Steve Harfst, Allegiant Air's chief operating officer since December 2014, abruptly resigned Friday.
The company offered no details of Harfst's departure beyond a four-paragraph statement that noted the resignation was effective immediately.
"We thank Steve for his contributions to Allegiant," Maurice Gallagher, chairman and CEO of Allegiant Travel Co., said in the statement announcing the departure. "We look forward to continuing his efforts to strengthen our operation."
Prior to joining Allegiant, Harfst was the chief operating officer at IndiGo Airlines based in New Delhi, India.
The Allegiant statement said the company would "use this leadership change as an opportunity to refocus on operational needs and areas for improvement. Allegiant is committed to operational excellence and looks forward to continued progress in this area."
Harfst had been the point man in Allegiant's efforts to negotiate a contract with Teamsters Local 1224, the company's pilot union.
Harfst played a significant role in April to avert a strike by pilots. The matter landed in U.S. District Court in Las Vegas and in May, U.S. District Judge Andrew Gordon released a decision issuing a preliminary injunction that blocked Allegiant Air pilots from striking.
Harfst also presided over several months of turbulent operational issues involving a series of emergency landings and aircraft diversions, but the Federal Aviation Administration has never stepped in to discipline the Las Vegas-based airline.