fuel, nearly half the aircraft's empty weight. The skins of the wings are produced using large milling machines, which in 1978 were claimed to be superior to anything else in North America. Many elements, such as the flaps, ailerons, and leading edge, feature conventional construction, but several parts, including the wing/fuselage fairing, flap shrouds, and wingtips, are moulded out of Kevlar, as are other elements of the aircraft. The original CL-600 Challenger was powered by two Lycoming ALF 502L turbofan engines, which were developed specifically for the Challenger. Subsequent models adopted other powerplants, including the General Electric CF34 engine. The engines are mounted on the rear fuselage close to the aircraft's tail, providing smooth airflow to the engines even when flown at high angles of attack, although this was in a lower position than the original LearStar concept had placed them to mitigate unfavourable pitch control characteristics. The engines are fitted with thrust reversers to decrease landing distances. An auxiliary power unit is also present for starting the engines and providing air conditioning within the cabin while on the ground. The fuselage comprises three sections — the nose, centre, and tail — which are manufactured separately in their own jigs and joined late on in the production process. It has been designed to be pressurised at a maximum differential of 9.3 lb/sq in. Various cutouts are present across the fuselage to accommodate various features, such as a large main door on the port side of the aircraft forward of the wing, multiple regulation-compliant emergency exits, a baggage hatch on the port-side aft of the wing, and numerous windows. The fuselage diameter was designed to accommodate an unobstructed cabin floor, a cabin height of 6 ft 1 in in the centre section, and space for the wing box, underfloor integral fuel tanks, air ducts, and various control cabling. It was also designed to easily accommodate Canadair's early plans to stretch the fuselage, for which equal-length plugs are installed fore and aft of the centre section to greatly increase the Challenger's capacity. Various avionics are present in the Challenger. As standard, the CL-600 is furnished with a dual-channel Sperry SPZ-600 automatic flight control system, incorporating a flight director and air data computers; more typical to larger commercial aircraft, this system is certifiable for conducting Category 3A automatic landings. The flight control system features significant redundancy, including three individual hydraulic systems; even with