Pacific Southwest Airlines (PSA) was an airline headquartered in San Diego, California. It is one of the four heritage airlines that form US Airways, the other airlines being America West Airlines, Piedmont Airlines and Allegheny Airlines. Prior to its merger, PSA was one of the first large discount airlines in the United States and is considered a precursor to Southwest Airlines. Its Boeing customer number was 14. Following the merger with USAir, the PSA name was given to Jetstream International Airlines in order to preserve the PSA name and trademarks.