Air travelers planning to take off for the holiday season will face a string of new security procedures and find that old rules no longer apply.

Even travelers who do not set off walk-through metal detectors will occasionally be pulled aside for pat-downs as part of changes that go into effect Dec. 22. A new procedure calls for screeners to routinely touch passengers in the mid-thigh area and arms -- not just their torsos. Security officers, newly trained in "behavior recognition" screening, will scan the crowds for travelers who appear to be nervous and will pull them aside for extra scrutiny.

The Transportation Security Administration yesterday announced those and other security measures as part of a new strategy designed to be less predictable to potential terrorists as well as frequent travelers. The changes are the biggest shift in airport security since the agency was created after the 2001 terrorist attacks.

"Our goal is to establish flexible protocols based on risk, so that terrorists cannot use the predictability of our security measures to their advantage when planning an attack," Edmund S. "Kip" Hawley, director the Transportation Security Administration, said at a news briefing in Washington yesterday. "In the past, security measures at every airport were pretty much the same. . . . With the changes we are implementing later this month, that predictability is gone."

Several aviation leaders yesterday applauded the changes, such as the decision to again allow small scissors and tools such as screwdrivers and wrenches in carry-on luggage, even though flight attendants and some members of Congress have objected. Security and aviation officials have complained that for too long the federal government spent few resources on the biggest carry-on threats to air travel, such as explosives. But at the same time, airport and travel experts questioned why the agency would make such major changes during one of the biggest holiday travel periods of the year.

"Initially when this goes into effect, there's going to be a lot more confusion as the screeners themselves try to adjust to these procedures," said David Stempler, president of the Air Travelers Association, which represents business travelers. "I wish they would do it earlier or later so they aren't starting during the busy travel season. A combination of inexperienced travelers mixed with inexperienced screeners is a formula for disaster."

TSA officials said they do not expect the new procedures to result in longer waits at the security checkpoints, and they advised that travelers give themselves the same amount of time they normally would during the holiday travel period, which begins the week of Dec. 19 and runs through the first week of January.

Among the changes the TSA announced yesterday:

* There has been a 70 percent increase in the number of bomb-sniffing-dog teams moving through airport terminals, with 420 teams now in 80 airports.

* Plans are in place to greatly increase the number of walk-through "puffer" machines that detect explosives on a person's clothing, from 43 machines today to 340 machines by fall 2006.

* Passengers who do not set off the metal detector may randomly be selected for additional screening, such as a brief search of their bags, screening shoes for explosives, hand-wanding with a metal detection device, or a pat-down.

* More passengers will be selected for additional screening, but some procedures may be briefer. A new pat-down procedure, for example, calls for more common sense: screeners will not pat down areas of the body covered by tight clothing or where it is obvious the passenger is not concealing items underneath.

* A new pat-down procedure calls for more attention to arms and legs, including touching the back of the mid-thigh.

* At a handful of airports, the TSA has trained screeners to look for suspicious behavior patterns in passengers and the agency plans to expand the program. Officials say the program uses observation techniques and does not single out people based on race, religion or nationality.

During the holiday travel period, Dulles International and Reagan National airport officials said that passengers should arrive at the airport two hours before their flight is scheduled to depart for domestic destinations and three hours for international destination. A Baltimore-Washington International spokesman recommends 90 minutes.

"The traveling public is accustomed to security protocols and procedures," said BWI spokesman Jonathan Dean, who said he was assured by the TSA that the changes would not cause any disruptions. "The new actual changes aren't dramatic."

Democratic and Republican members of Congress have expressed concerns about the TSA's plans to allow small scissors and other small tools back on airplanes, in response to concerns from flight attendants and relatives of victims of the Sept. 11, 2001, terrorist attacks. Rep. Edward J. Markey (D-Mass.) and Sen. Charles E. Schumer (D-N.Y.) said they plan to introduce bills that would ban the scissors and tools from airline cabins. Yesterday, the Senate Committee on Commerce, Science and Transportation announced plans to hold a hearing as soon as possible about the TSA's new security rules and procedures