Mitsubishi Uses Precision To Cut MRJ Costs

By Bradley Perrett
Source: Aviation Week & Space Technology

Each of the fuselage modules of the first prototype took about half as much time to put together as might have been expected with normal fabrication precision, Tatsuoka says. Thanks partly to prior practice on engineering prototypes, no major problems arose, and almost no adjustment was needed to make parts fit. Making shims—inserts that fill unintended gaps between parts—is a seriously time-consuming process in assembly, because measurements must be made for each one, which is then customized for its joint.

Normally, about 10% of holes will require reaming to get a fit when pieces are joined for riveting, laying one hole over another, Tatsuoka says, without specifying which parts of the aircraft that ratio applies to. For the MRJ the fraction was much less, “a very small number.” The tolerance of drilled holes is normal, however, because MHI is using standard rivets, says Akihiko Ishikawa, general manager of MHI's MRJ manufacturing program.

Availability of precise manufacturing machinery appears to have been a key enabler for this design and production strategy. MHI, Mitsubishi Aircraft's majority owner, needed only to buy few machines to build the MRJ's structure; its fabrication equipment could already achieve the necessary tolerances. In earlier programs, the precision capability had not been fully used, says Kawai. That may seem surprising, but he adds: “Almost all the machines in Japan are precise.”

Tight tolerances should also pay off in MRJ operation. Carefully manufactured skins with a surface finish within 10 microns (0.4/1000 in.) are mounted on precisely built frames, minimizing drag. The smoothness of the skin of the first MRJ prototype, the tight alignment of its panel joins and the regularity of its riveting were readily apparent during Aviation Week's visit to the Tobishima factory in southern Nagoya.

“The assemblies look very clean for early articles,” says an experienced production engineer from another manufacturer, examining photographs of the prototype. “There are no apparent tool marks [and] the fasteners are all the same size and in a consistent pattern, so it does not appear that there's been workmanship problems or even rework caused by an engineering issue or change.”

Visible fasteners on the MRJ prototype all appear to be seated properly, meaning there are no gaps underneath the head or collar. Accurate drilling has evidently kept them perpendicular to the surface, as they should be. From the outside, at least, sealing between panels looks like high-quality work.

A consultant production engineer who has also examined close-up photographs agrees that MHI has done a good job on the prototype.

The global aerospace industry is increasingly adopting moving assembly lines, in which the product continuously edges through the factory, creating a production beat and reducing wasted time and inventories. Since this is part of the lean manufacturing philosophy, largely learned from Japan and especially Toyota Motor, it is not surprising that MHI is applying the technique to MRJ production. The company has hired the Japanese consultant who advised Boeing on the process, says Kawai. MHI instituted its first moving line at Nagoya and then applied the process to production at its factory in Hanoi, where it makes flaps for Boeing 737s.

Although the MRJ is largely built of conventional aerospace aluminum, carbon-fiber reinforced plastic will be used for the moveable surfaces and for the tail. For the latter, MHI is using a process called A-Vartm—a variation of vacuum-assisted resin transfer molding. In this process, carbon fibers are infused with resin partly by suction, so the cost of an autoclave is unnecessary. MHI and material supplier Toray developed the A-Vartm variation.

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