There is not much difference between the Airbus A330 and the A340 in terms of structure, systems, and wings. The biggest difference is the engines. The A330 is a dual-engine aircraft, while the A340, its longer sibling, is a quad-engine aircraft. But why did Airbus need to launch two identical aircraft, with the longer one having smaller engines?
ETOPS (Extended-range Twin-engine Operational Performance Standards) were not yet developed while the A340 was being developed. While some airlines chose two engines because they had lower operating expenses, others wanted four engines because they had more dependability at a higher cost. A four-engine aircraft can carry more fuel for a given wing design because of the mass distribution throughout the wings and the good bending moment relief provided by the outer engines in particular. The fundamental justification is safety: An aircraft must be able to sustain flying even if one engine stalls to undertake long-haul flights that cross the sea for an extended period. Compared to a 4-engine aircraft, a 2-engine aircraft will lose just 25% of its power. Its four engines let it fly longer overwater trips, which the A330 could not do.
The debate about whether to use smaller engines on the bigger A340 can go on for eternity, and several hypotheses are developed in this regard. Airlines chose and desired a longer aircraft that could fly farther when the A340 was released. To meet demand, Airbus developed the A330 and the A340. In essence, Airbus requested it. The two or four-engine configuration is, of course, where the A330 and A340 diverge most. The A340 features a bigger wing and an extra center rear landing gear. Most heavy aircraft use two gears with larger wheels to handle the additional load, making this middle landing gear unusual.
The Boeing 707 and Douglas DC-8 were two outdated quad jets that the A340 was designed to replace. Airbus has not yet joined this industry but saw an opportunity to provide a more effective rival utilizing modern technology. The McDonnell Douglas MD-11 troubles gave the A340 a boost right away as well. Before the A340, this trijet widebody had its first flight in January 1990. But it soon became clear that it fell short in terms of range and fuel economy. The A340 was quickly eclipsed by the Boeing 777, and Airbus announced the end of the A340 program in 2011.