WASHINGTON - The Transportation Department's stopgap method for detecting bombs in checked bags - a system it intends to install at 429 airports by Dec. 31 - will not reliably detect explosives in a suitcase, European and U.S. experts say.

The technology, which is meant to detect trace amounts of explosives, is "not really operationally viable," said Norman Shanks, who once was in charge of security at the British Airports Authority, which rejected such a system a decade ago.

The chairman of the House Aviation Subcommittee, Rep. John L. Mica, a Florida Republican, called the planned system "semi-ineffective."

But it may be the best the department can do on the short notice given under the security law passed in November, Mica said. John Magaw, who is in charge of the Transportation Security Administration, told a Senate committee May 21 that the system the department planned to use was "equivalent" to what Congress wanted - thousands of giant bomb-detection machines - if used "with the proper protocol."

But the protocol itself can be flawed, some security experts say, depending on how many checked suitcases are opened for searching and how carefully searches are done.

The original plan of Congress was for the Transportation Department to buy thousands of explosive-detection systems, minivan-size machines that work like medical CAT scanners. Before Sept. 11, those machines were to be installed from 2007 to 2014. But after the attacks, Congress moved the deadline to Dec. 31 of this year.

In April, when the Transportation Department concluded that it could not procure and install enough of the big machines, it declared that trace detectors qualified as explosive-detection systems. The detectors look for microscopic quantities of explosives inside and outside a bag that are presumed to be present if it contains a bomb.

Friday, the Transportation Department announced that it had awarded a $508 million contract to the Boeing Service Co. to install 1,100 bulk machines and 4,800 to 6,000 trace machines by Dec. 31 and train at least 21,500 screeners.

Many experts say the department plans to use a "40-40-20" procedure, which is at the heart of the debate. The basic inspection technique for checked baggage will be the same as for carry-on bags: A technician will rub a muslin swab over a surface, then feed the swab into a detector.

Under the system, 40 percent of bags will be rubbed on the outside. Another 40 percent will be opened and the cloth rubbed around the inside. The remaining 20 percent will be treated under what some experts call the "Coke-can rule," meaning technicians will swab any object the size of a soft drink can or larger. The dimensions of the smallest object that can be skipped are secret. But for years, the Federal Aviation Administration has focused on the ability to detect explosives hidden in small electronic devices - a common terrorist tactic.

Mica, who favored a deadline of December 2003 instead of December this year, called a 40-40-20 system "a couple of notches above what we have now," but "not that effective."

Sergio Magistri, the chief executive of InVision Technologies, one of three companies that build bulk detectors, said the trace method had its place. Around the world, Magistri said, until recently it had been used "when you have to open a bag, to do a final check" after a bulk detector has indicated it might have a bomb.

Trace detection is a useful complement to bulk detectors, experts say, because the bulk machines measure density, not molecular content, and as a result can sound an alarm over harmless materials.

Airport officials like bulk detectors because they can process hundreds of bags per hour, compared with 20 or 30 an hour using the trace method.

Another problem for the Transportation Security Administration is that it is supposed to use certified machines. A certification standard is being written for the trace machines.

Then there is effectiveness. One way to improve that under a 40-40-20 protocol would be to combine trace detection with a program developed before Sept. 11 called the computer-assisted passenger profiling system. It was intended to identify passengers whose bags should be examined, based on their travel history and other characteristics, while other bags were loaded onto planes without analysis. Now it could be used to choose the passengers whose bags would get the Coke-can treatment. The problem is that lawmakers and others lack confidence that the system can reliably identify travelers whose bags should be examined.

The level of disruption to airports will depend on how thoroughly each bag is swabbed. At Salt Lake City International Airport, which used trace detection during the Olympics this year, Tim Campbell, the executive director, said it did not slow passenger processing. But trace detection was used only on the outsides of bags and on passengers' hands while passengers waited to check in.

Campbell said that he believed there would not be much disruption in airports that had enough space for the operation.