KNIVES ON planes: Can the Transportation Security Administration (TSA) be serious?

That has been the reaction of some members of Congress to the TSA’s announcement that it will loosen restrictions on what passengers can carry aboard airliners. Next month, federal airport screeners will no longer force you to surrender hockey sticks, golf clubs, toy baseball bats or pocketknives — as long as the blades don’t lock in place and aren’t too long.

In fact, it makes sense for TSA agents to be looking for bombs that could bring down a plane, not scouring luggage for penknives. The agency is not and cannot be in the business of protecting every passenger and crewmember from every conceivable threat. If it tried, it would have to ban a lot more than knives, and its effort would cost far more money, gobble up more of passengers’ already overbooked airport time or both.

Instead, government resources and travelers’ time should be spent with a sense of priority. Airplanes pose unique dangers that justify the frustration that comes with airport screening. Terrorists have committed mass murder by using planes as weapons: forcing them to crash and blowing them up in midair. In the post-Sept. 11 era of reinforced cockpit doors, savvy passengers, a more robust air marshal program and flight attendants trained in self-defense, allowing penknives onto planes would not result in such tragedies.

The change would, however, lead to less hassle for the tourist who forgot to take her knife off her key ring and less wasted time for the screener who must do the hassling. Opponents charge that the opposite could be true; instead of just confiscating knives, TSA agents might have to measure blade lengths. Yet if X-ray screeners were allowed simply to let items that are obviously pocketknives pass through, that would surely result in fewer time-consuming secondary bag inspections. The TSA, meanwhile, says that the screening process has gotten more efficient since it started allowing small scissors and knitting needles aboard in 2005 and lighters onto planes in 2007.

Some also wonder why passengers should be allowed to carry knives on planes when they still have to take off their shoes for X-ray screening. But terrorists can and have fit improvised explosive devices into shoes. They could combine certain liquids in an airline cabin and cause a disaster. TSA is right to loosen up on pocketknives before such items as shoes and liquids.

Instead of pouncing on the TSA for its rules change, lawmakers should enable — indeed, press — the agency to continue adding building a flexible and focused security system capable of blocking big and unexpected threats.