have not actually flown anywhere since before 9/11. dont care for flight, prefer to drive.
may have short trip BOS to LAS coming up. is it possible take any light on board in one’s carry-on?
Yep, As a test I took a DRY with 3 spare batteries on and they didn't care. That was one out of 5 flashlights I had. I also had 12 spare cr123a for my surefire g2x.
Yep. I fly to Central America all the time and always have at least 1 and sometimes as many as half a dozen lights with me. I’ve never once encountered a problem, not even when carrying my EagleTac T20 with the tactical crown (I usually take it off and put it in my checked baggage, but a couple of times I’ve forgotten to do so). Like scaru, I always have extra sleeves of batteries with me, too.
I flew out of Logan to Florida this winter I had my EDC Black Cat in my pocket and a cell phone charger 18650 flashlight in my carry on. No problems at all I also had an e-cig and an extra battery in my pocket again no problem. I brought down a 501A an A30B some 18350s, 18650s, 16340s and a charger in my checked bag all made it no problem. I did get bitched at for taking a couple hits off my e-cig on the return flight but no one seemed to care on the way down.
I don’t know if domestic USA airlines have some different regulations, but some international airflight companies have a new rule about batteries in hold baggage: Nicads and NiMh can be in hold baggage, but Lithium batteries can not. Lithium batteries must be held in hand baggage.
So as I travel for my job and also use a fair amount of flashlights for my job, in the last two months I’ve always put 3-4 of my flashlights (usually TK35 and V60C are always part of the team) in my small backpack or in small cabin trolley, and never had any problem with it. In a few countries at the metal detector I’ve been requested to show them out and switch on-off the light, but apart from this no problems at all. Oh, and I also had a smaller edc on my belt. Same check, light on-off and thanks you can go. 
I have flown in state and interstate at least 50 times in the past year and have had no problems carrying a TK41, L2P, BC40, MPP-1 & 2, L2M, TK70. All in carry on backpack. I even carry quite a few tools and as long as they are under 7" they have all passed through.
FWIW, the TSA needs to be disbanded.
Just want to back up Rockspider. Haven’t flown too much lately, but last week took a flight in the USA. Had one of my EDC lights with a 16350 and a couple extra batteries for my cell phone. All in my carry on with no problem.
There were however signs stating that due to new regs that all lith-ion batterys needed to be in your carry on and could not be checked. My guess is they are following in line with the new postal regs.
I don't think they would let you since it is a tool over 8 inches.
Edit: Please don't drag this off topic with varying interpretations of the above.
Sunk! Thanks Admin, quick work