In the more than seven years since Pan Am Flight 103 was blown out of the Scottish skies by a terrorist's bomb, U.S. aviation officials have adopted a wide array of security measures to prevent a repetition of that tragedy.

But most federal officials acknowledge that progress has been slow and spotty - and some critics charge that there has been no progress at all.

"The airport safety as it relates to international flights is terribly lacking," said Sen. Alfonse D'Amato (R-N.Y.) yesterday as he emerged from a previously scheduled briefing with federal aviation officials about airport security. "It presents no challenge to international terrorists to put bombs on a plane. That was a fact then [before the Pan Am bombing], it is a fact now ... It's not any harder at all."

Federal aviation officials can point to a variety of measures that have been implemented since the Pan Am tragedy, in which 270 people died.

Baggage on international flights is now routinely X-rayed, and passengers on such flights are questioned to screen for possible tampering with luggage. Travelers on all flights are supposed to be required to show picture IDs. Parking has been restricted at some terminals to forestall possible car bombings. At times, curbside baggage check-in has been suspended, and those without tickets have been barred from gate areas. Some terminals, such as Denver's new airport, have installed sophisticated electronic-and-computer-guided systems to control access to secure areas.

But the degree to which these measures have actually improved security is the subject of intense debate. For example, X-raying every bag on a flight is of little value if the X-ray machine cannot detect explosives - and that's exactly what was revealed in a 1994 test by the Federal Aviation Administration.

The test, conducted at four major U.S. airports, found that conventional X-ray machines have "a low probability of detecting a moderately sophisticated explosive device," according to a recent report by the congressional General Accounting Office.

Spurred by the recommendations of a presidential commission, the FAA has sought for years to find a new machine that could detect the presence of non-metallic explosives. It thought the search had ended in the late 1980s with the development of the thermal neutron analyzer, which screens baggage for the presence of nitrogen, a key ingredient in most bombs. The device was tested at six U.S. airports - including the TWA terminal at Kennedy Airport from 1989 to 1991 - and performed according to its design specifications.

But the machine was designed to detect bombs weighing 2.5 kilograms - 5.5 pounds - or more. Experts believe the Pan Am catastrophe was caused by a bomb weighing as little as 300 grams, or two-thirds of a pound. When the machine was asked to detect bombs that small, its performance plummeted. The FAA has shelved the device and is now seeking ways to adapt conventional X-ray machines or other technologies to detect explosives.

One expert in airline security believes the government's whole approach is flawed. Isaac Yeffet, who headed the legendary security operation of the Israeli airline El Al from 1978 to 1984, believes U.S. airlines and regulators rely too much on technology and too little on training of security personnel.

"Once you rely on machines, you are the loser," said Yeffet, now a Manhattan-based security consultant. "You deal with sophisticated terrorists today. The only way to stop the terrorist on the ground is to have security people that are well-educated, that know at least two languages and know how to question passengers."

Yeffet stressed the importance of baggage matching, a process by which every checked bag is matched with a passenger and removed if the passenger fails to board. U.S. airlines do this on all international flights, but not on domestic ones. The GAO report said that some FAA officials believe baggage matching should be extended to domestic flights, but that others disagree because they say the practice would be prohibitively expensive.

Other critics have questioned the competence of security personnel at U.S. airports. Mary Schiavo, who recently resigned as inspector general of the Transportation Department amid bitter criticism of her agency, yesterday said her staffers recently conducted tests at several airports - which she declined to name - and were able to penetrate security. "My staff was able to literally get out on the tarmac, get on planes, get in cockpits and witness a number of test devices go through security," Schiavo told ABC's "Good Morning America." "We found that a lot of it was just plain lax attitude."

Officials for the Port Authority, which runs all three major New York City airports, refused to comment yesterday on whether they were beefing up security in the wake of the TWA crash. A spokesman would say only that the authority was continuing to comply with FAA security standards at all of those airports.

While the debate rages, in Washington and elsewhere, about how best to protect passengers, the problem is only expected to grow larger. The number of bomb threats against U.S. aircraft and airports has risen steadily and sharply in recent years, according to FAA records - from 403 in 1993 to 673 last year.

And while the vast majority of those threats proved to be idle, federal intelligence agencies have warned that this may change. "The terrorism threat within the United States is increasing," the GAO said in its report, which was issued in March in response to inquiries from D'Amato and two other members of Congress. "Although no specific aviation threat is known, experts believe that aviation is likely to remain an attractive target for terrorists."