Atlantic Southeast Airlines, owned by SkyWest and flying for Delta Air Lines as a regional carrier, has grounded 60 of its 112 Bombardier CRJ200 jets after an internal audit raised safety concerns.

The Associated Press reports that the audit raised questions about whether the engines on Bombardier CRJ200 jets had been properly inspected according to the guidelines provided by the engines’ manufacturer.

ASA spokeswoman Kate Modolo said that “It could take 36 to 42 hours to complete the inspections.” The groundings, which represent 40 percent of ASA’s fleet, were expected to cause delays for those flying ASA.

Some of the planes are being inspected at Atlanta’s Hartsfield-Jackson Airport and the rest are scattered througout ASA’s other maintenance facilities.

Delta spokeswoman Betsy Talton can’t say how many Delta flights and passengers will be affected, but she said that Delta “was working to accommodate affected passengers and to cover as many ASA flights as possible with other regional carriers or Delta mainline planes.”