Southwest Airlines (WN, Dallas Love Field) has been given a dispensation by the US Federal Aviation Administration (FAA) allowing 128 B737-700s, voluntarily grounded by the carrier earlier this week, to remain in service so long as they undergo mandatory inspections of a backup rudder hydraulic system within the next five days.

Upon discovering the jets had flown beyond their required inspection hours, the Texan LCC, on Tuesday, grounded the affected aircraft before informing the FAA of the lapse.

"Once identified, Southwest immediately and voluntarily removed the affected aircraft from service, initiated maintenance checks, disclosed the matter to the FAA, and developed an action plan to complete all overdue checks," Southwest said in a statement. "The FAA approved a proposal that would allow Southwest to continue operating the aircraft for a maximum of five days as the checks are completed."

Southwest says approximately 80 cancellations occurred on February 24 as a result of the grounding.

The LCC's move to voluntarily ground the aircraft, while largely in the interests of maintaining customer and staff safety, follows a USD12 million civil penalty filed by the FAA in mid-2014 against the airline for failing to comply with Federal Aviation Regulations in three separate enforcement cases related to repairs on Boeing 737s Southwest operates.