John Liotine, the mechanic who first alerted authorities to potential problems at Alaska Airlines’ Oakland maintenance facility, has now filed a $20 million defamation lawsuit against the company. Liotine charged that information on the airline’s Web site suggested he was incompetent and he had accused his supervisors of misconduct because they passed him over for promotion. The mechanic’s claims about maintenance practices, which the airline has consistently denied, triggered ongoing investigations of Alaska Airlines’ operations by the Federal Aviation Administration and a grand jury in San Francisco.