Britain's Catherine, Duchess of Cambridge (C) arrives at an event to mark the 75th anniversary of the RAF Air Cadets, at St Clement Danes church in London, Britain February 7, 2016. REUTERS/Neil Hall 

Britain's Catherine, Duchess of Cambridge (C) is greeted by Air Commodore Dawn McCafferty (R) she arrives at an event to mark the 75th anniversary of the RAF Air Cadets, at St Clement Danes church in London, Britain February 7, 2016. REUTERS/Neil Hall 

Britain's Catherine, Duchess of Cambridge (R) is greeted by Air Commodore Dawn McCafferty she arrives at an event to mark the 75th anniversary of the RAF Air Cadets, at St Clement Danes church in London, Britain February 7, 2016. REUTERS/Neil Hall 

Britain's Catherine, Duchess of Cambridge (L) is greeted by Air Commodore Dawn McCafferty she arrives at an event to mark the 75th anniversary of the RAF Air Cadets, at St Clement Danes church in London, Britain February 7, 2016. REUTERS/Neil Hall 

A Tunisian soldier holds his position during an exercise along the frontier with Libya in Sabkeht Alyun, Tunisia February 6, 2016. Tunisia has completed a 200-km (125 mile) barrier along its frontier with Libya to try to keep out Islamist militants, and will soon install electronic monitoring systems, Defence Minister Farhat Hachani said on Saturday. REUTERS/Zoubeir Souissi TPX IMAGES OF THE DAY

An Air Force soldier marches with an eagle wearing a falconry hood on his shoulder, the symbol of Sri Lanka's Air Force, during Sri Lanka's 68th Independence day celebrations in Colombo, February 4, 2016. REUTERS/Dinuka Liyanawatte 

A "yellow dog" flight desk officer watches as a Rafale fighter jet prepares to take off aboard France's Charles de Gaulle aircraft carrier on missiion in the Gulf, January 28, 2016. REUTERS/Philippe Wojazer 

A sailor poses on the flight deck amongst Rafale fighter jets aboard France's Charles de Gaulle aircraft carrier in Abu Dhabi harbour, January 26, 2016. Picture taken January 26, 2016. REUTERS/Philippe Wojazer 

A sailor poses on the flight deck amongst Rafale fighter jets aboard France's Charles de Gaulle aircraft carrier in Abu Dhabi harbour, January 26, 2016. Picture taken January 26, 2016. REUTERS/Philippe Wojazer

A sailor poses on the flight deck amongst Rafale fighter jets aboard France's Charles de Gaulle aircraft carrier in Abu Dhabi harbour, January 26, 2016. Picture taken January 26, 2016. REUTERS/Philippe Wojazer

A sailor walks on the flight deck amongst Super Etendard fighter jets aboard France's Charles de Gaulle aircraft carrier in Abu Dhabi harbour, prior its departure for the Persian Gulf, January 27, 2016. REUTERS/Philippe Wojazer 

Sri Lankan military officers pose for photographs next to Indian Mig fighter jets on Indian Navy's largest aircraft carrier INS Vikramaditya at Colombo port in Sri Lanka January 21, 2016. REUTERS/Dinuka Liyanawatte 

U.S. President Barack Obama (L) laughs with U.S. Air Force Colonel John Millard as he arrives to board Air Force One for travel to Detroit from Joint Base Andrews, Maryland January 20, 2016. In Detroit, Obama will visit the North American International Auto Show and deliver remarks on the U.S. auto industry at the UAW-GM Center for Human Resources. REUTERS/Jonathan Ernst 

Soldiers holds their caps as a helicopter flies past during an operation, after a militant attack at Bacha Khan University in Charsadda, Pakistan, January 20, 2016. REUTERS/Fayaz Aziz 

A U.S. soldier stands guard in front of their Air F-16 fighter jet at Osan Air Base in Pyeongtaek, South Korea, January 10, 2016. REUTERS/Kim Hong-Ji 

Sergei Bainetov, deputy chief of the Russian Armed Forces' flight safety service, attends a news conference, dedicated to the crash of SU-24 fighter-bomber, a Russian warplane shot down by Turkey in November, in Moscow, Russia, December 21, 2015. REUTERS/Maxim ShemetovTPX IMAGES OF THE DAY

Sergei Bainetov (R), deputy chief of the Russian Armed Forces' flight safety service, and Nikolai Primak, Chairman of the Air Accident Investigation Commission, attend a news conference, dedicated to the crash of SU-24 fighter-bomber, a Russian warplane shot down by Turkey in November, in Moscow, Russia, December 21, 2015. REUTERS/Maxim Shemetov 

Sergei Bainetov (R), deputy chief of the Russian Armed Forces' flight safety service, and Nikolai Primak, chairman of the Air Accident Investigation Commission, attend a news conference, dedicated to the crash of SU-24 fighter-bomber, a Russian warplane shot down by Turkey in November, in Moscow, Russia, December 21, 2015. REUTERS/Maxim Shemetov TPX IMAGES OF THE DAY

