You’ve probably never thought too hard about it. As you board a plane, a flight attendant stands at the aircraft door, smiles, says “welcome,” maybe glances at your boarding pass. It feels like basic hospitality, like the aviation equivalent of a host greeting you at a restaurant.

But that brief interaction is doing a lot more than it seems.

“The smile and greeting you get at an airplane door is probably one of the most judgmental welcomes you’ll ever receive,” Jay Robert, a senior flight attendant and the founder of A Fly Guy Travels, told HuffPost. “We’re judging a lot in those few seconds, and believe it or not, that judgment is part of our job.”

It’s not for the sake of judgment, however. The purpose is actually safety.

“Greeting passengers at the door is our first line of defense in identifying threats that could be brought onto the aircraft,” Robert explained. “Whether it’s illness, intoxication or someone carrying prohibited dangerous goods, we’re constantly scanning for anything that could disrupt the flight or pose a safety risk.”

Of course, there is a customer service and hospitality element to the greeting as well.

“It’s the first opportunity to welcome passengers onboard and set a positive tone for the flight,” said Yuvraj Datta, the chief supply and revenue officer at the travel tech company Fareportal. “It’s also an important observational moment for the cabin crew. Flight attendants are making initial safety and operational assessments that can help identify potential issues before departure.”

He added that flight attendants begin assessing the cabin environment the moment they get on board.

“The greeting is actually a behavioural assessment,” said AirAdvisor founder and CEO Anton Radchenko. “The social framing is deliberate – passengers engage more naturally when they believe they’re being welcomed. Crew are trained to read each individual during that interaction – orientation, steadiness and anything that could become a liability in an enclosed environment at altitude with no exit available.”

In an era of increasing reports of unruly passenger behaviour and even abuse of flight attendants, airline crews are more vigilant than ever. And many of those incidents involve alcohol.

“Generally, flight attendants assess passengers during boarding for sobriety,” said Henry Harteveldt, president and airline industry analyst at Atmosphere Research Group. “If they notice a passenger who appears to be overtly drunk or high, they may ask to see their boarding pass, under the premise of directing them to their seat, but also to alert the crew who may be working where that passenger is sitting to monitor her or his alcohol intake on board.”

Robert echoed that intoxication is one of the biggest things flight attendants are checking for during that greeting process.

“It’s much easier to stop someone at the aircraft door than it is to remove them once they’re seated and settled in,” he said. “During boarding, we constantly communicate with each other. I’ll often call the crew at the back and ask them to keep an eye on a passenger I’ve noticed or get a second opinion before we decide whether someone is fit to fly.”

Flying with an intoxicated passenger can be a serious matter. Dan Bubb – a former airline pilot and professor in residence in the honors college at the University of Nevada, Las Vegas – explained why catching these situations early matters so much.

“You don’t want to be at altitude and have somebody who’s drunk and belligerent, and now you’ve got a problem,” he said. “You have to divert the plane. That’s going to cost money and time. So it’s better to try to catch those possible problems at the door before it closes.”

There are legal implications to getting it wrong as well.

“Boarding an impaired passenger creates significant legal and operational exposure,” Radchenko said. “By the time a passenger reaches the cabin door, gate agents have already conducted an initial screen. If you’re flagged at the door, you’ve already passed one assessment, which means the carrier has two separate documented grounds to work with if a denial gets challenged.”

For passengers, the legal calculus runs in the other direction.

“If you’re denied boarding after that interaction, aviation law starts from the assumption that the crew was right,” Radchenko said. “The discretionary authority aircrews hold on safety grounds is wide by design. To challenge it, a passenger has to demonstrate the decision was arbitrary, not just uncomfortable or inconvenient.”

He advised anyone who believes they are unjustly denied boarding to ask for a written reason before leaving the gate. “That document is the starting point for any claim worth filing,” he said.

Flight attendants are also trained to pick up on physical cues that passengers might be trying to hide.

“I’m looking for signs of illness, such as clammy or pale skin, excessive sweating or even visible contagious skin conditions that may require us to offload a passenger before the aircraft door closes,” Robert said.

“Passengers often try to hide the fact they’re unwell, so we have to use all the clues the body gives us to quickly determine whether someone is fit to make the journey by air.”

He’s also paying attention to the way passengers respond to his greetings.

“Slurred speech, confusion, difficulty maintaining balance or the smell of alcohol can all indicate intoxication or impairment,” Robert said. “Once passengers move into the cabin, the other flight attendants continue the assessment, watching for the same signs and making their own observations.”

