-busWASHINGTON (Reuters) - The U.S. Federal Aviation Administration on Saturday said it was prohibiting MD-11 planes from flying for now, pending inspections, after a crash earlier in the week of a UPS MD-11 cargo freighter killed at least 14 people in Kentucky.

UPS and FedEx ( FDX ) said they have already grounded their combined fleet of more than 50 McDonnell Douglas MD-11 cargo planes following a recommendation by planemaker Boeing ( BA ).

The FAA said the emergency airworthiness directive was prompted by the crash on Tuesday, when the left engine and pylon detached from the airplane during takeoff in Louisville. The cause of the detachment is under investigation.

"This condition could result in loss of continued safe flight and landing," the U.S. regulator said.

"The FAA is issuing this AD because the agency has determined the unsafe condition is likely to exist or develop in other products of the same type design."

UPS said it was in compliance with the directive because it has grounded its fleet of 26 MD-11 planes, representing about 9% of its fleet.

Boeing ( BA ) said on Saturday it supports the FAA order.

FedEx ( FDX ), with 28 MD-11s in operation out of its 700 planes, said it was "immediately implementing contingency plans" to minimize disruptions.

FAA LIKELY TO REQUIRE COMPREHENSIVE INSPECTIONS

Boeing ( BA ) acquired the MD-11 program through its 1997 merger with McDonnell Douglas.

Officials said the FAA is likely to require comprehensive engine and pylon inspections before allowing the planes to resume service. The FAA order requires any necessary fixes after the inspections.

A spokesperson for the U.S. National Transportation Safety Board stressed the agency had not reached any conclusions into the crash it is investigating. The 34-year-old freighter reached about 100 feet (30.5 meters) above ground level before plunging in flames off the runway and destroying two nearby businesses.

One of the plane's three engines detached from the left wing as it rolled down the Louisville airport runway.

On Friday, U.S. safety investigators said that as a warning bell sounded in the cockpit, the three UPS pilots on board had tried to wrestle control of the plane just before it crashed.

Earlier this week, the NTSB said it was probing the maintenance history of the plane, which was in Texas for repairs with the aircraft undergoing heavy maintenance weeks before the accident. The aircraft was at a facility operated by Singapore-based ST Engineering, according to a source familiar with the matter.

ST Engineering has said it provides airframe maintenance for UPS's MD-11 aircraft and operates a repair facility in San Antonio, but declined further comment.