WASHINGTON -- Although the Secret Service doesn't think that Frank Eugene Corder was trying to hurt President Clinton, the fact that someone without everyday flying experience -- and no airplane of his own -- could crash a Cessna within a few feet of the first family's sleeping quarters shattered the myth that the president is safe in his own bed.

Even security-conscious Washingtonians were asking each other how and why the Secret Service could let something like this happen. Where was the radar, they wanted to know? The anti-aircraft missiles? The famed, steely-eyed agents with their Uzis?

Carl Meyer, a Secret Service spokesman speaking at the White House, yesterday stressed that the Clintons weren't at the White House when this happened -- they were staying across the street at Blair House while minor repairs were being made -- and suggested that security is tighter when the president is there.

Nevertheless, what Mr. Corder did in his last act in life was to underscore a sobering reality: In a free country, even the president's protection has limits.

Those limits are made more complicated, security experts say, by the fact that he lives and works in a historic building in the center of an urban area, close to the landing path for a major airport.

Although the Federal Aviation Administration prohibits pilots from flying over or near the White House or the Capitol, enforcing that rule by shooting a plane out of the sky is not considered a very plausible course.

In early 1983, a rash of terrorist acts in the Middle East led to increased security for President Ronald Reagan and the White House. But widely circulated reports dating from that time of Secret Service agents in the adjacent Old Executive Office Building armed with shoulder-fired, heat-seeking missiles strikes some security experts as far-fetched.

'Not a lot you can do'

"Practically, there's not a lot you can do about" a stray airplane, said Brian Jenkins, an anti-terrorism expert. For one thing, he and others said, if the missile missed the target, it could very likely lock onto a civilian airliner -- the landing path for National Airport is less than two miles from the White House grounds.

Also, the missile or the flaming airplane it destroyed -- or both -- would be likely to land where they could cause a further tragedy.

"You have to be careful in a densely populated area," Mr. Jenkins said. "Is it [the aircraft] likely to crash into an apartment building at New Hampshire Avenue and M Street and kill a few hundred people?"

Yesterday's incident, in fact, may have underscored the danger of firing a missile at a plane: One witness told the Secret Service that the plane came in low over the White House fence with no lights and without its engine running. If so, it's uncertain a heat-seeking missile would have even locked in on the plane.

Also, with the airport so close, there is almost no time to make such decisions.

Early yesterday, Secret Service agents on the White House grounds had only seconds to determine what was going on. Just "enough time to run for cover," said Mr. Meyer.

Finally, there would be the dilemma posed by an airplane that veered into the prohibited airspace either by accident or because of mechanical trouble.

"It could be some poor [fool] with a wife and kids looking for an open area to land," said Neil Livingstone, a Washington-based terrorism and security expert.

"This is not a banana republic or a dictatorship where someone stands out in a crowd and we open fire on the crowd. You cannot wrap the president in an absolute cloak of security."

Somber-faced White House officials explained yesterday that Mr. Clinton has given this matter a lot of thought -- and come to terms with it.

Since the campaign of 1992, Mr. Clinton has often frustrated the Secret Service detail that is charged with protecting him. Although he has reportedly been paying more attention to security concerns of late, he still likes to leave secured areas to shake the hands of well-wishers, sometimes drops by a restaurant unannounced and often goes for regular jogs on city streets.

"You could make the security so overwhelming that it was completely oppressive to the president and his family," said the White House press secretary, Dee Dee Myers, "but in a nation with 200 million guns you still would never completely eliminate the risk that some wacko is going to try to do something."

Nevertheless, every close call, new attempt or even a perceived threat prompts a review of Secret Service procedures.

The private's attempt

One such review came in 1974 after a disgruntled 20-year-old Army private, Robert Preston, hijacked a helicopter from Fort Meade and landed it just a few yards from where the Cessna crashed yesterday. In that case, the Secret Service opened fire, wounding the helicopter pilot. In the early 1980s after the wounding of President Reagan and a rash of car bombings in the Middle East, a host of new security procedures were implemented.

These included stronger White House gates, metal detectors at the entrance to the West Wing, tighter checks by bomb-sniffing dogs of service vehicles entering the White House and an expansion of the bomb shelter underneath the executive mansion.

Most visible to the thousands of tourists who visit Washington each year are the concrete barriers erected around the White House to discourage would-be suicide car bombers.

Wartime and the threat of civil disturbances have generated similar responses over the years, according to the White House curator, Rex Scouten. After the bombing of Pearl Harbor and the United States' entry into World War II, machine-gun nests were installed on White House terraces, and the White House was outfitted with "blackout curtains."

But the threats to American presidents that have materialized have not been from foreign terrorists, but from assassins and would-be assassins who stalked various presidents with simple pistols or rifles.

Four American presidents have been assassinated, all by gunfire, and numerous attempts have been thwarted by the Secret Service or other law enforcement agencies, including the one on Mr. Reagan's life and two attempts on the life of Gerald R. Ford. "There are some very difficult situations and the Secret Service has measures to take care of just about every situation," said a former agent, Robert L. DeProspero.

Thwarting stalkers

"People say if someone wants to do something to the president they could do that -- but I would not agree with that," said Mr. DeProspero, who was an agent for 22 years and became chief of the presidential protection detail for President Reagan just months after Mr. Reagan was shot. "I think the best thing anybody could realize is there are many situations where people are stalking the president, planning some course of action, but because of all the measures the Secret Service has, they're thwarted."

Mr. Clinton, signaling confidence in the Secret Service, moved his family back into the residence last night.

"Unless you're going to remove the president and put him in Cheyenne, Wyo., he's going to be vulnerable," said Mr. Jenkins.