A deadly crash has forced UPS Airlines to ground a fleet of planes for inspections and possible repairs that could last through the peak delivery season, the company said Wednesday in an internal memo.

The airline grounded its McDonnell Douglas MD-11 fleet indefinitely as it works to meet Federal Aviation Administration guidelines, said the memo from UPS Airlines president Bill Moore to employees. The process was originally estimated to take weeks but is now expected to take several months.

A fiery MD-11 plane crash on Nov. 4 in Louisville, Kentucky, killed 14 people and injured at least 23 when the left engine detached during takeoff. Cargo carriers grounded their 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.

A UPS spokesperson said in a statement that the company will rely on contingency plans to deliver for customers throughout the peak season, and it "will take the time needed to ensure that every aircraft is safe."

The 109 remaining MD-11 airliners, averaging more than 30 years old, are exclusively used to haul cargo for package delivery companies. MD-11s make up about 9% of the UPS airline fleet and 4% of the FedEx fleet.

Boeing, which took over as the manufacturer of MD-11s since merging with McDonnell Douglas in 1997, said in a statement that it is "working diligently to provide instructions and technical support to operators" so that they can meet the FAA's requirements.

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