Possible Clues:

• UAL competitor
• Giant among low-cost carriers: Abbr
• Tex.-based carrier
• Budget carrier HQ'd in Dallas
• Dallas-based carrier: Abbr
• Namibia, formerly (initials)
• Flyer HQ'd in Dallas
• United competitor: Abbr.
• Namibia until 1990, briefly
• No-frills carrier: abbr.