Northwest Airlines Corp. pilots overwhelmingly authorized a strike if 2-year-old contract negotiations fail to produce an agreement. A spokesman for the Air Line Pilots Assn. said more than 99% of the 5,279 members voted in favor of the strike authorization. A Northwest spokesman said the carrier was not surprised by the vote and hopes to avoid a strike. A walkout couldn't take place until the National Mediation Board declares an impasse and either the union or management rejected arbitration. A 30-day cooling-off period would then follow. Major issues such as wages and job security are still being negotiated, and talks resumed Sunday. The St. Paul, Minn.-based company's stock fell $2.25, or 6.2%, to $46 after a third analyst this month reduced earnings estimates for the airline.