The RAF's upgraded BAE Systems Harrier GR.9 prototype development aircraft (ZD320), a follow-on to GR.7/7A ground-attack fighter, made its initial flight from Warton in Lancashire, on May 30.

The vertical take-off and landing-capable Harrier served in the U.K.'s Royal Air Force for five decades from 1969 to 2011. But why was it retired?