UPS’s fleet of planes has been grounded after a fatal accident that is expected to miss the peak delivery season

The fleet of aircraft grounded by UPS after a fatal accident is not expected to return to service during the peak holiday season due to inspections and potential repairs, the company said Wednesday in an internal memo.

The airline expects it will take several months before that McDonnell Douglas MD-11 fleet The memo from UPS Airlines President Bill Moore to employees said it was returning to service as it worked to meet Federal Aviation Administration guidelines. The process was initially expected to take weeks, but is now expected to take several months.

The November 4 crash of an MD-11 in Louisville, Kentucky, killed 14 people and injured at least 23 when the left engine separated during takeoff. The shipping companies grounded their McDonnell Douglas MD-11 fleets soon after, ahead of a directive from the Federal Aviation Administration.

“With respect to the MD-11 fleet, Boeing’s ongoing assessment shows that inspections and potential repairs will be more extensive than initially anticipated,” Moore wrote in the memo.

A UPS spokesperson said in a statement that the company will rely on contingency plans to provide services to customers throughout the peak season, and “it will take the necessary time to ensure that each aircraft is safe.”

The remaining 109 MD-11 aircraft, whose average age is more than 30 years, are used exclusively to transport cargo for parcel delivery companies. MD-11s make up about 9 percent of the UPS airline fleet and 4 percent of the FedEx fleet.

Boeing, which has been the manufacturer of the MD-11 aircraft since its merger with McDonnell Douglas in 1997, said in a statement that it is “working diligently to provide instruction and technical support to operators” so they can meet FAA requirements.

The FAA said Boeing will develop inspection procedures and any corrective actions, pending FAA approval.