Ryanair Holdings Plc Chief Operating Officer Michael Hickey will step down from Europe's biggest discount carrier after a management misstep resulted in the cancellation of more than 20,000 flights into next summer.

Hickey will step down at the end of October, the airline said in a statement. A search to identify and recruit a successor is underway.

Ryanair's botched rosters has affected flights for about 700,000 customers and reduced the company's growth plans by 6 million passengers this year and next. Chief Executive Officer Michael O'Leary has taken much of the responsibility for the mismanagement of its pilots' annual leave requirements that had left Ryanair without enough cockpit crew to operate its full schedules.

"Over the past 30 years Mick Hickey has made an enormous contribution to Ryanair," O'Leary said in the statement. "He will be a hard act to replace, which is why we are grateful he has agreed to continue in an advisory role to smooth the transition to a successor."

Hickey will conclude "a number of large projects" before leaving, such as arranging an engine maintenance contract and new hangars in Madrid and Seville, Spain, O'Leary said. The operating chief, who assumed the post in 2014, has been at the airline since 1988, joining as an engineer and rising to direct the department.

This is the first high profile resignation since the Sept. 15 start of the crisis, which has since emboldened Ryanair's pilots to air concerns about contracts and pay. O'Leary on Thursday published a letter promising pilots more money and better conditions to stem defections to competitors such as Norwegian Air Shuttle ASA and the U.K.'s Jet2.