Soldiers of German armed forces Bundeswehr Special Forces Command (KSK) attend an exercise at the airport in Trollenhagen, Germany, September 28, 2015. Picture taken September 28. TO GO WITH STORY DEUTSCHLAND-BUNDESWEHR/KSK REUTERS/Hannibal Hanschke

Soldiers of German armed forces Bundeswehr Special Forces Command (KSK) attend an exercise at the airport in Trollenhagen, Germany, September 28, 2015. Picture taken September 28. TO GO WITH STORY DEUTSCHLAND-BUNDESWEHR/KSK REUTERS/Hannibal Hanschke

Soldiers of German armed forces Bundeswehr Special Forces Command (KSK) attend an exercise at the airport in Trollenhagen, Germany, September 28, 2015. Picture taken September 28. TO GO WITH STORY DEUTSCHLAND-BUNDESWEHR/KSK REUTERS/Hannibal Hanschke

Army personnel attend a roll call prior to boarding a German airforce Airbus A400M military aircraft at German army Bundeswehr airbase in Jagel, northern Germany December 10, 2015. The first the German air force Tornado reconnaissance jets will take off for Turkey's Incirlik air base on Thursday, to support the military campaign against Islamic State. REUTERS/Fabian Bimmer

Army personnel attend a roll call prior to boarding a German airforce Airbus A400M military aircraft at German army Bundeswehr airbase in Jagel, northern Germany December 10, 2015. The first the German air force Tornado reconnaissance jets will take off for Turkey's Incirlik air base on Thursday, to support the military campaign against Islamic State. REUTERS/Fabian Bimmer

Army personnel board a German airforce Airbus A400M military aircraft at German army Bundeswehr airbase in Jagel, northern Germany December 10, 2015. The first the German air force Tornado reconnaissance jets will take off for Turkey's Incirlik air base on Thursday, to support the military campaign against Islamic State. REUTERS/Fabian Bimmer

Army personnel board a German airforce Airbus A400M military aircraft at German army Bundeswehr airbase in Jagel, northern Germany December 10, 2015. The first the German air force Tornado reconnaissance jets will take off for Turkey's Incirlik air base on Thursday, to support the military campaign against Islamic State. REUTERS/Fabian Bimmer

British Royal Air Force personnel work on a Tornado at RAF Akrotiri in southern Cyprus December 5, 2015. REUTERS/Darren Staples 

French President Francois Hollande (C) and French Defence Minister Jean-Yves Le Drian (L) arrive by helicopter on the Charles de Gaulle aircraft carrier deployed in the Mediterranean Sea off the coast of Syria, December 4, 2015. The carrier, which has 38 warplanes on board, was deployed to the east Mediterranean just days after Islamic State claimed responsibility for attacks in Paris that killed 130 people. The French carrier holds some 1,900 personnel and is accompanied by an attack submarine, several frigates, refueling ships, as well as fighter jets and surveillance aircraft. REUTERS/Pool TPX IMAGES OF THE DAY

RAF personnel work on a British Tornado after it returned from a mission at RAF Akrotiri in southern Cyprus December 3, 2015. REUTERS/Darren Staples TPX IMAGES OF THE DAY

Britain's Prime Minister David Cameron (3rd L) chats with Group Captain David Manning (2nd L) as they walk past a Royal Navy Merlin helicopter, and a Royal Navy Wildcat multi-role helicopter (R), at RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (3rd R) chats with Royal Navy personnel as they stand alongside a Royal Navy Wildcat multi-role helicopter, at RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (C) chats with Royal Navy personnel as they stand alongside a Royal Navy Wildcat multi-role helicopter, at RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (C) chats with Royal Navy personnel at RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (L) walks with Group Captain David Manning (R) past an RAF Eurofighter Typhoon fighter jet and a Royal Navy Wildcat multi-role helicopter during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (C) walks with Group Captain David Manning (R) past an RAF Eurofighter Typhoon fighter jet during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (C) walks with Group Captain David Manning (R) past an RAF Eurofighter Typhoon fighter jet and a Royal Navy Wildcat multi-role helicopter during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (3rd L) looks at an RAF Eurofighter Typhoon fighter jet during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (C) speaks with military personnel as he stands between an RAF Sentinel surveillance aircraft (L) and an RAF Eurofighter Typhoon fighter jet during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (C) speaks with military personnel as he stands between an RAF Sentinel surveillance aircraft (L) and an RAF Eurofighter Typhoon fighter jet during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (6th L) speaks with military personnel as he stands between an RAF Sentinel surveillance aircraft (L) and an RAF Eurofighter Typhoon fighter jet during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron looks at an RAF Eurofighter Typhoon fighter jet during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Britain's Prime Minister David Cameron (C) looks at an RAF Eurofighter Typhoon fighter jet during his visit to Royal Air Force station RAF Northolt in London, Britain November 23, 2015. Britain will invest an extra 12 billion pounds ($18 billion) in defense equipment over the next 10 years including nine Boeing submarine-hunting aircraft, Cameron said on Monday. REUTERS/Justin Tallis/pool

