WASHINGTON -- It doesn't take much these days to draw a pair of jet fighters off your wingtips -- an unruly airline passenger, a wrong turn into restricted airspace or deviation from a flight plan.

Since Sept. 11, concerns about more terrorism in North America's skies have drawn investigating U.S. and Canadian fighters 292 times, military officials say.

Fighters also come when planes stop communicating with ground controllers, when they present inadequate identification or head into unusual flight patterns, said Maj. Barry Venable, a spokesman for North American Aerospace Defense Command (NORAD), the Colorado-based military command with authority to protect U.S. and Canadian airspace.

Some jet fighters are sent from airborne patrols over New York, Washington and other U.S. locations. Others are scrambled from one of the dozens of bases that have had armed aircraft on alert since September. The job usually falls to U.S. F-15s and F-16s, or to Canadian CF-18s.

Venable said most of the calls for fighters are precautionary, taken once civilian authorities have exhausted other options to investigate events in the sky. Regional air traffic control centers under the Federal Aviation Administration usually make the call.

"A military option is an option of last resort," he said.

In a few cases, fighters are called because of concerns over an airline passenger. Last week, Canadian and U.S. fighters shadowed an Air India airliner because a ground screener in London decided one of the passengers looked like one of the FBI's "most wanted" terrorists -- after the flight had taken off from Heathrow. The FBI later questioned and released two passengers, saying it was a case of mistaken identity.

Combat aircraft are sometimes summoned when a problem isn't in the sky. A security breach at an airport -- as when a metal detector loses power and fails to screen a batch of passengers -- usually results in a patrol being dispatched, Venable said.

"Some days there are more than one and some days none," he said.

In some cases, fighters pull alongside an unidentified plane to call in its tail number to authorities. In others, they signal for the aircraft to land, Venable said. There are no reports of planes refusing to comply once fighters have reached them.

Fighters scrambled as 15-year-old Charles Bishop slammed a Cessna into a Tampa office building on Jan. 5. Investigators said Bishop committed suicide and had no terrorist ties. The summoned fighters -- sent from a base near Miami -- did not reach Tampa's airspace until after Bishop had crashed.

More than 13,000 missions

U.S. military aircraft have flown more than 13,000 missions for Operation Noble Eagle, the military's name for the effort to protect U.S. airspace since Sept. 11. This figure includes missions by fighters, refueling tankers and airborne radars, Venable said.

In February, Air Force Secretary James Roche proposed doing away with the constant combat air patrols over U.S. cities, but leaving fighters on alert at bases around the country. The patrols are wearing down U.S. planes and crews, he said.

Senior Bush administration officials have said the fighters would be authorized to shoot down a hijacked airliner to prevent it from crashing into a city.

Colorado-based NORAD was created during the Cold War as a joint command of the United States and Canada to provide early warning of a Soviet attack, either by long-range missile launches, or by bombers coming over the Arctic Circle.

Armed interceptors waited on standby at bases to fight the opening rounds of World War III. But with the end of the Cold War, continental defense fell by the wayside. In early September, 20 fighters in all of North America were on alert -- two each at ten bases in the United States and Canada.

A few times a month, a pair would be launched to investigate an aircraft approaching North American airspace, Venable said. Some of those fighters chased the hijacked planes on Sept. 11, but none got close before the planes crashed.