At Error Statistics Philosophy, D. G. Mayo has an interesting discussion of changes that just went into effect to Transportation Security Administration rules about what air travelers can bring in their carry-on bags. Here's how the TSA Blog describes the changes:

TSA established a committee to review the prohibited items list based on an overall risk-based security approach. After the review, TSA Administrator John S. Pistole made the decision to start allowing the following items in carry-on bags beginning April 25th:

  • Small Pocket Knives – Small knives with non-locking blades smaller than 2.36 inches and less than 1/2 inch in width will be permitted
  • Small Novelty Bats and Toy Bats
  • Ski Poles
  • Hockey Sticks
  • Lacrosse Sticks
  • Billiard Cues
  • Golf Clubs (Limit Two)

This is part of an overall Risk-Based Security approach, which allows Transportation Security Officers to better focus their efforts on finding higher threat items such as explosives. This decision aligns TSA more closely with International Civil Aviation Organization (ICAO) standards.

These similar items will still remain on the prohibited items list:

  • Razor blades and box cutters will remain prohibited in carry-on luggage.
  • Full-size baseball, softball and cricket bats are prohibited items in carry-on luggage.

As Mayo notes, this particular framing of what does or does not count as a "higher threat item" on a flight has not been warmly embraced by everyone.

Notably, the Flight Attendants Union Coalition, the Coalition of AIrline Pilots Associations, some federal air marshals, and at least one CEO of an airline have gone on record against the rule change. Their objection is two-fold: removing these items from the list of items prohibited in carry-ons is unlikely to actually make screening lines at airports go any faster (since now you have to wait for the passenger arguing that there's only 3 ounces of toothpaste left in the tube, so it should be allowed and the passenger arguing that her knife's 2.4 inch blade is close enough to 2.36 inches), and allowing these items in carry-on bags on flights is likely to make those flights more dangerous for the people on them.

But that's not the way the TSA is thinking about the risks here. Mayo writes:

By putting less focus on these items, Pistole says, airport screeners will be able to focus on looking for bomb components, which present a greater threat to aircraft. Such as:

bottled water, shampoo, cold cream, tooth paste, baby food, perfume, liquid make-up, etc. (over 3.4 oz).

They do have an argument; namely, that while liquids could be used to make explosives sharp objects will not bring down a plane. At least not so long as we can rely on the locked, bullet-proof cockpit door. Not that they’d want to permit any bullets to be around to test… And not that the locked door rule can plausibly be followed 100% of the time on smaller planes, from my experience. ...

When the former TSA chief, Kip Hawley, was asked to weigh in, he fully supported Pistole; he regretted that he hadn’t acted to permit the above sports items during his reign service at TSA:

“They ought to let everything on that is sharp and pointy. Battle axes, machetes … bring anything you want that is pointy and sharp because while you may be able to commit an act of violence, you will not be able to take over the plane. It is as simple as that,” he said. (Link is here.)

I burst out laughing when I read this, but he was not joking:

Asked if he was using hyperbole in suggesting that battle axes be allowed on planes, Hawley said he was not.

“I really believe it. What are you going to do when you get on board with a battle ax? And you pull out your battle ax and say I’m taking over the airplane. You may be able to cut one or two people, but pretty soon you would be down in the aisle and the battle ax would be used on you.”

There does seem to be an emphasis on relying on passengers to rise up against ax-wielders, that passengers are angry these days at anyone who starts trouble. But what about the fact that there’s a lot more “air rage” these days? ... That creates a genuine risk as well.

Will the availability of battle axes make disputes over the armrest more civil or less? Is the TSA comfortable with whatever happens on a flight so long as it falls short of bringing down the plane? How precisely did the TSA arrive at this particular assessment of risks that makes an 8 ounce bottle of conditioner more of a danger than a hockey stick?

And, perhaps most troubling, if the TSA is putting so much reliance on the vigilance and willingness to mount a response of passengers and flight crews, why does it look like they failed to seek out input from those passengers and flight crews about what kind of in-flight risks they are willing to undertake?