Korean Air will sell five of its planes to US aerospace firm Sierra Nevada Corporation, the airline said in an exchange filing on Wednesday. Sierra Nevada recently won a $13 billion US Air Force contract to develop a successor to the E-4B Nightwatch, known as the Doomsday plane due to its ability to survive a nuclear war and act as a command and control centre during emergencies. Sierra Nevada did not immediately respond to a request for comment. The Korean Air planes being sold are four-engined Boeing 747-8s, a source familiar with the matter said. The four current E-4 planes, which have been in service since the 1970s, are modified Boeing 747-200s. The sale, valued at 918b Korean won ($674m), is in line with Korean Air's medium to long-term plan to dispose of older aircraft and replace them with newer generation ones, the Korean Air filing said.<br/>