ATLANTA, Georgia: United Parcel Service (UPS) has stated that several of its aircraft have been grounded and are not expected to return to service any time soon, affecting deliveries during the peak season.

The fleet of planes that UPS grounded after a crash is undergoing inspections and possible repairs, the company said this week in an internal memo.

A memo from UPS Airlines president Bill Moore to employees said the company is working to meet Federal Aviation Administration guidelines and that its McDonnell Douglas MD-11 fleet may not return to service for several months. What the company initially thought would take weeks may now take months, the memo added.

On November 4 in Louisville, Kentucky, an MD-11 plane crashed, killing 14 people and injuring at least 23 when the left engine detached during takeoff. Cargo carriers grounded their entire McDonnell Douglas MD-11 fleets shortly after, ahead of a directive from the FAA.

"Regarding the MD-11 fleet, Boeing's ongoing evaluation shows that inspections and potential repairs will be more extensive than initially expected," Moore wrote in the memo.

The company would rely on contingency plans for deliveries throughout the peak season, a UPS spokesperson said, adding, "It will take the time needed to ensure that every aircraft is safe."

The 109 remaining MD-11 airliners, which are more than 30 years old, are used to haul cargo for package delivery companies. MD-11s make up about nine percent of the UPS airline fleet and four percent of the FedEx fleet.

Boeing, which took over MD-11 manufacturing after its 1997 merger with McDonnell Douglas, said in a statement that it is "working diligently to provide instructions and technical support to operators."

The FAA said Boeing will develop the procedures for inspections and any corrective actions, pending FAA approval.