ASHINGTON, June 7 — 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 American 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, John L. Mica, Republican of Florida, 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 last November, Mr. Mica said. John Magaw, who is in charge of the Transportation Security Administration, told a Senate committee on 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 shortened the deadline to Dec. 31, 2002.
In April, when the Transportation Department concluded that it could not procure and install enough of the big machines, it declared that trace detectors also 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.
On Friday, the Transportation Department announced that it had awarded a $508 million contract to the Boeing Service Company 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 trace detector.
Under the system, 40 percent of bags will be rubbed on the outside. Another 40 percent will be opened and the cloth will be rubbed quickly around the inside, too. 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 can of soda 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 tactic for terrorists.
Mr. Mica, who favored a December 2003 deadline instead of a December 2002 one, 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, Mr. Magistri said, until now 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 work by measuring density, not determining the molecular content, and as a result can sound an alarm over harmless materials.
Airports 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 last year, Tim Campbell, the executive director, said it had not slowed passenger processing. But trace detection was used only on the outsides of bags and on passengers' hands and was done while passengers waited to see airline check-in agents.
Mr. Campbell said that he believed there would not be much disruption in airports that had enough space for the operation. Inadequate space will be a problem at many airports, airport executives say.
Bruce Baumgartner, the manager of aviation at Denver International Airport, was even less optimistic. Processing will take a minute a bag if the outside is done and significantly longer if the bag is opened, Mr. Baumgartner said.
"We know the proper procedure is to open every bag and swab all items in the bag that meet a certain characteristic," he said. For security reasons, he would not say what that characteristic was.
"If you do it properly, the flow rate is going to be even slower and the effect even more significant," Mr. Baumgartner said. He compared the likely effect to the actions taken by the Federal Aviation Administration right after Sept. 11, when the flow rate through passenger checkpoints was cut by three-quarters, producing waits of two to three hours.
But it is not clear what options exist. The British use high-capacity X-ray machines, but these do not meet American standards. If a high-capacity machine finds a suspicious bag, usually it is bounced to a machine that meets American standards, the kind that the Transportation Department is now buying. That handles most of the "false positive" problem, experts say.
But the British machines, which work like the bulk machines, also have a "false negative" rate, clearing bags that have test objects representing bombs. These would go on planes undetected. Mr. Shanks, the former British airport official, said that his machines let through fewer of those than a 40-40-20 protocol using trace would.