During the late 1980s Boeing developed the 747-400, with the same fuselage length and extended upper deck as the 747-300, but with a larger wingspan, winglets, a highly modernised digital flight deck, higher weights and a lower fuel consumption thanks to improved engines. The 747-400 made its maiden flight on 29 April 1988. It is also available in a 'short-bulge' cargo version (747-400F)and as a combi (747-400M). The 747-400D (Domestic) for short-range flights doesn't have winglets. The 747-400ER is an improved extended range version, of which also a freighter variant exists, the 747-400ERF.Boeing sold more than 1300 aircraft of all 747 versions. 724 aircraft were so-called 'Classics' (747-100, 747-200, 747-300 and 747SP). Today a growing number of passenger aircraft is being converted to freighters. Some Boeing 747 aircraft are used for special purposes, like two 747-200s designated 'Air Force One' as the U.S. presidential aircraft and four 747-200s designated E-4 as airborne emergency command and control posts for the US Air Force. One 747 was modified to ferry space shuttles between California and Florida and one modified 747SP is flying as an astronomical observatory with an infrared telescope on board