This question is surprisingly more complex than you might think. The official TSA website says flashlights are allowed in both carry-on luggage and checked bags. However, the reality is TSA officers often confiscate several types of flashlights which they believe violate FAA regulations.
Large Maglite flashlights, or any flashlights exceeding seven inches in length are not permitted in your carry on bag. This is due to the TSA’s guidelines concerning bludgeoning weapons. According to the TSA, “sports equipment that can be used as a bludgeon such as bats or clubs is prohibited in the cabin of the plane and must be transported in your checked baggage.” This is why the TSA will allow passengers to carry bowling balls in their carry-on luggage, but not bowling pins (a pity for those who want to play ten-pin at 35,000 feet).
Lithium-ion batteries like CR123As carried outside a device are another TSA no-no for carry-on luggage. Loose spare batteries should be stowed away into your check-in baggage to prevent them from being confiscated. Airlines have problems with Lithium batteries since they cause inextinguishable fires if they short circuit. There are very few exceptions.
Most modern tactical flashlights are about five inches long, which is well under the TSA’s seven inch limit. However, some of them have very aggressive crenellated bezels, glass breakers or self-defense accessories like fold-out blades or tasers. The TSA interprets aggressive features on a flashlight to be a form of weapon. Weapons aren’t allowed in the cabin for obvious reasons, and if anyone argues that they’re merely bringing a “flashlight,” the TSA argues that if the flashlight can be used like a weapon, it’s a weapon.
However, the TSA will almost always allow pen lights and compact plastic flashlights, especially when those lights are free of any aggressive texturing or extra features. On the other hand, the moment TSA officers see what they determine to be a “black and scary” flashlight with ridges, your tactical flashlight’s fate is in their hand. While many people have kept their flashlights on their person from the United States all the way to Europe, Asia and all points beyond, many have had their expensive tactical lights confiscated simply because a TSA agent deemed them to be too “aggressive.”
Inforce TFx lights are devoid of any overly aggressive texturing and don’t come equipped with tasers or foldout blades, which should make them TSA friendly in theory. However, to be safe, avoid bringing your tactical flashlight on a plane. It would be safer to store it in your check-in baggage along with any spare batteries.