Flight deck crew work around a Super Etendard fighter jet as a French flag flies aboard the French nuclear-powered aircraft carrier Charles de Gaulle before its departure from the naval base of Toulon, France, November 18, 2015. France's Charles de Gaulle aircraft carrier will be deployed today to support operations against Islamic State in Syria and Iraq. REUTERS/Jean-Paul Pelissier

French Navy crew members walk past Rafale fighter jets (R) aboard the French nuclear-powered aircraft carrier Charles de Gaulle before its departure from the naval base of Toulon, France, November 18, 2015. France's Charles de Gaulle aircraft carrier will be deployed today to support operations against Islamic State in Syria and Iraq. REUTERS/Jean-Paul Pelissier 

Flight deck crew work on Rafale fighter jets aboard the French nuclear-powered aircraft carrier Charles de Gaulle before its departure from the naval base of Toulon, France, November 18, 2015. France's Charles de Gaulle aircraft carrier will be deployed today to support operations against Islamic State in Syria and Iraq. REUTERS/Jean-Paul Pelissier

The French nuclear-powered aircraft carrier Charles de Gaulle is seen before its departure from the naval base of Toulon, France, November 18, 2015. France's Charles de Gaulle aircraft carrier will be deployed today to support operations against Islamic State in Syria and Iraq. REUTERS/Jean-Paul Pelissier

A Philippines Air Force personnel rests next to riot shields as they stand guard near the venues of the Asia-Pacific Economic Cooperation (APEC) summit in the capital city of Manila, Philippines November 17, 2015. REUTERS/Edgar Su

A Philippines Air Force personnel rests on riot shields as they take a break from standing guard near the venues of the Asia-Pacific Economic Cooperation (APEC) summit in the capital city of Manila, Philippines November 17, 2015. REUTERS/Edgar Su

Residents walk past Philippines Air Force (white) and Police personnel standing guard near the venues of the Asia-Pacific Economic Cooperation (APEC) summit in the capital city of Manila, Philippines November 17, 2015. REUTERS/Edgar Su

Britain's Prince Charles (2nd R) and Camilla, Duchess of Cornwall (C) meet with Australian Air Force Air Commodore Stephen Meredith (2nd L) and his wife Susan Meredith in Adelaide, Australia, November 10, 2015. REUTERS/William West/Pool 

U.S. Paratroopers from the 82nd Airborne Division based in Fort Brag, N.C. , participate in a massive airdrop from C-17 Globemaster aircraft as part of the NATO Exercise Trident Juncture 2015, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

U.S. Paratroopers from the 82nd Airborne Division based in Fort Brag, N.C. , participate in a massive airdrop from C-17 Globemaster aircraft as part of the NATO Exercise Trident Juncture 2015, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

An AH-64 Apache helicopter takes part in Exercise Trident Juncture 2015, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

An AH-64 Apache helicopter flies through smoke while taking part in Exercise Trident Juncture 2015, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

An AH-64 Apache helicopter flies over smoke while taking part in Exercise Trident Juncture 2015, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

Helicopters take part in Exercise Trident Juncture 2015, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

NATO forces in AH-64 Apache helicopters take part in Exercise Trident Juncture 2015, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

U.S. Paratroopers from the 82nd Airborne Division based in Fort Brag, N.C. , participate in a massive airdrop from C-17 Globemaster aircraft as part of the NATO Exercise Trident Juncture 2015 military exercise, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

U.S. Paratroopers from the 82nd Airborne Division based in Fort Brag, N.C. , participate in a massive airdrop from C-17 Globemaster aircraft as part of the NATO Exercise Trident Juncture 2015 military exercise, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

U.S. Paratroopers from the 82nd Airborne Division based in Fort Brag, N.C. , participate in a massive airdrop from C-17 Globemaster aircraft as part of the NATO Exercise Trident Juncture 2015 military exercise, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna 

U.S. Paratroopers from the 82nd Airborne Division based in Fort Brag, N.C. , participate in a massive airdrop from C-17 Globemaster aircraft as part of the NATO Exercise Trident Juncture 2015 military exercise, NATO's largest joint and combined military exercise in more than a decade, at the San Gregorio training grounds outside Zaragoza, Spain, November 4, 2015. Some 36.000 personnel from more than 35 nations, including all NATO Allies will have participated in Exercise Trident Juncture 2015 which began on 21 October, in Italy, Portugal and Spain, including their adjacent waters and airspace and runs till November 6. REUTERS/Paul Hanna TPX IMAGES OF THE DAY