The next time a terrorist aims a jet at building, maybe the plane should say no.

From computers that could steer airliners away from skyscrapers to face-recognition devices already used to spot card counters in casinos, technology could provide ways to make the skies safer, but at a cost, experts said.

Current technology, which focuses on weapons searches, was bested by terrorists armed with knives and box cutters who crashed hijacked planes into the World Trade Center and the Pentagon this week. To prevent this, new security systems could be buttressed with devices that look for terrorists before boarding and ones that keep the plane safe, experts said.   

Autopilots could be linked to terrain warning systems, which the Federal Aviation Administration already requires in new U.S. aircraft, that show pilots what is below and recognize major landmark buildings, for instance.

"It's only a small step to take that, link it into the flight control system, and voila, you have something that you can stop things flying into buildings," said Ian Sheppard, of British airline consultant Air Claims.

But putting that technological fix into service would require extensive testing and raise questions about taking control from pilots, said Ron Crotty, a spokesman for Honeywell International, which makes the terrain systems.

"Technically it is possible," he said. But it would take expensive certification as well as modification of the systems, which sell for around $70,000.

"Every time you take control of the plane away from the pilot, it is a major problem," Crotty said.

FAA spokesman Les Dorr said his agency took a different approach: "We really don't think that that sort of technology would be the solution to the hijacking issue. Where technology can be brought to bear is keeping those kinds of individuals and any weapons off the aircraft."

The FAA is already rolling out new technology, including million-dollar, three-dimensional scanners that probe checked baggage for bombs, comparing the density of items inside to those of explosives. The scanners are made by InVision Technologies, a security device maker based in Newark, Calif.

Airports single out suitcases for scrutiny with an FAA computer system that winnows out better-known passengers. Experts say that probably means allowing frequent flyers to pass, for instance, although the FAA declined to give details.

New X-ray machines for carry-on luggage also will be phased in over the next year, in which software will randomly generate images of fake weapons to keep security staff who watch machines on their toes, the FAA said.

"To stay in business they are going to have to screeners got that meet our standards," FAA spokeswoman Rebecca Trexler said.

The new machines will tell workers when they've caught fakes without requiring packages to be opened and allow the FAA to monitor how contract security firms are performing. The agency is seeking other ways to improve security as well, Trexler said.

The augmented X-ray machine addresses a main concern--the low level of training for many airport workers.

"They can either work for McDonald's or be screeners," said Bob Monetti, an air security advocate whose son died on Pan Am flight 103, which exploded in December 1988 over Lockerbie, Scotland. Monetti has since become a member of the FAA's Aviation Security Advisory Committee.

Screeners face a tougher task now that the FAA has banned knives in the aftermath of the World Trade Center attacks.

"Everybody carries a little pocket knife," Monetti said. "To say we will not allow anybody to carry one is somewhere between really silly and unenforceable."

A solution would be to check the people in addition to what they are carrying, said Monetti and Francois Mesqui, chief technology officer of InVision.

"If it is true that you could hijack a plane with a small knife, there must be another security feature that we are using to make sure that passengers going on a plane (are) offering a certain guarantee, by themselves, that they are not dangerous people," he said.

Evening the odds

Casinos have already found a way to even the odds in their business. More than 100 casinos--and two European airports--have bought face-recognition systems from Littleton, Mass.-based Viisage Technology.

"We use it to filter players and suspicious people," said Derk Boss, who uses the system to find card counters and cheats at the Stratosphere casino in Las Vegas. He expected it would catch terrorists, if the computer had a good database to use.

"People use different colored glasses and mustaches and things like that. They'll grow a beard. And it certainly worked on that," he said.

A major airport could outfit itself with cameras and computer systems at all the gates for hundreds of thousands of dollars, said Tom Colatosi, Viisage CEO.

His software takes 128 readings of each face and compares the result to a database. That would mean searching for individuals, rather than using computers to find unknowns, which some civil libertarians object to as intrusive. But Colatosi said those concerns have faded since the World Trade Center bombing.

"I see the airports here in Boston are saying no more curbside check-in, you can't drive within 300 feet of the door, and all those types of things," he said. "They are all going to increase the inconvenience for the traveling public, and none of them would have prevented the hijacking at Logan."