A man photographs the main engines of the space shuttle orbiter Discovery on display at the Udvar-Hazy Smithsonian National Air and Space Annex Museum in Chantilly, Virginia August 28, 2015. Shuttle Discovery had 27 years of service and flew 39 times before being retired in 2011. (Photo by Gary Cameron/Reuters)

A policewoman wipes her tears as she stands in front of metal caskets containing the bodies of Special Action Force (SAF) police who were killed in Sunday's clash with Muslim rebels, at Villamor Air Base in Pasay city, metro Manila January 29, 2015. Philippine President Benigno Aquino urged legislators on Wednesday not to abandon a plan for autonomy for Muslims to end a decades-old insurgency after the clash in which dozens of people were killed, saying doing so would dash hopes for peace. (Photo by Romeo Ranoco/Reuters)

A military AWAC aircraft lands at the British RAF Akrotiri airbase on March 20, 2011 in Cyprus. Defence analysts said the sovereign military airfield at Akrotiri could be used by the RAF to help enforce a no-fly zone over Libya against Col Gaddafi’s offensive against outgunned Libyan rebels. (Photo by Matt Cardy/Getty Images)

A Polish MiG-29 flies beneath dark skies at the annual RNAS Yeovilton Air Day on July 9, 2011 in Yeovil, England. The annual air day at RNAS Yeovilton in Somerset, attracts over 35,000 people to what is one of the biggest air shows in the region and gives visitors the chance to see a host of aircraft as well as a number of historic and classic cars. (Photo by Matt Cardy/Getty Images)

A plane carrying a wing walker crashed at an air show and exploded into flames Saturday, killing the pilot and stunt walker, authorities said. The crash of the 450 HP Stearman happened at around 12:45 p.m. at the Vectren Air Show near Dayton in front of thousands of horrified spectators. No one else was hurt.

Photo: A wing walker performs at the Vectren Air Show just before crashing, Saturday, June 22, 2013, in Dayton, Ohio. The crash killed the pilot and the stunt walker instantly, authorities said. (Photo by Thanh V. Tran/AP Photo)

The second of six C-17 Globemaster III airlifters for the United Arab Emirates Air Force and Air Defence is being assembled at the Boeing assembly facility on May 10, 2011 in Long Beach, California. The UAE will take delivery of three more C-17s this year and two in 2012 as it modernizes its airlift capabilities. (Photo by Kevork Djansezian/Getty Images)

A hot air balloon in tortoise shape flies in the air during the Tazaungdaing air balloon festival in Pyin Oo Lwin, Myanmar, 25 November 2015. During the festival, hot air balloons made of multicolored papers and hung with paper lanterns, fireworks and fire sticks are exploded in mid-air. (Photo by Hein Htet/EPA)