The sizing point in Figure 11.3 shows a wing-loading, W/SW = 64 lb/ft2, and a thrustloading, T/W = 0.32; there is little margin given for the landing requirement. The maximum landing mass for this design is at 95% of the MTOM. If for any reason the aircraft OEW increases, then it is better if the sizing point for the W/SW is somewhat lower than 64 lb/ft2 – for example, 62 lb/ft2 and/or increase T/W to 0.35. A quick iteration resolves the problem; however, this choice is not exercised to keep the wing area as small as possible. Instead, an aircraft is allowed to approach landing at a slightly higher speed because the LFL is generally shorter than the TOFL. This is easily achievable because the commonality of the undercarriage for all variants starts with the design of the heaviest (i. e., for the growth variant), and then the bulky components are reduced for lighter weights. The middle variant is used as the baseline version; its undercarriage can be made to accept the MTOM growth as a result of the OEW growth instead of making the wing larger.
Civil aircraft are recommended to come in a family of variants in order to cover wider market demands to maximize sales. However, none of the three variants is optimized, although the baseline is carefully sized in the middle to accept one larger and one smaller variant. Even when development costs are front-loaded, the variant aircraft cost is low by sharing the component commonality. The low cost is then translated to a lowering of the aircraft price, which absorbs the operating costs of the slightly nonoptimized designs.
It is interesting to examine the design philosophy of the Boeing 737 and the Airbus 320 families of aircraft variants in the same market arena. Together, more than 8,000 aircraft have been sold in the world market, which is no small achievement in engineering. The cost of these aircraft is about $50 million each (in 2005). For airlines with deregulated fare structures, making a profit involves complex dynamics of design and operation. The cost and operational scenario changes from time to time (e. g., increases in fuel cost and terrorist threats).
As early as the 1960s, Boeing recognized the potential for keeping component commonality in offering new designs. The B707 was one of the earliest commercial – transport jet aircraft to carry passengers. It was followed by a shorter version, the B720. Strictly speaking, the B707 fuselage relied on the KC135 tanker design of the 1950s. From the four-engine B707 came the three-engine B727 and then the two – engine B737, both of which retained considerable fuselage commonality. This was one of the earliest attempts to utilize the benefits of maintaining component commonality. Subsequently, the B737 started to emerge in different sizes of variants, maximizing the component commonality. The original B737-100 was the baseline design; all other variants that came later, up to the B737-900, are larger aircraft. This posed certain constraints, especially on the undercarriage length. Conversely, the A320 (serving as the baseline design) was in the middle of the family; its growth variant is the A321 and its smaller variants are the A319 and A318. Figure 4.7 illustrates how the OEW is affected by the two examples of family variants. A baseline aircraft starting in the middle of a family is better optimized; therefore, in principle, it provides a better opportunity to lower production costs of the variants.
The simultaneous failure of two engines is extremely rare. If it happens after the decision speed is reached and there is not enough clearway available, then it is a catastrophic situation. If the climb gradient is not in conflict with the terrain of operation, it is better to take off with higher flap settings. If a longer runway is available, then a lower flap setting can be used. Takeoff-speed schedules can slightly exceed FAR requirements, which stipulate the minimum values. There have been cases of all-engine failures occurring at cruise due to volcanic ash in the atmosphere, as well as in the rare event of fuel starvation. Fortunately, the engines were restarted just before the aircraft would have hit the surface. An all-engine failure due to a bird strike occurred in 2009 – miraculously, all survived after the pilot ditched the aircraft in the Hudson River in New York.