Southwest Airlines discovered a required maintenance check on the standby hydraulic system, which serves as a back-up to the primary hydraulic systems, was overdue on 128 Boeing 737-700 aircraft. 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. On the evening of February 24th, the FAA granted Southwest approval to continue operating the aircraft for a maximum of five days as the checks are completed. Approximately 80 cancellations occurred Tuesday, February 24th, as Southwest awaited FAA’s response. The airline is anticipating very minimal impact to its Wednesday operation as remaining checks are completed.