Boeing said Friday that hairline cracks have been found in the wings of some of its 787 Dreamliners that are still in production, due to a manufacturing problem. A Boeing spokesman told AFP the cracks were found in the composite wings made by Mitsubishi Heavy Industries of Japan, but insisted it was "confident that the condition does not exist in the in-service fleet." Boeing had been notified by MHI that "a change in their manufacturing process may have led to hairline cracks in a limited number of shear ties on a wing rib in the 787." The tiny cracks may be present in a limited number of airplanes still in production, the Chicago-based aerospace giant said. MHI was not immediately available to comment. Boeing said it was completing inspections of the roughly 40 airplanes that are potentially affected. Deliveries may be delayed because of inspection and repairs, but the company expected no impact to its overall 787 Dreamliner delivery expectations this year. The company plans 110 deliveries. "This will have no material impact on revenue," the Boeing spokesman said. It was the latest in a series of problems that have beset the high-tech 787 put into service two years ago, including a months-long global grounding over battery problems last year. Lightweight composite materials are used in 50 percent of the all-new jetliner, which uses 20 percent less fuel with fewer emissions than today's similarly sized airplanes, according to Boeing. Boeing is using foreign suppliers for many parts of the Dreamliner, which is assembled at plants in Seattle, Washington state, and North Charleston, South Carolina. The company has ramped up production of the twin-aisle plane, which entered service in September 2011 with Japan's All Nippon Airways. In January, Boeing rolled out its first 787 built at the rate of 10 airplanes per month, the highest rate ever for a twin-aisle plane. It was the third production rate hike in just over a year. Boeing has booked more than 1,000 orders for the 787 from 60 customers worldwide. Shares in Dow member Boeing slipped 0.3 percent to $128.54 on the New York Stock Exchange. Boeing was down 0.5 percent at $127.91 in after-market trade.