Ryanair chief executive Michael O'Leary insisted Thursday he's serious about making passengers pay for the right to relieve themselves on flights - and is flush with interest in the idea of mounting credit-card-operated toilets. O'Leary, whose Dublin-based airline has pioneered the practice of filling air travel with extra charges, divided opinion last week by suggesting that his next target would be coin-operated toilets. Aviation analysts and the traveling public alike couldn't tell whether O'Leary - a cut-throat operator with an entertainingly bombastic style - was poking fun at his own reputation. The Ryanair boss told a dumbfounded Dublin press conference Thursday the days of the unlocked potty are indeed numbered. "Eventually it's going to happen. It's just we can't do it at the moment because we don't have a mechanism for charging you," he said. O'Leary said he has asked engineers at US aircraft maker Boeing - which supplies Ryanair's entire fleet of 737-800s - to design toilets with doors that open only if you swipe a valid credit card through the locking mechanism. He conceded that his earlier idea, to make them coin-operated, wouldn't work in part because Ryanair operates heavily in areas using both the euro and British pound. "We have looked into this before, and the problem is Boeing can't come up with a mechanism on the toilet door to take coins," he said. "We're suggesting they go back and look at a mechanism where you'd swipe the credit card for a quid [British pound] on the toilet door. They've gone off to look at that... We are serious." He didn't seek to soften the impression that Ryanair was happy to make its passengers suffer. "Most people would go to the loo [toilet] before they get on the plane, or they hold it until they land. You would only have to deal with the people who absolutely have to go," he said. One positive effect, he said, would be to "reduce an awful lot of the unnecessary visits to the toilet that [expletive] so many passengers off." It could also become the next serious moneymaker at Ryanair, already Europe's most profitable and aggressively expanding airline. O'Leary estimates if 20 percent of passengers pay Â£1 to use the rest room, this would generate Â£15 million annually - which he characterizes as future "fare savings to the traveling public." Ryanair is famous for offering officially "free" flights that end up costing â‚¬50 ($70) or more once fees for check-in, luggage and on-line payment are applied. On board, the airline hawks bingo cards and duty-free goods on short-hop flights that can last barely an hour. It offers no snacks or drinks for free, not even tap water.