United CEO Oscar Munoz and Chairman Henry L. Meyer III are backed by Air Line Pilots Association, and unions representing United mechanics and flight attendants n fighting the attempt by a pair of hedge funds to gain six seats on United's 15-member board.

"United's pilots have grave concerns about the sudden attempt by two activist investors to gain effective control of United's board of directors," said Todd Insler of the Air Line Pilots Association in a news release.

"This coup attempt, being done for their own benefit and without publicly stating their intentions for the future of the airline, unnecessarily distracts all employees from our commitment to improve customer service and grow United Airlines," added Insler, who himself has a seat on the airline's board and represents 12,000 members.

Munoz, who underwent a heart transplant after falling ill five months ago, is set to return March 14 when United will hold a summit of its various labor unions. The meeting originally had been scheduled for Oct. 15, the very day Munoz suffered a heart attack. He went on sick leave shortly after falling ill, and general counsel Brett Hart had served as acting CEO

"Munoz has demonstrated by his actions, not merely words, that he believes employees make the difference in the success of an airline," said International Association of Machinists and Aerospace Workers President Mike Klemm,   which represents 28,000 United workers and is negotiating a contract with United "The progress we have already made in working with his team is proof of his commitment to United's most important asset - its employees. This challenge to United's board has the potential to distract the company and derail the progress we have made."

Sara Nelson, president of the Association of Flight Attendants, which represents 24,000 United employees who are in arbitration over their new contract with United, said "Even during his recovery and sick leave, Munoz engaged meaningfully in repairing labor relations and getting flight attendant contract negotiations on track to conclusion,"

"These investors are creating a distraction at just the wrong time," she added in a statement.