Pilots at American Airlines and US Airways have approved a single labor contract, a step toward combining workforces at the two carriers, which merged in December 2013. The multiyear deal gives the carriers’ 15,000 pilots a 23 percent pay raise retroactive to Dec. 2. The Allied Pilots Association said Friday that the contract was approved 66 percent to 34 percent, with 95 percent of eligible pilots casting a vote. The outcome could help American avoid some of the labor-integration issues that have dogged other airlines after mergers. United Airlines has a joint contract with pilots but not with mechanics or flight attendants. The pilots at US Airways did not approve a joint contract after the 2005 merger with America West Airlines, forcing the combined carrier to operate with separate crews.