Southwest Airlines is investigatingwhy a required inspectioninterval in its routine maintenance program was left out of its maintenance tracking system, causing it to miss deadlinesfor checking backuphydraulic systems on 128 Boeing 737s.

The carrier discovered the issue on Tuesday and immediately informed FAA. The affected aircraft were voluntarily removed from service while Southwest and the agency, in consultation with Boeing, came up with a plan to perform the required checks.FAA said it evaluated therisk and determined the aircraft couldremain in service for up to five dayswhile awaiting checks. News of the missed checks, which affectedone of three 737NG hydraulic systems, was first reported by The Wall Street Journal.

With the immediate issue of getting the aircraft airworthy apparently settled, Southwest is examining what went wrong.

“While updating our maintenance tracking system, we discovered that we inadvertently omitted the lowest of three intervals for the maintenance check established within Southwest’s FAA-approved maintenance program,” the carrier explained to ATW. “We are investigating the events and will implement any necessary policy or procedure changes.”

The missed inspections are part ofworkpackagesperformed in-house, the carrier confirmed.The missing interval was discovered internally, though was not immediately clear whether the issue was uncovered by happenstance or as part of a routine quality check.

The issue caused Southwest to cancel about 80 flights Tuesday as it worked with FAA to gain approval on aplan.

Analysts at Cowen & Co. calculated that the entire issue could cost Southwest $3.6 million in lost bookings, assuming 200 cancellations that drove most customers to book on other carriers. It also suggested the carrier could incur $64 million in added maintenance costs toexpedite the checks.

The actual costs are likely to be lower, as the carrier had fewer cancellations Wednesday than Tuesday and was able to accommodate a sizable portion of the disrupted passengerson other Southwest flights.