The tank has to have the reg/valve removed for two reason. One, you got a big pressure change while flying, last thing they need is you blowing a burst disk at 30,000 feet. Taking the valve off guarantees that the tank has no pressure in it. Though the the main reason is so they can look inside of it. An air tank a very spacious, air tight, x-ray proof container. They're going to want to look inside of that puppy, just to make sure you're not sneaking anything that you're not supposed to onto the plain.
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