U.S. regulators said yesterday they would conduct a special investigation of Seattle-based Boeing Corporation after a string of recent aircraft quality problems, including improperly tightened bolts and parts that failed federal flame tests. The Federal Aviation Administration said the inspection of production lines in Seattle and Boeing’s quality control procedures would begin December 2 and likely run through February. The FAA’s aircraft certification director did not rule out the possibility of fines for the Seattle-based aerospace giant.