“Once you are trained and skilled at doing it… I would challenge any pilot to explain how this is a difficult job or how it is they are overworked or how anybody who by law can’t fly more than 18 hours a week could possibly be suffering from fatigue,” he said at Ryanair’s annual shareholder meeting on Thursday, according to The Guardian.

O’Leary’s comments come days after the discount carrier announced that it would be cancelling up to 50 flights a day for the next six weeks due to an internal mix-up in the planning of pilots’ time off. The mishap affected over 300,000 passengers and 2% of all Ryanair flights in the period. The airline said on Wednesday that it expected to re-accommodate over 175,000 of those customers by the end of the day.

Read: RyanAir’s Stock Is Tumbling As It Cancels Hundreds of Flights

To ease the on-going crisis, Ryanair (RYAOF) had offered pilots a tax-free one-time bonus of £12,000 (about $16,000) if they agreed to work extra days, which would have headed off further cancellations. But a group of pilots rejected that offer on Thursday and threatened to launch a “work to rule” campaign. “Working to rule” means pilots would refuse to work outside the constraints of their employment contracts. They’d refuse to show up early for shifts or work overtime to fill in for missing colleagues, for example.

Read: Massive Pilot Sick-Out Makes Air Berlin Cancel 100 Flights

O’Leary said on Thursday that pilots scheduled for a four-week break in coming months could be required to delay one of those weeks until January due to the terms of their contract.

Also on Thursday, SkyNews reported that Ryanair plans to hire 125 new pilots in the next week or two as it faces the pilot shortage.

Ryanair did not immediately return a request for comment.