And while falling jet fuel prices have helped United save money, the airline said the weakness in the energy sector is now hurting demand as fewer oil workers take business trips.

Then there's the internal turmoil at United.

New CEO Oscar Munoz, who just joined the company in September, suffered a heart attack on Thursday.

United (UAL) said on Monday that Munoz, 56, would be taking a medical leave and named its general counsel, Brett Hart, to be its acting CEO.

In a statement Thursday, Hart thanked United's employees for their "hard work, professionalism and contributions" and added that "the United family has had a challenging few weeks."

Hart said United would continue to work on the agenda put into place by Munoz to try and boost the airline's profit margins.

Related: United names acting chief following CEO's heart attack

Munoz replaced Jeff Smisek, who resigned last month in the wake of some questionable dealings with the Port Authority of New York & New Jersey.

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