We've been through this scenario before. Airlines invoke a policy of limiting bags to 50lbs (23kg); passengers put their luggage on scales that show that the baggage is overweight; the airline charges the passenger for the excess. Is it simple? Yes it is, and it's probably annoying too. However, few passengers seem to question the accuracy of the scale, and that oversight can be a costly mistake.

A team of reporters from the U.S. television network ABC and its "Good Morning America" program accompanied inspectors from the Los Angeles County Weights and Measures Bureau to check the accuracy of baggage scales at several busy U.S. airports. While baggage scale accuracy scored most poorly at the two New York City airports – JFK International and LaGuardia Airport – some of the busiest airports in the country scored surprisingly well considering the number of passengers handled daily. Atlanta-Hartsfield, Chicago O'Hare and Dallas-Fort Worth International Airport collectively had only 12 faulty scales. Overall, the data for over 2600 scales at 10 U.S. airports studied by the reporters showed that the inaccuracy rate hovered around 5 percent. It doesn't seem like much considering the number of airport scales tested, but if the bad one is the one that your bag is sitting on, one is all it takes.

Some inaccurate scales can read a piece of luggage as being lighter than it actually is, which can work in the passenger's favor. But the opposite is true as well, and that false sense of security on your way to your destination could become a nasty surprise on your return trip. How often have you heard this phrase: "but they didn't charge me on the way over here"?

Airlines lease their terminal facilities from the airport authorities so accusations of airline trickery in this regard may fall on deaf ears. The airports are largely responsible for testing and certifying the accuracy of the scales. Depending on the prevailing certification rules, there will likely be a sticker on or near the scale indicating the date of the last inspection. The airline agent and the passenger should be aware of the readout of the scale before any baggage is placed.

To play it safe, passengers should consider the following tips:

• Weigh your luggage at home or purchase a digital hanging scale. Though the accuracy is probably not much better than a home bathroom scale, at least you can establish a base weight from which to work. This also gives you time to add or remove articles in your luggage while you're at home instead of holding up the queue at the airport.
• Don't forget that you should know the weight of your baggage before you get to the airport for your return flight as well, especially if you have done some shopping during your journey.
• At the airport, ask the agent to reset the scale if it reads anything other than zero.
• Try weighing your luggage on more than one scale if possible. Many check-in desks have a neighboring counter with its own scale. You can ask the agent to try another scale if you do not agree with the indicated weight.
• Try to get your baggage to be underweight by at least 5lb (2.3kg) just to err on the side of caution. Coming too close to the permitted maximum is asking for a dispute if the airport scale is slightly off.