[After three U.S. helicopter were shot down by insurgents in November, 2003], American commanders ordered pilots to fly evasively at all times. American helicopters routinely fly at tree-top level, bobbing and weaving on their way to their destination. Like the Super Stallion that went down Wednesday, Army and Marine helicopters often fly at night, when the threat of attack is diminished. Helicopter pilots say that they are still routinely shot at from the ground but that the tactics have largely prevented the insurgents from hitting them.
Because the helicopters fly so low, one of the principal dangers is electrical and telephone wires, which the choppers often leap over in flight.
The CH-53E Super Stallion involved in the crash is the largest and heaviest helicopter used by the American military.
"Look at its sheer size - it's huge," said Richard Aboulafia, a military industry analyst at the Teal Group, a northern Virginia aerospace and consulting firm. "It's a monster, and with size comes the fact that it is not very maneuverable."
Weather, too, presents special problems.
"Helicopters are fairly fragile pieces of equipment," said Ivan Oelrich, director of the Strategic Security Project at the Federation of American Scientists, a Washington nonprofit group. "It's rough for them to operate in a dusty, desert environment where the dust can get into the machinery. And they are vulnerable to ground fire because they fly at slow speeds, close to the ground..."
Before Wednesday's crash, the CH-53E Super Stallion had a strong safety record, something analysts said was due to the maturity of its design and the reliability of its equipment.
The helicopter first came into service in 1981, although it is based on a design that dates to the Vietnam War. Produced by the Sikorsky Aircraft Corporation, the helicopter was bought almost exclusively by the Marine Corps. Production ended about five years ago.
A three-engine craft, the helicopter is designed to operate in bad weather, day and night. It can lift more, carry it farther and fly faster than other helicopters in the Pentagon's fleet. Equipped with night vision ability, it is designed to operate in harsh terrain.
"This is a craft that can operate day or night, in all types of weather," said John Milliman, a spokesman for the Naval Air Systems Command at Patuxent River, Md. "It is a very big, very rugged helicopter than can carry a very heavy load."
Still, for all its bulk, the craft remains vulnerable. If forced to fly evasively in bad weather, a pilot could become disoriented.
Some American officials have expressed worry that the harsh conditions of Iraq and Afghanistan, and the frequency with which the helicopters are deployed, could have rendered them vulnerable.
At an October 2003 hearing of the House Armed Services Committee, Representative Joel R. Hefley, Republican of Colorado, the chairman, said the typical Super Stallion returning from service in Afghanistan and Iraq was found to have 150 pounds of sand spread throughout its interior.
Sand is thought to be one of the worst enemies of the helicopter in Iraq, wearing down rotors and seeping into engines and electronics. It can blind pilots, especially on landing, when the helicopters kick up huge clouds of dust. It mixes with lubricants and turns them into sticky masses of gum.
"The conditions were harsh," Mr. Hefley said. "The heat, the sand, the operational tempo together resulted in our troops taking a beating."
