mission codes when it is more suitable (e.g., M in MH-53J Pave Low III). The defined codes are: A: Attack aircraft (for tactical air-to-surface mission) B: Bomber (for strategic air-to-surface mission) C: Cargo (i.e. Transport) E: Special electronic installation F: Fighter K: Tanker (dropped between 1977 and 1985) L: Laser-equipped O: Observation (Forward Air Control) P: Maritime patrol R: Reconnaissance S: Anti-submarine warfare T: Trainer U: Utility X: Special research The rise of the multirole fighter in the decades since the system was introduced has created some confusion about the difference between attack and fighter aircraft. According to the current designation system, an attack aircraft (A) is designed primarily for air-to-surface missions (also known as "attack missions"), while a fighter category F incorporates not only aircraft designed primarily for air-to-air warfare, but also multipurpose aircraft designed also for attack missions. The Air Force has even assigned the F designation to attack-only aircraft, such as the F-111 Aardvark and F-117 Nighthawk. The only A designated aircraft currently in the U.S. Air Force is the A-10 Thunderbolt II. The last front line A designated in the U.S. Navy and Marine Corps was the A-6 Intruder, with the only strictly A designated fixed-wing aircraft remaining is the A-29 Super Tucano leased under the Imminent Fury program. When the new mission codes were implemented, the numerical series were restarted, causing some redesignated naval aircraft and subsequent new designs to overlap disused USAAC/USAAF designations. Of these code series, no normal aircraft have been assigned a K or R basic mission code in a manner conforming to the system. Vehicle type The vehicle type element is used to designate the type of aerospace craft. Aircraft not in one of the following categories (most fixed-wing aircraft) are not required to carry a type designator. The type categories are: D: Unmanned aerial vehicle (UAV) control segment G: Glider H: Helicopter Q: Unmanned aerial vehicle S: Spaceplane V: Vertical take-off/short take-off and landing (VTOL/STOL) Z: Lighter-than-air A UAV control segment is not an aircraft, it is the ground control equipment used to command a UAV. Only in recent years has an aircraft been designated as a spaceplane, the proposed MS-1A. Design number According to the designation system, aircraft of a particular vehicle type or basic mission (for manned, fixed-wing, powered aircraft) were to be numbered consecutively. Numbers were not to be assigned to avoid confusion with other letter sequences