Alaka'i Technologies unveils a model of a working prototype of Skai, a hydrogen fuel cell powered multi-rotor aircraft the company designed to carry up to five passengers, during an event in Newbury Park, California, U.S., May 29, 2019. REUTERS/Andrew Cullen 

Alaka'i Technologies unveils a model of a working prototype of Skai, a hydrogen fuel cell powered multi-rotor aircraft the company designed to carry up to five passengers, during an event in Newbury Park, California, U.S., May 29, 2019. REUTERS/Andrew Cullen 

Alaka'i Technologies unveils a model of a working prototype of Skai, a hydrogen fuel cell powered multi-rotor aircraft the company designed to carry up to five passengers, during an event in Newbury Park, California, U.S., May 29, 2019. REUTERS/Andrew Cullen 

Alaka’i Technologies board member Dr. Bruce Holmes speaks before unveiling a model of a working prototype of Skai, a hydrogen fuel cell powered multi-rotor aircraft the company designed to carry up to five passengers, during an event in Newbury Park, California, May 29, 2019. REUTERS/Andrew Cullen 

Men look at KC-390 aircraft as they visit the Embraer's exhibition stand during LAAD, the biggest military industry expo in Latin America in Rio de Janeiro, Brazil April 2, 2019. REUTERS/Ricardo Moraes 

A woman gets inside an aircraft model during LAAD, the biggest military industry expo in Latin America in Rio de Janeiro, Brazil April 2, 2019. REUTERS/Ricardo Moraes 

The Super Guppy is seen on the Shuttle Landing Facility at NASA's Kennedy Space Center in Cape Canaveral, Florida, U.S. March 11, 2019. This aircraft will ferry the Orion capsule to different locations. REUTERS/Mike Brown 

The Super Guppy is seen on the Shuttle Landing Facility at NASA's Kennedy Space Center in Cape Canaveral, Florida, U.S. March 11, 2019. This aircraft will ferry the Orion capsule to different locations. REUTERS/Mike Brown 

The Super Guppy is seen on the Shuttle Landing Facility at NASA's Kennedy Space Center in Cape Canaveral, Florida, U.S. March 11, 2019. This aircraft will ferry the Orion capsule to different locations. REUTERS/Mike Brown 

The Super Guppy is seen on the Shuttle Landing Facility at NASA's Kennedy Space Center in Cape Canaveral, Florida, U.S. March 11, 2019. This aircraft will ferry the Orion capsule to different locations. REUTERS/Mike Brown 

Plane seats made by Jiatai Aircraft Equipment Co are seen on display at an Aviation Industry Corporation of China (AVIC) booth during the China International Aviation and Aerospace Exhibition, or Zhuhai Airshow, in Zhuhai, Guangdong province, China November 7, 2018. Picture taken November 7, 2018. REUTERS/Brenda Goh 

Plane seats made by Jiatai Aircraft Equipment Co are seen on display at an Aviation Industry Corporation of China (AVIC) booth during the China International Aviation and Aerospace Exhibition, or Zhuhai Airshow, in Zhuhai, Guangdong province, China November 7, 2018. Picture taken November 7, 2018. REUTERS/Brenda Goh 

An Airbus A330-800 aircraft flies during a flight event presentation in Colomiers near Toulouse, France, November 6, 2018. REUTERS/Regis Duvignau 

An airplane flies over an electricity pylon at sunset in Aranda del Duero, central Spain, October 2, 2018. REUTERS/Sergio Perez 

An airplane flies over an electricity pylon at sunset in Aranda del Duero, central Spain, October 2, 2018. REUTERS/Sergio Perez 

Two planes fly in front of the setting sun in Shanghai, China October 2, 2018. REUTERS/Aly Song 

A passenger plane is seen with the moon behind as it begins its final landing approach to Heathrow Airport in London, Britain, September 25, 2018. REUTERS/Toby Melville 

A passenger plane is seen with the moon behind as it begins its final landing approach to Heathrow Airport in London, Britain, September 25, 2018. REUTERS/Toby Melville 

A passenger plane is seen with the moon behind as it begins its final landing approach to Heathrow Airport in London, Britain, September 25, 2018. REUTERS/Toby Melville 

A passenger plane passes the moon as it comes into land at the international airport in Chennai, India, September 25, 2018. REUTERS/P. Ravikumar 

A passenger plane is seen with the full moon behind as it begins its final landing approach to Heathrow Airport in London, Britain, September 24, 2018. REUTERS/Toby Melville TPX IMAGES OF THE DAY 

