In the U.S. Air Force, "WSO" also refers to a USAF combat systems officer (CSO, formerly USAF navigator) who is the aft crew member in the F-15E Strike Eagle. In the B-52, the CSO crew members on the lower deck of the aircraft who were previously designated as navigator and radar navigator (bombardier) are now known as weapon systems officers while the CSO on the upper deck is an electronic warfare officer. In the B-1B Lancer bomber, there are 2 weapon systems officers manning crew positions aft of the pilot and co-pilot. They are known as the Offensive Systems Officer (OSO), and the Defensive Systems Officer (DSO). WSOs also flew in the now-retired F-111 Aardvark strike bomber, flown by the US Air Force and Royal Australian Air Force, and the now retired USAF EF-111A Raven airborne jammer. In the F-111, the WSO (EWO in the EF-111) was seated directly to the right of the pilot/aircraft commander. The WSO integrates with the pilot to collectively achieve and maintain crew efficiency, situational awareness and mission effectiveness. In the U.S. Air Force fighter aircraft such as the F-15E Strike Eagle, the WSO can pilot the aircraft when required, although this is typically during non-tactical portions of the mission (e.g., en route to mission area or returning to base). When designated as the mission commander (MC), the WSO is also responsible for all phases of the assigned mission, especially if there are multiple aircraft involved. For example, the aircraft pilot could be the junior member of a flight crew such as a first lieutenant or captain, and the weapon systems officer could be a senior officer such as a major, lieutenant colonel, or colonel; regardless of rank, the WSO would be the mission commander if he/she is qualified.