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Maximum Takeoff Mass versus Operational Empty Mass
Figure 4.6. OEM versus MTOM
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MTOM (kg)
decreasing). In the midrange (i. e., 70- to 200-passenger class – single-aisle, narrow – body), the OEMF is around 0.56. At the higher end (i. e., more than 200 passengers – double-aisle, wide-body), it is leveling out at around 0.483; the MTOM is slightly more than twice the OEM. The decreasing trend of the weight fraction is due to better structural efficiencies achieved with larger geometries, the use of lighter material, and the more accurate design and manufacturing methods of more recent designs.
The OEM is a function of aircraft load experienced on both the ground and in the air, which depends on the MTOM. The load in the air is a result of aircraft speed-altitude capabilities, the maneuverability limit, and wind. A higher speed capability would increase the OEMF to retain structural integrity; however, the OEM would reflect the range capability for the design payload at the MTOM (see Figure 4.5). Payload and fuel load can be exchanged to reach the MTOM from the OEM.
Figure 4.6a is represented in higher resolution when it is plotted separately, as shown in Figure 4.6b for midrange-size aircraft. It also provides insight to the statistical relationship between the derivative aircraft of the Boeing 737 and Airbus 320 families. The approaches of the two companies are different. Boeing, which pioneered the idea, had to learn the approach to the family concept of design. The Boeing 737-100 was the baseline design, the smallest in the family. Its growth required corresponding growth in other aerostructures yet maintaining component commonality as much as possible. Conversely, Airbus learned from the Boeing experience: Their baseline aircraft was the A320, in the middle of the family. The elongated version became the A321 by plugging in constant cross-section fuselage sections in the front and aft of the wing, while retaining all other aerostructures. In the shortened versions, the A319 came before the even shorter A318, maintaining the philosophy of retaining component commonalities. The variants were not the optimized size, but they were substantially less costly, decreasing the DOC and providing a competitive edge.
