The Vietnam-era CH-46 Sea Knight -- one of the military's most storied helicopters -- is older than most of the men who fly it today, and it is showing its age.

The CH-46 has been beset in recent years by mechanical troubles that have forced more frequent inspections and driven up operating costs. The Marine Corps has complained it needs a new helicopter.

On Thursday, a Marine CH-46 crashed and burned in Kuwait, killing all 12 American and British soldiers aboard in the first coalition casualties of the war. The cause of the crash is under investigation, but officials said hostile fire had not been reported in the area.

Built by Boeing, the Sea Knight entered service in 1964.

The Marine Corps relies on the Sea Knight, a bus-like helicopter with two large rotors, as its primary combat assault helicopter, using it to fly troops from ships at sea or base camps to forward positions. The Navy uses it to ferry equipment to ships and for search-and-research missions.

It made its combat debut in Vietnam, carrying troops and cargo to and from Navy ships. It flew thousands of missions rescuing wounded Marines, including during the siege of Khe Sanh in 1968.

It played a role in every major U.S. military operation since then; in Afghanistan in 2001 it carried supplies ashore when Marines set up the first U.S. land base, south of Kandahar.

At nearly 40 years old, however, the Sea Knight has to be inspected more frequently for mechanical flaws, and the Marine Corps version is limited to delivering 12 troops instead of the 25 as originally designed. The Marines say its cost per flight hour has risen 75 percent in recent years.

Last August, a crack found in a rotor component during an inspection led the Navy and Marine Corps to ground the entire fleet of 291 Sea Knights. Inspections found only one more helicopter with such a flaw, and the fleet was promptly returned to service.

In February 2001, the Navy temporarily grounded the Sea Knight fleet after discovering a crack in another part of the rotor assembly.

At the time, the Marine Corps lamented the deteriorating state of the helicopter and underscored the urgency of replacing it with the V-22 Osprey, a hybrid helicopter-airplane that has been hampered by fatal crashes in testing.

"Maintenance personnel are finding that components that have never before failed are beginning to break due to increased age and fatigue," the Marines said at the time.

Among some recent accidents involving the Sea Knight: In 2001, a CH-46 crashed into a river during a nighttime exercise near Camp Lejeune, N.C., killing three Marines and injuring two. In 1999, a CH-46 went down off San Diego, killing seven Marines.