Washington (CNN) -- Small pocketknives and an array of sporting equipment -- banned from aircraft cabins in the wake of the September 11, 2001, terrorist attacks -- will once again be allowed in U.S. planes, the head of the Transportation Security Administration said Tuesday.
Knives with blades that are 2.36 inches (6 centimeters) or shorter and less than a 1/2 inch wide will be permitted on U.S. airline flights as long as the blade is not fixed or does not lock into place. Razor blades and box cutters are still not permitted.
Two golf clubs, toy bats or other sports sticks -- such as ski poles, hockey sticks, lacrosse sticks or pool cues -- will also be allowed in carry-on luggage.
Transportation Security Administration head John Pistole said the changes will take effect April 25 and will bring the United States into alignment with international rules.
Some security experts say security improvements since the attacks -- most notably reinforced cockpit doors, better intelligence and motivated passengers -- have made the strict prohibition of small pen-knives unnecessary.
Pistole said the changes are in keeping with his "risk-based security" initiative and will permit airport screeners to focus on looking for bomb components, which present a greater threat to aircraft, instead of lesser threats.
But a union representing 90,000 flight attendants called the measure "a poor and short-sighted decision by the TSA."
"Continued prohibition of these items is an integral layer in making our aviation system secure and must remain in place," according to a statement by the Coalition of Flight Attendant Unions.
Although it was widely reported that the 9/11 hijackers used "boxcutters" in their attack, the weapons were not recovered, and investigators believe other types of knives were used.
At a January 2004 hearing of the 9/11 Commission, a commission staff member said, "Our best working hypothesis is that a number of the hijackers were carrying -- permissible under the regulations in place at the time -- permissible utility knives or pocket knives."
The commission displayed a Leatherman knife, saying, "We know that at least two knives like this were actually purchased by the hijackers and have not been found in the belongings the hijackers left behind."
Immediately after the attacks, the TSA prohibited a wide range of items from aircraft, but it has gradually been reintroducing some banned items such as cigarette lighters and matches. On occasion, such as a liquid bomb scare in August 2006, the TSA has expanded the list, banning large containers of liquids and gels.
Under the TSA's risk-based security, Pistole has sought to "reduce the haystack," giving modified screening to travelers under age 12 and over age 75. It has also expanded its TSA PreCheck initiative, which expedites checks of known passengers.
TSA spokesman David Castelveter said the changes announced Tuesday will not slow down the screening process by requiring screeners to measure knife blades and weigh plastic bats. Screeners will use "common sense" when applying the rule, he said.
A major pilot's union on Tuesday declined to comment on the specific changes announced by the TSA, saying it had not had time to study them. But it applauded the TSA's efforts to harmonize U.S. rules with those overseas, and its embrace of risk-based security.
"Unfortunately, the reality has been that we've had this patchwork of regulations and systems that in some part... have been carried on the shoulders of the airlines," said Capt. Sean Cassidy, first vice president of the Air Line Pilots Association International.
Cassidy said risk-based security benefits the industry, the airlines and travelers.