Editor's note: CNN Contributor Bob Greene is a bestselling author whose 25 books include "Late Edition: A Love Story"; "Duty: A Father, His Son, and the Man Who Won the War"; and "Once Upon a Town: The Miracle of the North Platte Canteen."  (CNN) -- How would you like to be Janet Napolitano on the day the first person is stabbed or slashed on a commercial airline flight?  Usually, it would be unfair to personalize the question like that; second-guessing a public policy decision after the fact is always easy.  But in the case of the dimwitted decision to lift the prohibition against passengers carrying knives onto airplanes, Napolitano, the secretary of the Department of Homeland Security, will be the one to get the direct blame if something terrible happens in the air, because she has been given ample notice that people who make their living flying think the idea is inexplicable and highly dangerous.  In early March, the Transportation Security Administration -- a part of the Homeland Security bureaucracy -- announced that it would soon allow knives with blades shorter than 2.36 inches in length, and no wider than a half-inch, to be carried onto flights.  There was immediate outcry from members of Congress, pilots and flight attendants. Many people may have assumed that, because the decision was so nonsensical, it would soon be scrapped. But it hasn't been; April 25 has been set as the day when, for the first time since restrictions were instituted after the September 11, 2001, terror attacks, passengers carrying knives will be permitted to board any commercial flight. Napolitano and the TSA have shown no inclination to reconsider.  It was passengers carrying blades, of course, who carried out the 9/11 attacks. But Napolitano has been adamant, even flippant, in dismissing the concerns about the wisdom of welcoming blades back onboard.  The Christian Science Monitor reported that, at a breakfast sponsored by that publication, Napolitano rebuffed the critics by comparing them to a comedic character once played by a popular actress in the early years of "Saturday Night Live":  "It's kind of like Gilda Radner, you know, if it is not one thing, it's another."  Napolitano's chief official at TSA, John Pistole, has said that by freeing TSA screeners from looking for small knives, they will be able to "better focus their efforts on finding higher threat items such as explosives." He also said: "A small pocket knife is simply not going to result in the catastrophic failure of an aircraft."  Become a fan of CNNOpinion  Stay up to date on the latest opinion, analysis and conversations through social media. Join us at  . We welcome your ideas and comments.  Greene: Ebert's sheer love of life  U.S. Rep. Michael Grimm of New York -- he, like Pistole, is a former FBI agent -- recalled those blades (not explosives) that resulted in the 9/11 carnage, and characterized the refusal of Homeland Security and TSA to rethink their position as "borderline idiocy."  Grimm is wrong only about the "borderline" part.  Will every passenger with a knife present a danger? Of course not, and that isn't the issue. An infinitesimal percentage of passengers board a flight with deadly intentions. The enormously expensive and technologically sophisticated post-9/11 security measures are in place precisely because their presence is meant to deter those few passengers bent on destruction. Why would you possibly go to the trouble of electronically frisking everyone boarding a flight, and then wave the passengers carrying knives right onboard?  Last week, members of the Association of Flight Attendants handed out leaflets at large airports around the country, asking for the public's help in trying to convince Homeland Security and the TSA to change their minds. The association said: "The people on the front lines of aviation security know that allowing knives on planes is a bad idea."  And the Flight Attendants' Union Coalition released a letter from the family of Sara Elizabeth Low, an American Airlines flight attendant killed on 9/11. The family addressed the letter to TSA Administrator Pistole:  "We are astounded by the lack of understanding and thoughtlessness that this terrible decision reflects. ... The terrorists have to be laughing at how naïve our government continues to be. ... For the safety of flight crews, passengers and those potentially affected on the ground, please reconsider this terrible decision. Take a page from the doctors' oath and 'first do no harm.'"  The plan to allow the knives won't even speed up the airport screening process. Try to imagine the scene at the front of the lines, as already overworked TSA agents are forced to take tape measures or rulers and gauge the length and width of knives, as travelers insist their blades are shorter than 2.36 inches or that they aren't more than a half-inch wide. Try to envision the arguments that will ensue.  So stubborn is Homeland Security and the TSA about refusing to reconsider the new policy that officials -- why they would choose to say these things out loud is incomprehensible -- are rationalizing their knives-on-planes initiative by publicly declaring that other items onboard could also be used to harm passengers and crew: "If you are talking about a small knife, there are already things on a plane that somebody can convert into a small, sharp object," Napolitano said.  Summer is coming, and with it flights that are jammed to capacity, passengers who are irritable and hot, delays that make blood pressure rise. It won't even have to be a terrorist -- just a drunken, enraged or emotionally disturbed passenger who has been given permission by the government to bring a knife along with him.  Napolitano, at that Christian Science Monitor breakfast, was quoted as saying that it was the handling of the announcement of the new rules that was the problem: "Where we could have done better, quite frankly, was a little more legislative and public outreach before we announced the decision. Try to give it a softer landing, as it were."  She couldn't be more wrong. It wasn't the handling of the announcement that was the problem. It wasn't a lack of "public outreach." The problem was -- and is -- the policy itself.  A policy that will -- unless Congress or the White House steps in -- put knives back into the hands of air travelers before this month is out.  Follow us on Twitter @CNNOpinion.  The opinions expressed in this commentary are solely those of Bob Greene.
Bob Greene says a rule change that will soon allow passengers to take small knives on planes is a terrible, dangerous idea.