Scent is part of these assessments for multiple reasons.

“We’re checking for the smell of alcohol but also for offensive body odour,” Robert said. “Both can result in you being denied boarding, as BO is a no-go. Many people don’t realise that excessive body odour can significantly affect the comfort and wellbeing of everyone else on the flight, and airlines have the right to refuse carriage if it becomes an issue.”

Flight attendants also keep an eye on people’s luggage to ensure carry-ons meet the airline’s size and quantity requirements and to identify bags that need to be gate-checked. Robert added that they look out for anything that could be considered a dangerous item as well.

“In recent years, we’ve also added another important responsibility during boarding – looking for potential signs of human trafficking,” he said. “Cabin crew around the world now receive training to recognise indicators that someone may be a victim and know how to discreetly report their concerns.”

These assessments can be wide-ranging, but the practice becomes second-nature over time. And it’s a subtle but important function.

“Airlines often position their most experienced cabin crew at the aircraft door because we’ve developed the ability to rapidly identify potential issues,” Robert said. “As a purser, or cabin manager, while I’m making a good first impression and welcoming guests onboard, I’m also continuously assessing whether each passenger is fit to travel.”

Beyond looking for potential issues, flight attendants are also quietly scanning for potential assets.

“We’re also identifying passengers who may be able to assist in an emergency if they’re seated in an exit row,” said Paula S. Adams, a former crew member for Etihad Airways who now works as a private flight attendant and aviation and business trainer.

Indeed, the boarding process is when crew members identify what they call able-bodied passengers or ABPs.

“These are the passengers we’re most likely to ask for assistance if we ever have an emergency,” Robert said. “I naturally take note of people who appear physically capable, law enforcement officers, military personnel, firefighters, medical professionals or simply someone who looks strong and confident enough to help if a passenger becomes disruptive or assistance is needed during an evacuation.”

Harteveldt noted that another pre-flight step contributes to this process.

“The FAA requires flight attendants to give a verbal briefing to passengers sitting in exit rows and by exit doors about the requirements to operate the doors in an emergency, if so directed by cabin crew, and to receive a verbal confirmation from each passenger sitting in those seats that they understand and agree to the safety-related requirements they are expected to fulfill,” he said. “The flight attendants also use this briefing to determine that the passengers meet the safety requirements and, again, are not impaired by alcohol or other substances before take-off.”

The boarding process is also a time when flight attendants identify travelers who might need additional assistance during the flight due to disability, for example.

“It helps crew members build situational awareness about who is on the aircraft. In an emergency, knowing which passengers may need extra help, or which travelers appear particularly capable of assisting others, can be valuable,” Datta said, adding, ”The few seconds spent greeting each passenger can help crew members identify potential issues before the aircraft ever leaves the gate.”

For all the safety assessment happening at that door, flight attendants also genuinely enjoy greeting travellers.

“While we’re scanning the cabin, we’re also looking for passengers we genuinely want to connect with,” Robert said. “Contrary to popular belief, many flight attendants actually like people. We enjoy chatting with our guests, hearing their stories and helping make their flight memorable.”

“That brief greeting is often our first opportunity to build rapport with passengers,” Adams said. “A friendly interaction at the door helps set a positive tone for the entire flight while allowing us to quietly assess the cabin before departure.”

Robert noted that he’s usually spotted his “in-flight friends” during boarding.

“And if you want a little galley gossip, flight attendants also tend to spot the ‘BOB’ during boarding,” Robert added. “That’s the ‘Best On Board,’ otherwise known as the passenger who catches our eye. It’s usually harmless fun, and you’ll often hear a little discussion in the galley about who we think is the most attractive passenger in our section.”

Now and then, these kinds of connections turn into something more as well.

“I know several cabin crew who met their husbands or wives because of a spark that started during boarding,” Robert said. “So you never know. That warm smile and eye contact at the aircraft door could one day lead to you enjoying one of the best perks of dating a flight attendant. As we often joke in the industry, ‘Marry me and fly for free.’”

Marriage prospects aside, there’s something to be said for simply being kind to the crew.

“If you can just greet the flight attendant, say hi or ask how their day is going, believe it or not, those little comments go a long way with flight crews because they have very long days,” Bubb said. “Before I go on a trip, I even buy a small stack of $5 Starbucks gift cards, and I give them out to the flight crew. It really brightens people’s days. One time I did that, the captain actually came back, shook my hand and gave me a handwritten note. It’s all about little things.”