I know some airlines need you to check it in, but I'm not sure if it has to be in a separate bag.
“Can I take my longboard on an airplane with me?”
Yes. Probably. My friends and I have been traveling with skateboards for years and haven’t had a problem bringing skateboards with us. That said, every airline and airport is different. Your mileage may vary.
Preface: In general, you want to be polite and pleasant with airport people. They deal with stressed-out travelers constantly and have the power to really ruin your day, so a smile and a bit of empathy goes a long way. Don’t make their lives difficult and they won’t make your life difficult.
You have options for bringing your skate on board: check it through or carry it on.
Carrying it on is easy. Just carry your board through security and bring it onto the plane with you. If anyone at the security screening gives you guff–sometimes they think your skateboard could be used as a weapon, but the TSA’s page on sporting equipment doesn’t say anything about that and skateboards aren’t on the list of prohibited items–just respectfully tell them the baggage agent told you it was ok to carry the board on.
Once you’re at the gate, the agent there might ask you to gate check your board to conserve overhead bin space. Gate checking is free, but if you don’t want to let your board out of your sight you can ask to put it in the coat compartment at the front of the plane. When you’re on the plane, just throw your board wheels-up in the overhead bin and you’re good to go. (note: this advice does not apply to atrocious low-cost carriers like Spirit airlines or RyanAir that charge you to bring a carry-on. Don’t fly with those companies. Spending a little more on the fare can save you money in the long run by avoiding fees at the airport.)
If you’re planning to carry multiple boards, you’ll want to take the trucks and wheels off and tape the decks together to conserve space.
I travel with way too much camera gear, so I always check my skate gear through. You’re going to want a bag big enough to fit your board inside (Sector 9 and Decent Hardware make good board-specific bags, and golf bags are good if you’re on a budget), as TSA agents sometimes get sticky fingers and some friends of mine have lost boards on the outside of their bags. To avoid costly surprises at the airport, check your airline’s baggage policy before you leave and make sure your bag is within the size and weight requirements.