(FORTUNE Small Business) – In our October issue, FSB wrote that small-plane operator Atlantic Coast Airlines (ACA) was trying to launch its own discount airline after a contract dispute with its biggest customer, United Airlines. On Oct. 6, Mesa Air Group, a Phoenix-based carrier, made a hostile offer to buy ACA for $571 million in stock. ACA rejected the bid, but not before Mesa called on shareholders to unseat the board. Some speculate that United, which has contracts with Mesa, is behind the bid. Mesa denies it: "We have discussions with United every day, but not about this," says CEO Jonathan Ornstein. Ray Neidl of Blaylock & Partners says that ACA may be forced to renegotiate its United contract. These skies aren't friendly.--N.S.