Dallas-based Southwest Airlines has removed 44 aircraft from service as its own internal investigation of allegations that violated FAA rulemaking continues.

The decision triggered cancellations of about 4% of its daily flights today, the carrier says.

Flight’s ACAS database shows Southwest flies a mix of 416 classic and next-generation Boeing 737s with an average fleet age of 8 years.

FAA levied a $10.2 million fine against the Southwest after the carrier flew 46 aircraft last year without completing mandatory inspections of fuselages for cracks that were required in a September 2004 airworthiness directive.

During a review of maintenance records yesterday, Southwest says it discovered an "ambiguity related to required testing". The airline made the decision to remove the aircraft from service to immediately start re-inspecting the aircraft.

Of the 44 affected aircraft, one was retired, five were in scheduled maintenance checks, and the remaining 38 were removed from scheduled service.

Southwest says a portion of those planes have cleared inspection and have returned to service, and the remaining aircraft should be inspected by early this evening.

The lapse in inspections reemerged last month after US Representative James Oberstar ordered DOT’s Inspector General to determine whether FAA properly handled the inspection gap after whistleblowers came forward claiming the fuselages were not properly checked for cracks.

Yesterday Southwest placed three employees on administrative leave, and CEO Gary Kelly expressed concern "with some of our findings as to our controls over procedures within our maintenance airworthiness directive and regulatory compliance processes.

"I have insisted that we have the appropriate maintenance organizational and governance structure in place to ensure that the right decisions are being made."

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Source: FlightGlobal.com