SAN ANTONIO — You’ve probably already booked those summer airline tickets and are dreading those baggage fees the airlines charge. If your bag is too heavy, you have to pay even more. So have you ever wondered how accurate those airline scales are?

If you’re like most people, you try to fit as much stuff in your bag as possible without going over 50 pounds since, on most airlines, going over that triggers an extra charge. The bag fee jumps from about $25 up to $90 or $100. So, if the scale is off by even one pound, that’s a big deal. With that much money riding on the accuracy of the scales, how often do you think they’re tested?

One air traveler told us, “I think they should check them once a week.” Another guessed, “Once a month.” But the Texas Department of Agriculture only comes out to check the accuracy of the scales at San Antonio International Airport once every four years.

“That sounds like a long time. Shouldn’t they be inspected more often?”, Avila asked a TDA official.

“The routine schedule for inspections is once every four years,” said Veronica Obregon, Chief Communications Officer for the TDA. “The good news is, we have a high compliance rate. Just at this airport, in the last round of inspections we did, we had a 94 compliance rate.”

She’s right about that. The News 4 WOAI Trouble Shooters obtained the inspection records and found that 66 scales were inspected at San Antonio International Airport last year, and four scales failed – two United Airlines scales, one Delta scale, and one used by Southwest.

But inspectors also found four other scales being used by Continental, Airtran, Aeromexico, and U.S. Airways hadn’t been registered with the state, as is required. So, they had never been inspected. Some travelers think the scales need to be policed more carefully.

“They should all be in compliance,” said airline passenger Mike Lawrence. “Look at all the people standing in line right now to get their bags weighed. They should all be in compliance, and they should all be checked regularly.”

Since the scales at our airport aren’t scheduled to be checked again until 2014, we decided to test them ourselves. We used the same kind of weight state inspectors use, which has a stamp showing it is certified by the state to weigh exactly 50 pounds. Then, we went from airline to airline with a hidden camera, testing each one of their scales. At first we were impressed, because we only found one scale, at the Delta Airlines counter, that failed. It measured one pound without anything on it, and 51 pounds when we place the 50 pound weight on top.

But it turns out the Department of Agriculture had informed the airport ahead of time we were investigating baggage scales, and some of the airlines had their scales serviced by a private company just days before we conducted our test. There’s no way to tell how that effected the results. The Department of Agriculture claims it doesn’t give advance warning when it does its inspections.

“We do not alert the airlines when we are coming. We come, do the inspection, and if it fails, we put an out-of-order tag on it.” said Obregon.

Delta Airlines did not call us back about its scale that appeared to be inaccurate. However, United Airlines, which had some scales fail inspection in 2010, told us it does its own inspections annually. United says sometimes when a scale slips out of calibration, it under-weighs baggage, which works to the traveler’s advantage.

United says airlines want the scales to be accurate, because weight determines how much fuel the plane needs and how to distribute the bags safely in the cargo hold.

If you suspect you were cheated by an airline scale you can complain to the Texas Department of Agriculture to have it inspected. Call 1-800-TELL-TDA (1-800-835-5832) or email the TDA at TellTDA@TexasAgriculture.gov.

Last year the airlines collected more than $2.5 billion in baggage fees from travelers.

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