The Navy and Marine Corps grounded their fleet of 291 CH-46 Sea Knight helicopters after an inspection of one in North Carolina revealed a crack in part of the rotor assembly.

A Navy spokeswoman, Lt. Cmdr. Dawn Cutler, said Wednesday the grounding order was issued late Monday. She said none would fly again until they had undergone inspection, which normally takes about three hours per aircraft. Those deployed aboard ships will take a bit longer to check, she said.

Cutler said there was no specific timetable for completing the inspections.

A recent inspection of a CH-46 at Marine Corps Air Station Cherry Point, N.C., revealed a crack in the bellcrank, which is part of the rotor assembly. In line with standard practice, that led to a decision to keep all the troop-ferrying aircraft out of the air until they have been inspected, said Capt. Shawn Turner, a spokesman at Marine Corps headquarters.

Pentagon spokesman Lt. Col. Dan Stoneking said the Army, which also uses CH-46s, had not grounded them, to his knowledge.

The CH-46 is used mainly for carrying troops, although the Navy also uses them for search-and-rescue missions. The helicopter has been in use for more than three decades. The Marines want to replace their fleet with the V-22 Osprey, the hybrid helicopter-airplane that is now in testing.

Cutler said there is no indication that the discovery of a crack in the bellcrank of the CH-46 at Cherry Point had any relation to recent crashes.

A Navy CH-46 crashed into the Atlantic Ocean off the coast of Virginia in February. The four crew members were rescued.

In July 2001, a Marine Corps CH-46 crashed into a North Carolina river during a mock nighttime ship landing, killing three Marines and injuring two. The same model crashed in 1999 during a training flight 15 miles off San Diego, killing seven Marines from Miramar Naval Air Station.

A CH-46 also was involved in a 1996 collision of two Marine helicopters at Camp Lejeune, N.C. Fourteen people were killed.