DALLAS (AP) -- Pilots for American Airlines on Wednesday sued some of their counterparts at TWA over the sensitive issue of seniority, which affects pay and rank in the cockpit. The Allied Pilots Association wants to place about half of TWA's pilots at the bottom of the seniority list when the two sets of workers are combined -- American's parent bought Trans World Airlines last year. The union said it filed the lawsuit so that a federal judge in Fort Worth can rule on the plan's legality. Seniority is a crucial issue for pilots. It determines their assignments, including whether they fly as captain or flight officer. Also, airlines cut from the bottom of seniority lists when laying off employees. ``Given the importance of the seniority integration agreement, APA is entitled to know, as we begin to enforce the agreement, whether it is valid and enforceable,'' said John Darrah, president of the 11,000-member union. American Airlines agreed to the seniority plan. But TWA pilots, who are represented by the Air Line Pilots Association, objected that they were treated unfairly by the deal. American's union ``unilaterally concocted this integration scheme, and now they are trying to get a Texas federal judge to rule on this,'' said Jeff Darnall, a spokesman for the TWA pilots. ``Even in their own mind they have reason to feel they need some sort of declaratory statement by a responsible party'' to uphold the plan. Darnall said his union's lawyers were studying the lawsuit but had not yet decided on a response. Darnall said the American pilots' plan would put 1,250 of TWA's 2,250 pilots at the bottom of American's seniority list and leave even veteran TWA captains behind American pilots with less experience. The Allied Pilots Association did not actually sue its rival union. Instead, it sued Bud Bensel, leader of a vocal group of TWA pilots protesting the seniority plan. The union said Bensel's group, calling itself the Aviation Workers Rights Foundation, was set up as a ``litigation vehicle'' to derail the seniority plan. Bensel could not be contacted. American's pilots also named the airline as a defendant, but in such a way that it ruled out damages against the carrier, the union said. A spokeswoman for American said the airline would not comment. American and TWA pilots still operate on separate seniority lists. But American's pilots have asked the National Mediation Board to declare American and TWA a single carrier, which American's pilots believe would make their preferred seniority plan official. American's pilots are confident that the mediation board will rule soon in their favor. The Aviation Workers Rights Foundation, fearing that result, has asked for an independent arbitrator to decide seniority issues.