A DHL Boeing 757-2Q8 airplane climbs into the sky at sunset after taking off from Malta International Airport, Malta September 24, 2018. REUTERS/Darrin Zammit Lupi 

Passenger planes leave contrails in the sky as they fly over Malta at sunset, as seen from Valletta, Malta September 24, 2018. REUTERS/Darrin Zammit Lupi 

Passenger planes leave contrails in the sky as they fly over Malta at sunset, as seen from Valletta, Malta, September 24, 2018. REUTERS/Darrin Zammit Lupi 

Passenger planes leave contrails in the sky as they fly over Malta at sunset, as seen from Valletta, Malta, September 24, 2018. REUTERS/Darrin Zammit Lupi 

A passenger plane is seen with the full moon behind as it begins its final landing approach to Heathrow Airport in London, Britain, September 24, 2018. REUTERS/Toby Melville 

A passenger plane is seen with the full moon behind as it begins its final landing approach to Heathrow Airport in London, Britain, September 24, 2018. REUTERS/Toby Melville 

A firefighter films a plane as it drops water on a fire next to the village of Monchique, Portugal August 8, 2018. REUTERS/Pedro Nunes 

A military helicopter flies near flooded areas after the Xepian-Xe Nam Noy hydropower dam collapsed in Attapeu province, Laos July 26, 2018. REUTERS/Soe Zeya Tun 

A landing gear of an Airbus Beluga XL transport plane is pictured on the tarmac in Colomiers near Toulouse, France, July 19, 2018. REUTERS/Regis Duvignau 

Taiwanese President Tsai Ing-wen (C) attends a ceremony to kick start the building of Blue Magpie advanced jet trainer aircraft, at a factory of Aerospace Industrial Development Corporation, in Taichung, Taiwan June 1, 2018. REUTERS/Tyrone Siu TPX IMAGES OF THE DAY 

Taiwanese President Tsai Ing-wen attends a ceremony to kick start the building of Blue Magpie advanced jet trainer aircraft, at a factory of Aerospace Industrial Development Corporation, in Taichung, Taiwan June 1, 2018. REUTERS/Tyrone Siu 

Taiwanese President Tsai Ing-wen attends a ceremony to kick start the building of Blue Magpie advanced jet trainer aircraft, at a factory of Aerospace Industrial Development Corporation, in Taichung, Taiwan June 1, 2018. REUTERS/Tyrone Siu 

Taiwanese President Tsai Ing-wen (C) attends a ceremony to kick start the building of Blue Magpie advanced jet trainer aircraft, at a factory of Aerospace Industrial Development Corporation, in Taichung, Taiwan June 1, 2018. REUTERS/Tyrone Siu 

An aircraft flies past a building under construction on a stormy evening in Moscow, Russia, April 21, 2018. REUTERS/Maxim Shemetov 

Reacquired Volkswagen and Audi diesel cars sit in a desert graveyard near Victorville, California, U.S. March 28, 2018. Volkswagen AG has paid more than $7.4 billion to buy back about 350,000 vehicles, the automaker said in a recent court filing, and is now storing thousands of vehicles around the United States. Picture taken March 28, 2018. REUTERS/Lucy Nicholson TPX IMAGES OF THE DAY 

The nose of a Lufthansa Airbus A330-300 aircraft is seen in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

Technicians work inside a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician inspects the engine cover of a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A Lufthansa Airbus A330-300 aircraft is brought into a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician works inside a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

Technicians work inside a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A Lufthansa Airbus A330-300 aircraft undergoes maintenance in a hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

Technicians work on a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician works inside a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

Technicians work inside a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician works under the wing of a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

Technicians works on a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician works on the undercarriage of a Lufthansa Airbus A321 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

Technicians work on a Lufthansa Airbus A321 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician works on the undercarriage of a Lufthansa Airbus A321 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

Workers carry out maintenance on the seats of a Lufthansa Airbus A330-300 aircraft in a workshop at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician works on the engine of a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A Lufthansa Airbus A330-300 aircraft is brought into a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician works on the engine of a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A Lufthansa Airbus A330-300 aircraft is brought into a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician looks out of the cockpit of a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A Lufthansa Airbus A330-300 aircraft is brought into a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician boards a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi 

A technician boards a Lufthansa Airbus A330-300 aircraft in a maintenance hangar at Lufthansa Technik Malta at Malta International Airport outside Luqa, Malta, March 27, 2018. REUTERS/Darrin Zammit Lupi