NEW YORK (AP) — United Airlines CEO Oscar Munoz says he plans to return to work sometime in the first quarter of 2016.

Munoz, 56, has been out on medical leave following an Oct. 15 heart attack. The company’s general counsel, Brett J. Hart, has led the company in his absence.

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“My time away will be a little longer than I would like, but based upon discussion with my doctors I will be back in the first quarter,” Munoz said in a letter to employees released by the company.

Munoz had spent a month leading the Chicago-based United Continental Holdings before having the heart attack. He became CEO on Sept. 8 after Jeff Smisek was pushed out amid a federal investigation into the airline’s dealings with the government agency running one of its most important airports.

During his short tenure, Munoz acknowledged that United had failed its passengers with chronic delays and computer outages and that it had poor relations with its workers. He promised to fix both.

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Munoz served as president and chief operating officer of CSX Corp. before accepting the CEO job with United. He joined the United Continental board in 2010.

United has been plagued by technology challenges since its 2010 merger with Continental. More recently, federal prosecutors began investigating whether under Smisek the airline gave preferential treatment to a former chairman of the Port Authority of New York and New Jersey, which operates the New York-area airports.

United has only said that it made the decision to oust Smisek and two other senior executives following an internal investigation into the Port Authority matter.

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