lufthansa stand in their parking positions at Frankfurt airport July 28, 2008. A strike by Lufthansa ground and cabin staff over pay has barely disrupted flights to and from German airports, the airline said on Monday, but union officials said the impact would grow in coming days. REUTERS/Alex Grimm (GERMANY) 

German Chancellor Angela Merkel gestures upon her arrival at New Chitose International Airport near Sapporo July 7, 2008. for the Group of Eight (G8) Hokkaido Toyako Summit which begins on Monday. REUTERS/Kim Kyung-Hoon (JAPAN) 

U.S. President George W. Bush gives thumbs up as he goes to board the Air Force One at the Tegel airport in Berlin, June 11, 2008. Bush threatened Iran on Wednesday with more sanctions if it fails to stop enriching uranium and said all options were on the table to thwart Tehran's nuclear ambitions. REUTERS/Tobias Schwarz (GERMANY) 

Crew prepare a Virgin Atlantic Boeing 747 aircraft before the world's first commercial biofuel flight to Amsterdam from Heathrow Airport in London February 24, 2008. Nuts picked from Amazon rainforests helped fuel the world's first commercial airliner flight powered by renewable energy on Sunday. REUTERS/Luke MacGregor (BRITAIN) 

U.S. President George W. Bush (top L) and first lady Laura Bush wave goodbye to Tanzania's President Jakaya Kikwete and his wife Salma as they depart Julius Nyerere International Airport on Air Force One in Dar es Salaam February 19, 2008. REUTERS/Jim Young (TANZANIA)