Washington ? The sound is jarring and unmistakable, an electronic trill that pilots of the 121st Fighter Squadron recognize in a quickened heartbeat. Nearly every day, the warbling alarm reverberates through bunks and hangars in a remote corner of Andrews Air Force Base, signaling trouble in the sky above the nation’s capital.

In seconds, the airstrip tenses with choreographed vigilance. Inside a trailer at the runway’s edge, pilots wrestle into their flight suits. Mechanics dash into the hangars to inspect the fully armed F-16 fighters before firing up their deafening engines. Within minutes, the pilots are out on the runway while the jets whine, waiting for final takeoff orders.

“When that ‘tweedle’ goes off, everything stops,” said Lt. Col. Bob Montgomery, mission commander for the Capitol Guardians, a team of Air National Guard pilots deployed to respond to air alerts over Washington. “The first time you hear it, it sounds kind of funny, like a high-pitched bird call. But from then on, it’s automatic: just drop and run.”

With the daily crush of commercial jets, military aircraft and small planes filling the region’s air corridors, the 121st is placed on alert more often and intercepts more approaching aircraft than any other air defense unit in the nation. From Sept. 12, 2001, through the end of 2004, flight path incidents around Washington accounted for 43 percent of the 3,400 airspace violations recorded across the United States, the Government Accountability Office reported last month.

When their luck holds, the chaos is resolved within minutes and the 121st’s pilots stand down. But at least three times in recent months, they have raced skyward into nerve-racking confrontations with civilian planes that blundered near prohibited airspace over the White House and Capitol grounds. There have been other incidents that have not been disclosed. “We have more intercepts than the public realizes,” Montgomery said.

In May, two pilots from the 121st fired four warning flares near an errant Cessna from Pennsylvania that had come within three miles of the White House before veering away for a forced landing.

In June, Montgomery and Lt. Col. Mike Synoracki – another F-16 pilot from the 121st – intercepted a Cessna that had strayed into the 16-mile-wide restricted zone around Washington. In the tense minutes before the pilots escorted the small plane to a Virginia airfield, the White House went on red alert, evacuating President Bush and his staff and ordering a mass exodus of the Capitol.

Three days later, the squadron’s pilots headed off another intruding plane.

“We only hear about the evacuations when we get back down,” Synoracki said. “That’s when we realize how freaked out people are on the ground.”

Unlikely role

Ever since terrorists caught the capital’s air security off guard nearly four years ago, the pilots of the 121st have been preparing themselves for the day they receive a Defense Department order to use their firepower against civilian aircraft. But they also have found themselves thrust into the unlikely role of air traffic enforcers.

“It’s not supposed to be the Wild West up there,” Montgomery said, “but we’ve had more than our share of tough days.”

Unnerved by the repeated Capitol evacuations, House lawmakers recently proposed legislation aimed at reducing incidents by increasing fines on errant pilots. If the bill passes, violators who breach the restricted zone around Washington could be fined up to $100,000; pilots straying into a 50-mile-wide area could be fined $5,000.

But the financial threat is unlikely to have much effect on the daily congestion as passenger jets and business shuttles descend on Washington’s airports and military aircraft sweep over the region on drills and surveillance rounds.

The GAO study found that 88 percent of the “no-fly” zone violations logged by the Federal Aviation Administration involved business jets and small civilian aircraft.

But errant civilian planes remain the most vexing problem. They come out in force on sunny days, when visibility is high, luring joy riders and student pilots from hundreds of miles away.

“We only catch a break when it rains,” Montgomery said.

The breaks rarely last long. On a typical day, the alarm sounds at least once or twice, Montgomery said. Several times over the last four years, the 121st’s pilots have had to scramble as often as six times in a 24-hour period.

“We’re like Pavlov’s dogs when the bell rings,” Lt. Col. Gary Akins said. “Your heart jumps and you’re out the door.”

One morning last month, just hours after terrorists had bombed London’s transit system, Akins and Lt. Col. Timothy Lehmann began their 24-hour shift, standard for the squadron.

On the runway, flight crews tested the engine of Akins’ F-16, throttling it from a shuddering roar to an ear-splitting howl. Facing a long night of flight drills, Akins and Lehmann retreated to the mustard-colored trailer that houses pilots when they are on duty.

Air Force guidelines require the F-16 pilots to be in the air within 15 minutes after an alert sounds. The pilots say they routinely shave minutes off that mark, although their actual times are classified.

“Sometimes when the horn goes off at night, I’m already in my boots before the wall lights start flashing,” Lehmann said.

When aloft, the F-16 pilots maintain contact with military and FAA controllers on the ground. But there have been communications glitches. During several recent alert situations, the 121st pilots were unable to immediately connect with U.S. Customs and Coast Guard helicopters also responding to the intruders.

In the May incident, F-16 pilots aiming for a clear path to the errant Cessna from Pennsylvania had to contact ground controllers to relay warnings to the helicopters below. “Our pilots knew the copters were there,” Montgomery said, “but we had to get word to them to clear out of the way so we could get in.” Direct communication lines, he added, “are being worked out.”