FREDERICK, Md. -- Kathleen Roy banks her single-engine Piper Archer into a slow circle 1,500 feet above fields and housing developments, careful not to cross a white line on a navigational screen on the plane's control panel.
She punches ''5353" into the plane's transponder, a code she needs to cross into airspace around Washington, D.C., and land at an airport in Gaithersburg, Md. Now she waits for air traffic controllers to give her clearance to go over the line.
''Transponder observed. Maintain on course," a controller says a few minutes later.
The transponder signal, known as her ''squawk," is critical. Without it, Roy is an unknown to the controllers and regarded as a possible threat to the nation's capital 30 miles to the south. If she gets too close, fighter jets could chase her down and blow her out of the sky.
Hundreds of small planes have violated the airspace restrictions imposed around Washington after the Sept. 11, 2001, attacks. Most are minor incidents, but some have prompted evacuations of the White House and Capitol for what turned out to be confused pilots.
The Federal Aviation Administration says the protected zones covering 2,000 square miles are needed to secure the nation's capital from attack.
But many private pilots say the rules are rigid and confusing, and they worry that a strong gust of wind, a problem with their transponder, or a flying error could have grave consequences.
''Every time I roll down the runway, I am keenly aware that one wrong push of a button or one small lapse of attention to where I am could bring my dream to an ugly end," said Meredith Saini, a Bethesda pilot who recently started a flight school.
The restricted areas form overlapping circles that extend deep into Virginia and Maryland. Airspace above the presidential retreat Camp David in western Maryland is also restricted, and the zone expands when the president is there.
An inner ring, the Flight Restricted Zone, extends about 15 miles from the Washington Monument. Private pilots who fly into three small airports in that zone need a background check and a personal identification number. They must register their flight plans with the FAA, use a specially assigned transponder code, and maintain constant radio contact with controllers.
There is also the Air Defense Identification Zone, or ADIZ, created in 2003. It consists of three rings, each 60 miles across and centered on the region's three commercial airports. Pilots flying from these areas do not need a background check but still must file a flight plan, stay in radio contact, and use a special transponder code.
Elsewhere, private pilots fly using a universal transponder code, the number 1200, and are not required to file flight plans in advance or stay in radio contact with controllers.
''It certainly keeps you on your toes," said John Luke III, a pilot and manager of the Montgomery County Airpark in Gaithersburg, an airport within the ADIZ. ''You used to be able to just jump in your plane and go. You can't do that anymore."
Punishments for breaching the restricted zone range from a warning to revocation of a pilot's license.
The FAA pulled the license of a Pennsylvania pilot after he and a student flew a Cessna 150 within 3 miles of the White House on May 11. The FAA later determined the pair were lost.
The Pentagon has installed a system that flashes red and green lights from the ground to notify pilots when they have entered the restricted airspace.
The Aircraft Owners and Pilots Association has also tried to educate its 400,000 members about the restrictions.
The association blames the regulations for a roughly 20 percent decline since 2003 in the number of planes based at the 20 airports within the ADIZ, a drop of about 500 planes.
The association president, Phil Boyer, fears even stricter rules after four recent episodes near Washington and one near Camp David.
A proposed Senate measure would allow a $100,000 fine, a five-year loss of license, and confiscation of the airplane of a pilot who violates the inner circle and prompts evacuations.
Boyer wants to ease restrictions on smaller aircraft, such as single-engine planes, that he said do not pose much of a risk because of their size and low speed. Larger aircraft, such as private jets, should still be subject to the restrictions, he said.
After landing at Gaithersburg, Roy called air traffic controllers again to get another transponder code for the return trip to Frederick. A pilot for 13 years, Roy, who works for the association, is used to the tighter regulations.
But as she pulled her plane back into the air, she said, ''This airspace around D.C. is complicated."