Southwest Airlines canceled 80 flights on Tuesday, and expects to cancel more flights on Wednesday because of an overdue required maintenance check on the the standby hydraulic system of 128 737-700 aircraft. As of December 31, 2014 Southwest operated 665 Boeing 737s, and 447 of their 737s are the 700 series aircraft.

Southwest Canceling Flights As 128 737-700s Pulled From Service Due To Overdue Maintenance Checks
Southwest Airlines offered this statement about the incident:

Southwest Airlines discovered an overdue maintenance check required to be performed on the standby hydraulic system, which serves as a back-up to the primary hydraulic systems. As a result of this discovery, 128  -700 aircraft were identified as having overflown a required check. 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. 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. Approximately 80 cancellations occurred today as a result of the events and the airline is anticipating a very minimal impact to their operations tomorrow. The Safety of our Customers and Employees remains our highest priority and we are working quickly to resolve the situation.

Bottom Line:

It sounds like Southwest quickly cut a deal with the FAA for additional time to complete the checks, and this will keep operational disruptions at a minimum for the airline and its passengers. But be sure to check your flight before heading to the airport if you are flying Southwest over the next few days.