Smugglers' air force? Drug war sees rise in use of ultralight planes.

On a cold winter night in the high desert, an Immigration and Customs Enforcement agent instinctively looks to the moonlit sky for signs of ultralight aircraft, the motorized hang gliders that daring drug smugglers fly low over the US-Mexican border.

"Basically, ultralight smuggling is a quick, easy, and efficient way of smuggling marijuana into the US," says Kevin Kelly, assistant special agent in charge of the Nogales, Ariz., office.

Drug traffickers trying to conquer a fortified Southwest border have used catapults, tunnels, and boats to get their product into the United States. In recent years, they have increasingly turned to ultralights, which can carry several hundred pounds of illicit cargo ready to drop to accomplices on the ground.

In the fiscal year that ended Sept. 30, Customs and Border Protection recorded 223 ultralight incursions along the 2,000-mile border. That is fewer than in 2010 but about double the 2009 numbers.

Congress has taken notice of the cross-border ultralight flights in areas from Arizona to California and Texas. Last month, the Senate unanimously passed legislation to toughen penalties for those who use ultralights to smuggle drugs, a concept Rep. Gabrielle Giffords (D) of Arizona had pushed before she was wounded in a mass shooting last January in Tucson. The House now will consider the legislation.

Because the Federal Aviation Admin­is­tra­­tion does not categorize ultralights, the aircraft do not fall under existing aviation smuggling laws. A new law would establish the same penalties for trafficking whether by plane, car, or ultralight – up to 20 years in prison and a $250,000 fine.

The aircraft are hard to spot because radars "typically do not provide coverage down to ground level in remote areas," says Ian Gregor, an agency spokesman.

For pilots of the aircraft, the goal is to remain elusive for as long as it takes to deliver the goods and return home, sometimes in just a matter of minutes. If all goes well, ground crews pick up the drugs before authorities take note.

"Before you know it, they're gone," says Tony Estrada, sheriff of Arizona's Santa Cruz County. "It's a very fast maneuver."

The ultralights usually take off from a clandestine location in Mexico, their wings often spray-painted black and their pilots clad in dark clothing as they glide across the international boundary toward a desert patch, an agricultural field, or even a backyard.

Although some wear night vision goggles and carry a small global positioning system, most pilots rely solely on the light of the moon to guide them.

The hum of a slow-moving ultralight reminds Agent Kelly of a lawn mower. "You can hear it whining as it gets closer and closer to you," he says.

After dropping to as low as 200 feet to release the marijuana bundles, which are tightly wrapped in cellophane and duct tape to keep them from bursting on impact, the pilot quickly heads back to Mexico.

It's easy for the pilots to evade capture, even when border agents can detect the aircraft.

"We can't shoot them down," Kelly says. The ultralights don't pose an imminent threat. And reaching a drop-off site before the ground crew absconds with the drugs is challenging.

The flights are not without risk for the brazen ultralight pilots. One died after crashing in a lettuce field in southwestern Arizona, and another was paralyzed after clipping power lines and crashing in the southern part of the state.

"They had some trial-and-error periods," but they finally got it right, Kelly says.

The ultralights no longer carry the marijuana on either wing, which sometimes caused the aircraft to flip. Smugglers now use a centerline compartment to hold the 250-to-280-pound load. More weight could cause problems.

An ultralight is typically made of aluminum and carbon fiber, which have a low radar signature, says Tim Sestak, a professor of aerodynamics at Embry-Riddle Aeronautical University in Prescott, Ariz. "The engine is very small. They're flying very low in what you call ground clutter when you're looking at the radar."

Although the ultralight is now one of the preferred aerial methods to smuggle drugs, Professor Sestak says traffickers in the future may move to unmanned aerial vehicles, which could result in more drug interdiction but spare pilots from being arrested and interrogated.

In a way, the use of ultralights shows the growing difficulty of getting large amounts of marijuana across the border, says Richard Bloom, Embry-Riddle's associate vice president for academics, who teaches security and intelligence.

The smugglers have had to adapt.

"It's a never-ending spy-counter-spy phenomenon," says Mr. Bloom.