Subcontractors are emerging as the weakest link in Boeing’s supply chain. The WSJ’s Sharon Terlep and Andrew Tangel report that a Boeing engineer warned at an internal symposium in 2001 that entrusting other companies with the production of parts was a big risk, but the company plowed ahead regardless. The strategy is being questioned in the wake of a string of quality problems that include complaints of loose rudder bolts and a recent blowout of a door plug on an Alaska Airlines flight. Dozens of factories build key pieces of 737 and 787 jets before they are assembled by Boeing. The system has slashed costs by letting production lines maximize output and eliminate waste. But, as the Boeing engineer pointed out, it adds risks because the final product is only as good as "the least proficient of the suppliers.”