Southwest Airlines says it has grounded 128 planes after failing to inspect backup hydraulic systems used to control the rudder if the main system fails. With about 20% of the airlines’ fleet grounded, Southwest spokesperson Brandy King said the airline canceled about 80 flights Tuesday and expected to cancel about 19 more on Wednesday while inspections are completed. She said the airline expected to finish “a good portion” of the inspections by Wednesday morning.

The missed inspections have forced Southwest to scramble late Tuesday in negotiating unusual arrangements with federal regulators in an effort to keep the planes flying while the checks were completed. When the carrier informed the Federal Aviation Administration (FAA) that certain hydraulic inspections were missed on 128 of the company’s Boeing 737-700 models, the planes were voluntarily taken out of service right away, cancelling flights as a result.

Southwest flies more than 3,400 flights a day. The company’s updated statement anticipated “very minimal impact” on Wednesday’s operations.

Officials from Southwest, the FAA, and plane maker Boeing Co. worked out a plan to complete the inspections as quickly and efficiently as possible, with the goal of returning the planes to service in the interim. Early Wednesday Southwest said the FAA approved “a proposal that would allow the planes to keep flying for a maximum of five days” while inspections are done. An FAA spokesman confirmed the compromise.

The FAA released its own statement about two hours prior saying that the carrier had “missed some required inspections on the standby rudder system” on 128 aircraft, adding that FAA officials, working with representatives of the airline and manufacturer Boeing Co., were still evaluating the proposal “that would allow the airline to continue flying the planes until the inspections are completed over the next few days.”

Southwest Airlines is the nation’s fourth largest, with 665 jets, all of which are some version of the Boeing 737. “The safety of our employees and customers remains our highest priority and we are working to resolve this matter swiftly,” the carrier said.