Ryanair has grounded two planes after Boeing recommended a vital component be replaced on some of its next-generation 737 aircrafts.
Ryanair says the planes were grounded last night while the replacement part - in the flight control modules - was fitted.
The planes have now been returned to service.
Boeing says fewer than 50 jets worldwide were affected by the potential fault.
Eight Ryanair flights were delayed as a result of the problem while back-up aircraft were being prepared.
Each plane has two modules that control hydraulic fluid to the flight control system.
A failure of both modules could significantly affect the flight control systems, according to the US-based Federal Aviation Administration.
A Boeing spokesman said: "Operators have been instructed to check the serial numbers of the Flight Control Modules. If both parts are within the identifiable number block then Boeing recommends operators should take the plane out of service until the part is replaced."
He added: "If it is only one part, then operators are instructed to perform specific pre-flight to ensure the module is operating properly. Fewer than 50 aircraft within a fleet of more than 1,300 may be affected."
A Ryanair spokeswoman said: "This affected just two of Ryanair's 23 new 737 800-series aircraft. These two planes were immediately grounded on Boeing's recommendation last night. The parts were fitted last night and the planes have been returned to service this morning."
PA