Rough terrain, land mines and other obstacles won't keep them away.

Along with the advantages of a helicopter's agility, however, come greater dangers _ as demonstrated in the deadly firefight in Somalia that left 18 American soldiers dead in 1993.

"Helicopters can be shot down by very simple weapons, like machine guns or rocket-propelled grenades," said Kalev Sepp, a former special forces officer with combat experience in Central America.

Indeed, it was guerrillas armed with just such weapons and trained by Osama bin Laden's terror network who brought down two Black Hawk helicopters in the Somalia firefight.

As the U.S.-led operation moves forward in Afghanistan, special operations forces are sure to have significant roles, relying on specially equipped helicopters to get them in and out and to provide fire support. Such forces could be inserted to collect intelligence, could conduct clandestine raids to capture individuals and could help train and equip Afghan rebels.

Small teams of British and U.S. special reconnaissance teams already were inside Afghanistan before this week's airstrikes. The next deployment is expected to be much larger.

The forces would likely rely on Black Hawk helicopters, which can carry up to 14 commandos and their gear and have equipment allowing them to fly low and fast at night or in bad weather. Black Hawks, a workhorse of the Army's helicopter fleet, carry machine guns to defend themselves and can be fitted with air-to-ground rockets. They also are heavily armored.

Even so, U.S. helicopters "traversing the areas where the Taliban are likely to be holed up are in great danger," said Will Garner, a former Army Airborne Ranger. "The Soviets showed us that."

The Soviets suffered heavy helicopter losses during their failed 1979-89 war with Afghanistan. The victorious ragtag Afghan armies, which included the Taliban, came away equipped with U.S.-supplied Stinger missiles that now could be turned against American helicopters. Further, Taliban soldiers can be readily seen moving around with cone-shaped rocket-propelled grenades strapped to their backs.

With Afghanistan's huge expanse, rough terrain and extensive land mines, helicopters can provide important ways to move Americans around quickly and with relative stealth.

"You can stay below the sight of things, pop over the top of a hill and land and then take off again," said Jack Moroney, a former special forces colonel from Vermont. "Helicopters can be a great advantage to move a decisive force in at the right time."

With their ability to hover, helicopters also can use their firepower more precisely than can a high-flying jet shooting past at hundreds of miles an hour, an ideal circumstance for shooting at targets such as people on the move or trucks. Some are equipped with small-caliber cannon, rockets and guided missiles.

Still, risks abound, altitude, weather and groundfire among them.

Helicopters lose maneuverability and can't carry as much weight at higher altitudes such as those of Afghanistan's mountainous regions. The mountains also could tend to funnel aircraft into channelized canyons, where Afghan fighters could try to pick them off.

"It doesn't take a lot to knock a helicopter down," said Moroney. "They're not a tank with a rotor. They do have some vulnerabilities."

Further, military experts say, even "all-weather" helicopters can have trouble in some winter conditions, particularly with high winds and poor visibility.

"Even with sophisticated navigation tools, you still have to be able to see what you're doing," Moroney said.

Another likely limitation on the use of helicopters in Afghanistan is created by long distances from U.S. bases. Helicopters can fly several hundred miles, and some can refuel in flight, but long distances make their missions more dangerous.

The aircraft carrier USS Kitty Hawk is in the Indian Ocean and could provide a base for such forces, although officials said they also would set up a forward base closer to Afghanistan. They would not say which country, but U.S. forces have arrived in Uzbekistan as part of this month's deployments. Pakistan also is seen as a potential jumping-off point.

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On the Net: Facts on the Black Hawk:

http://www.army-technology.com/projects/black_hawk/