The overhead bins are empty. Why do airlines make you check your bag?

On the Sunday after Thanksgiving, Camila Tellez was first in line with her boarding group at the airport in San Luis Potosi, Mexico. The college student had one carry-on bag that fit American Airlines’ size dimensions. She approached the counter to scan her pass when the gate agent told her she would have to check her bag because the overhead bins on the plane were full.

But they weren’t.

“I start walking to the back and I kid you not, there are fully empty overhead bins. There must have been 10 to 15 of them,” said Tellez, who was flying to Boston via Dallas. “They were completely, completely empty.”

Airlines are very clear about their carry-on bag policies. Passengers can bring onboard one personal bag and one carry-on of a certain size. But even when travelers follow the rules, airline employees sometimes require them to surrender their bags for what can appear to be dubious claims of unavailable overhead space.

“I scroll through the social media exchanges of the major airlines with passengers pretty much every day, and the two most common complaints that I see are damaged bags and overhead bins with plenty of space when passengers have been asked to check their bags anyway,” said Gary Leff, founder of the View from the Wing blog.

Evidence abounds on social media, where aggrieved travelers share photos of lightly occupied or completely barren bins. So we asked aviation experts and the airlines: What gives with the empty bin space?

Why your carry-on gets rejected

Airlines want to leave on time. One delayed flight can cause a chain reaction that leads to even more disruptions.

Carriers typically allow 30 minutes to an hour for boarding, depending on the aircraft, airport and destination. To stay on schedule, the gate agents will try to anticipate - and preemptively eliminate - any speed bumps.

“Those delays of a few minutes matter because it can be the difference between some passengers making their connections and not. It can mean delaying the next flight. It also usually means planes aren’t as clean, because they don’t spend time removing trash from the plane to try to get back on schedule,” Leff said. “They’re really trying to get the plane out to keep its schedule, and not having to deal with carry-on bags is a key way of doing that.”

In a perfect world, everyone’s luggage would effortlessly slide into the overhead space. But that’s not always the case.

Luggage is not uniform. Some travelers ignore size limits or stick their personal items and coats in the overhead compartment instead of by their feet. Others place their roller bags horizontally instead of wheels out. The one exception to the latter guideline: Smaller planes with shallow bins, which require all passengers with rollaboards to gate-check their bags.

Traffic jams can form in the aisles as passengers puzzle out the possibilities until flight attendants swoop in with the solution. If all else fails, the passenger will have to march their bag to the jet bridge for a gate check. All of this puts pressure on the ticking clock.

“Ideally there should be enough overhead bin space for every passenger to store one carry-on bag. That’s why we’re seeing airlines invest in new, larger overhead bins,” said Henry Harteveldt, a travel industry analyst and the president of the Atmosphere Research Group. “But this will rely on everyone doing their part. Overhead bin space is finite.”

Several factors have contributed to a surge in carry-on bags. In 2008, some major airlines started charging for checked baggage. The original $15 fee has since doubled or more. (Southwest, which allows free checked items, is the exception.) Also, planes are jamming more people into cabins by adding extra rows without necessarily expanding overhead space.

Gate agents are in charge of keeping the boarding process on track. A plane’s late departure can earn them a demerit, said a Delta Air Lines flight attendant, who spoke on the condition of anonymity for professional reasons.

“They’re the only department that gets penalized for that. It’s mark on their record, and if you get however many, you can get fired,” the flight attendant said. “So they’re under a lot of pressure.”

As the eyes inside the aircraft, the flight attendants will send updates about the overhead space to the gate agents. When making their calculations, the flight attendants will factor in the bags already stowed and en route, as well as the style of luggage, since some types consume more space than others. As the number of open spots starts to dwindle, the gate agents - with the flight attendants’ blessing or independently - will decide to cut off all carry-on bags.

“If it’s a really chaotic boarding for the gate agents, they may assume it’s really chaotic for us on the plane, so then they use their best guess to when to start checking if they don’t hear from us,” the Delta flight attendant said.

For gate-checked bags, the gate agent will tag your bag, and you will either roll it to the end of the jet bridge or load it onto a cart. Depending on the aircraft and airport, you will either retrieve your bag on the jet bridge at your next stop or at the baggage carousel in your final destination (unless you have to go through customs).

American Airlines, which replied by email to questions about empty compartments, said its staff uses its best judgment when determining bags-to-bin space. However, the carrier acknowledged that sometimes its estimation is slightly off. Delta said by email that it switches from voluntary to mandatory bag-checking once the flight attendants have alerted the gate agents to diminishing space.

Harteveldt said airlines’ intentions are virtuous - they are trying to avoid delays - but their tactics are imperfect. “No one wants to check bags when it’s not necessary, and no one wants to be lied to, either,” he said.

Later boarding groups often feel the brunt of this practice. For Delta passengers, this could mean the second half of the four divisions. For her American Airlines flight, Tellez was in Group 7 (out of nine) when the agent informed the remaining passengers they would have to check their bags. I was in Group 6 when an American Airlines agent made a similar announcement on a flight from Knoxville, Tenn., to D.C. The agent told us that the 90-seater plane could only accommodate 25 bags, despite several empty compartments.

“The gate agents get a threshold of how many bags they are supposed to allow on before they start checking,” the flight attendant explained. " You think, ‘Oh, 25 bags. There’s more than 25 overhead bins.’ The problem is that people don’t listen [to the carry-on rules].”

How to avoid getting gate-checked

Obviously, travelers who board early will have ample space for their bags. But to get that priority, passengers must pay for it or earn it through credit card memberships or loyalty status.

If you qualify for any of the earlier boarding groups, such as military or families with young children, take advantage of them. Try to be the first in your group, but don’t try to pass yourself off for a higher number. That is akin to cutting the cafeteria line on pizza day. Also, avoid basic economy, which kicks you to the back of the queue.

Leff said to be aware of the size and bulk of your bag. Make sure it is the correct dimension, including the wheels and handle. Don’t overstuff your bag. You might be able to avoid scrutiny by carrying an unobtrusive or squishy bag, such as a duffel or backpack.

The Delta flight attendant said carry-on bags are more often checked on routes with heavy weekend and business travelers, and during busy travel periods, such as holidays and summer. So choose your flight times and connections wisely.

“I started noticing it more right before the summertime, but it doesn’t happen much in the slower traveling season,” they said.

Some people ignore the directive to gate-check their carry-on and head straight for the plane to look for an open spot. Or they may retrieve their bag from the jet bridge after noticing a cubby hole for their bag.

“If you see one or two spaces open and your bag is still sitting on the bridge and the ramp hasn’t been taken in yet, just talk with a flight attendant,” the flight attendant said.

Once your bag is settled in, hand over the gate-check tag to a flight crew member, so they can adjust the weight and balance tally.

“That’s more like asking for forgiveness than permission,” she said. “Like you found a spot for it. It’s already up. Here’s the tag. Sorry.”