NEW YORK (MainStreet) — Frequent business class flyer George Hobica, air industry analyst and president of airfarewatchdog.com, recently had his carry-on bag rejected in the American Airlines bag sizer before even getting to the TSA security line. He was sent back to check-in to check his one-inch-too-large carry-on bag. And, he almost missed his flight because of the long check-in line which he usually avoids as a frequent flyer.

Also See: Top 10 Travel Sites

He's been flying with his favorite over-head-bin-fitting 20 inch by 15 inch by 8 inch carry-on bag for at least ten years when the bag suddenly stuck out of American Airline's 22 inch by 14 inch by 9 inch bag sizer by the middle dimension one inch. Hobica was sent back to check-in even though the overall measurement of his small carry-on bag, at 43 linear inches (adding the height, width and depth including wheels and handle), was under American's 45 inch total size limit.

"It's not just about the price to check the bag, running up to $100 depending on the airline, which was free for me since I'm a frequent flyer," he said. "But I almost missed my flight. And that really would have cost me."

If you miss your flight, which is easy to do if you have to enter the check-in line all over again to check your bag, you could be open to all sorts of additional mark-ups such as the ticket-change fee, walk-up fee, fair difference fee and the bag check fee, all of which are even more expensive on international flights, depending on the airline, advises Hobica.

Airlines have different carry-on size requirements

Southwest and JetBlue have a more generous 24 inch by 16 inch by 10 inch carry-on limit and no carry-on fees. Spirit also has a more generous size restriction (22 inch by 18 inch by 10 inch) but charges $26 to $50 for all carry-on bags beyond a personal computer bag, back pack or purse, up to $100 if checked at the gate.

Also See: Americans Spend on Average $800 per Year for Trips They Don't Want to Take

Hobica says he flew United earlier this year and had no problem with his carry-on bag size. Prior to March 2014, United Airline's carry-on bag restriction stated carry-ons must not exceed total linear dimension of 45 inches and may not be longer than 22 inches in any single dimension. Now the carry-on bag restrictions stipulate for a bag that is no larger than 22 inches by 14 inches by 9 inches in any dimension. The company says bag-sizers are located in the lobby to check your bag's size compliance prior to check-in.

Hobica even received a tweet recently from Delta flyer who was sent back to check-in and charged $100 for the international bag check fee over that same middle-dimension one inch.

According to the FAA, the maximum size carry-on bag for most airlines is 45 linear inches, when all dimensions are added. But the FAA does not mandate carryon-on bag size. Instead, the FAA requires airlines to enforce their own baggage restrictions if they have them, advises Hobica.

"Adding to this issue is the merger between American Airlines and U.S. Air," says Hobica. "The FAA does scrutinize all operations of each airline in a merger and that may have led to this current crack-down on carry-on bag size for American."

Airlines are cracking down on carry-on size requirements

Maybe it's airlines trying to collect more bag check fees or the FAA watching a merger like a hawk. Or maybe airlines are working to improve passenger complaints of full overhead bins and delays caused by gate-checking carry-ons (by passengers whose carry-on bags do not fit). "Whether it's any of those or all of the above, there is definitely a crack-down on carry-on bag size restrictions," says Hobica.

Also See: Winning the Airline Seating Game

The Transportation Security Administration (TSA) screens any carry-on baggage that fits through the X-ray machine; it is up to individual air carriers whether the baggage fits the size restrictions for your flight.

The point is, do you want to be the one missing your flight or paying fees at the gate or sent back to check-in over an inch difference in any one dimension of your bag size?

Even if you have been flying with the same bag and many different airlines for years, be sure that bag exactly fits the restrictions for each airline every time you fly:

  • Measure your bag width, length and depth (including handles and wheels) and add it all up to be sure you fit each separate dimension requirement as well as the total linear size requirement listed on the airline's website.
  • Weigh your bag in case there are any weight restrictions to carry-on bags.
  • If you have connecting flights under another airline, check those specific carry-on restrictions.
  • Allow extra-time in the check-in line for all those being sent back to check their bags.
  • If you need to, buy a new carry-on bag and check the actual product dimensions that fit the smallest carry-on requirements, which is 22 inches by 14 inches by 9 inches.
  • Don't just rely on the overall linear dimension. The bag must fit both the requirements of each dimension and the overall linear dimension.

--Written by Naomi Mannino for MainStreet