of roll, exacerbated by the insufficient wing stiffness, especially with heavy underwing ordnance loads. The first production F/A-18A flew on 12 April 1980. After a production run of 380 F/A-18As (including the nine assigned to flight systems development), manufacture shifted to the F/A-18C in September 1987. Improvements and design changes In the 1990s, the U.S. Navy faced the need to replace its aging A-6 Intruders and A-7 Corsair IIs with no replacement in development. To answer this deficiency, the Navy commissioned development of the F/A-18E/F Super Hornet. Despite its designation, it is not just an upgrade of the F/A-18 Hornet, but rather, a new, larger airframe using the design concepts of the Hornet. Hornets and Super Hornets will serve complementary roles in the U.S. Navy carrier fleet until the Hornet A-D models are completely replaced by the F-35C Lightning II. The Marines have chosen to extend the use of certain F/A-18s up to 10,000 flight hours, due to delays in the F-35B variant. Design The F/A-18 is a twin engine, midwing, multimission tactical aircraft. It is highly maneuverable, due to its good thrust-to-weight ratio, digital fly-by-wire control system, and leading-edge extensions, which allow the Hornet to remain controllable at high angles of attack. The trapezoidal wing has a 20-degree sweepback on the leading edge and a straight trailing edge. The wing has full-span, leading-edge flaps and the trailing edge has single-slotted flaps and ailerons over the entire span. Canted vertical stabilizers are another distinguishing design element, one among several other such elements that enable the Hornet's excellent high angle of attack ability, including oversized horizontal stabilators, oversized trailing-edge flaps that operate as flaperons, large full-length leading-edge slats, and flight control computer programming that multiplies the movement of each control surface at low speeds and moves the vertical rudders inboard instead of simply left and right. The Hornet's normally high angle of attack performance envelope was put to rigorous testing and enhanced in the NASA F-18 High Alpha Research Vehicle (HARV). NASA used the F-18 HARV to demonstrate flight handling characteristics at high angle-of-attack (alpha) of 65–70 degrees using thrust vectoring vanes. F/A-18 stabilators were also used as canards on NASA's F-15S/MTD. The Hornet was among the first aircraft to heavily use multifunction displays, which at the switch of a button allow a pilot to perform either fighter or attack roles or both. This "force multiplier" ability gives the operational commander