Five incursions into the no-fly zones that protect the White House and other important Washington landmarks have Federal Aviation Administration officials considering ways to further prevent planes from breaching the space and defending the penalties for pilots who violate the restrictions.

But at least one pilot said air traffic controllers are as much to blame for security breaches as pilots are.

Planes departing to the north from Washington's Reagan National Airport are supposed to veer away from these restricted areas. But just this week, a Frontier Airlines jet entered the no-fly zone and flew just a little west of the White House. Further out, the plane crossed another no fly zone above Vice President Dick Cheney's official residence.

The pilots have been suspended while the FAA and Frontier Airlines investigate.

The Secret Service, which privately lobbied to keep Reagan National permanently closed after Sept. 11, had no comment.

Pilots have flown through the prohibited airspace at least 94 times over the past decade, illustrating the challenges of thwarting a terrorist airstrike on the nation's capital.

In most cases, pilots who violated the airspace protecting the White House, vice presidential mansion and Capitol have received penalties less severe than a parking ticket.

Of the 111 pilots on the 94 flights, just one was fined $1,000. Nine other pilots had their licenses suspended temporarily. Administrative action, mostly warnings or correction letters, were the result of 90 other incursions.

Authorities who run Reagan National said violations aren't necessarily a sign of lax security. Pilots can be knocked off course by something as simple as a heavy wind.

"It is fairly typical. The FAA really doesn't like to do enforcement actions, particularly any carrier infringement," Former U.S. Transportation Department Inspector General Mary Schiavo said.

FAA Deputy Administrator Monte R. Belger said Thursday that the agency recognizes there's little time to react once planes penetrate the safety zone and so the government has imposed numerous other precautions to prevent planes from coming close to landmarks.

"The restricted area is kind of the last line of defense," Belger said. "The additional on-the-ground security procedures and in-flight protocols put in place give us a much higher level of confidence."

Borders have been tightened; pilots, flight crews and passengers are screened to weed out possible terrorists, and planes approaching Washington must complete authentication procedures, including providing passwords.

Almost three dozen planes that failed to get verification have been turned away since Sept. 11. Most authenticated themselves immediately, officials said.

One pilot caught in the airspace blamed air traffic controllers, saying they are so busy they sometimes order flight maneuvers that send pilots into the prohibited zone.

"The D.C. controllers are absolutely horrible. Washington National is absolutely the worst place to fly into, period," said Happy Wells, a 30-year veteran pilot from Oklahoma who was cited in July 1997 for flying his charter plane through Washington's prohibited zone. The FAA rescinded his penalty after he complained.

One person who studies homeland security issues believes that, despite this incident, reasonable precautions are being taken and there is no way to eliminate all potential risk.

"We're never going to have 100 percent security in any of our forms of infrastructure, whether it be railroads, oil and gas or the aviation industry. Even if we had closed National and a similar event occurred out of Dulles or (Baltimore-Washington International Airport) — they are only 30-40 miles away from the capital — at a couple hundred miles an hour, that's not much time to react either," said Mike Scardsville, a homeland security analyst at the Heritage Foundation.

"Practically speaking, by the time a violation is discovered, it is too late to do anything to prevent a crash into the White House," former FAA security chief Billie H. Vincent said.

All flights operating in or out of Reagan National do carry federally trained sky marshals. Pentagon sources confirmed that there are still military aircraft flying combat air patrols over the nation's capital at all hours of the day, every day of the week.

It seems unlikely in this instance that a military jet could have intervened in a timely fashion since the Frontier jet ended up in the wrong place in a matter of seconds, which is a reminder that if it had been an attack, rather than a mistake, the situation could have gone very badly, very quickly.

The Associated Press contributed to this report.