Why would airlines not let a US citizen with an expired passport board a plane back to the US?

So this guy rocks up to the airline counter:

Guy: Here's my US passport.

Airline Employee: It's expired

Guy: Yeah, but it's a legit US passport! Honest!

AE: You got any other ID?

Guy: Nah, but look at my passport. It's a genuine US passport!

AE: So you want me to assume that because you are in possession of an expired US passport in your name, that you are still eligible to enter the US because you are a citizen. But you can't provide me with any other proof that would support that assumption?

Guy: Well, yeah. But look at how shiny and blue the passport is!

This reads like a Monty Python sketch.

While it's true that a US citizen can not be refused entry to the US, how do you expect $RandomAirline to validate that traveller is a current US citizen if all they can proffer up is an expired travel document?

Given that an airline can be fined for allowing passengers on a flight who are ineligible for admission at a destination, it behooves the airline to reject any passenger who cannot adequately prove who they are or that they have the correct travel documents.

Because the US government wants it this way.

Rule #1: The government requires all citizens to travel with a valid passport (or equivalent). See https://www.usa.gov/enter-us

Rule #2: CBP (Customs and Border Protection) cannot deny entry to a US citizen: see for example https://www.axios.com/us-citizens-rights-at-the-border-430039f3-724b-4a26-8ad8-976346c95431.html

So what happens if a citizen without passport (or expired passport) shows up at the border? Turns out rule #2 trumps rule #1 and so CPB has to admit them. The government doesn't like that and so they basically use the airlines to enforce rule #1 for them.

A CBP liaison office has told IATA to write in TIMATIC that a valid passport is required, and TIMATIC is what airlines go by. Whilst entry with an expired US passport can't be refused, the carrier would be heavily fined for transporting the passenger.

Equally, in South Africa, South Africans are legally supposed to have a valid passport; there, failure to comply results in a fine for the passenger as well as the airline.

For many other countries TIMATIC does say expired passports/other proof of citizenship is accepted, even though national law may stipulate you're supposed to have a valid one. In that case a carrier would not be fined, but the passenger might.

For instance, for Bosnia TIMATIC says valid or expired Bosnian passports or ID cards are accepted. However, if using an expired document, or a valid ID card other than if arriving on a direct flight from Belgrade, a Bosnian citizen could receive an administrative fine.

The problem with this scenario is the same one as that person will have at the immigration checkpoint: while the law says that US citizens cannot be denied entry, how does the immigration officer know that they are, in fact, a US citizen? Now, the immigration officer has four things that the gate agent does not have:

• Access to the full resources of the US government, so they could, for example, use a birth certificate, driver's license, social security number, etc. to verify that person's identity, and from verifying the identity go on to verify the citizenship status.
• Knowledge of US immigration and citizenship law.
• Time. The law says that US citizens cannot be denied entry, it doesn't say that they can't wait a little bit while their citizenship status is verified.
• Experience in recognizing, reading, interpreting, and verifying various documents that can be used to establish identity or citizenship, such as birth certificates.

The gate agent has none of that. They are not US government agents, they cannot take an arbitrary amount of time (the plane's not waiting, but the other passengers in the queue are!)

How would the person prove to the gate agent that they are, in fact, a US citizen, and do that in a way that is fast to verify, easy to verify, and possible to verify for a random gate agent of a random airline at a random airport in a random country? Where I fly from, some gate agents don't even work for a single airline, they are provided by the airport as a service to the airlines. They might handle an Asiana flight to Seoul right now, an hour later a South African flight to Johannesburg, and then a Delta flight to Atlanta.

Do you expect them to memorize the immigration law of every country on the planet, and be familiar with the citizenship laws of every country on the planet, and be able to recognize, read, interpret, and verify every possible document that can be used to prove every possible citizenship of every country on the planet? Heck, people in the US cannot even agree with each other whether Barack Obama was born in the US or Kenya!

It is simply impractical to expect a gate agent to be able to verify that someone is a US citizen who has no document showing that they are a US citizen.

It is just much easier, and much cheaper, to rely on an automated system that contains a set of simple, easy to follow, and easy to check rules, and that system says that in order for a US citizen to fly to the US, they need a valid passport. The US government could have put a different rule in the system, but they decided not to.

Note that there is another aspect that was not mentioned in the question nor in any of the answers so far that has nothing to do with immigration or citizenship: airlines are also, in at least some cases, required to verify the identity of their passengers. Typically, travel documents (and certainly passports) also pull double-duty as identity documents. So, even if this hypothetical passenger were allowed to board the plane for immigration reasons, they would still need to prove their identity using a valid, internationally recognized identity document … most likely a passport.

This is the TL;DR version :-)

US customs can determine if the traveller is the bona fide holder of the expired passport and whether they are a US citizen.

The airline can do neither with confidence.

If the airline allows the passenger to travel to the US and the claim is not valid the airline may be fined.

Not allowing the passenger to travel is the superior option for the airline.

Here's an analogy: Let us suppose, for a moment, that you are a Season Ticket holder for a local Sports Team. Today is a really important game, and the stadium is going to be packed.

Unfortunately, there was a mix-up with this year's Season Ticket, and it wasn't sent out to you properly. They'll get it to you as soon as they can, and for today they have a runner waiting to check your ID, and escort you to the VIP box as an apology for the situation. Nothing can stop you from getting in to watch the game!

Except, your taxi driver is a bit put out by all of the traffic - going all the way there, with zero chance of a return-fare (because everyone is staying to watch the game) isn't going to make them as much money as doing other trips around the city. However, as a fellow fan of Local Sports Team, your taxi driver will take you to the Stadium - if, and only if, you can show a valid Ticket for the game.

Which, you don't have.

A Season Ticket Holder [Citizen] will not be denied entry to the Stadium [Country]. But, first, they have to get there. And the Taxi Driver [Airline] is under no obligation to take you if they think it will be a waste of both your time.

Being a US citizen, and being able to prove that you're a US citizen, are two different things.

If you show up at the border then, as a citizen, they have to let you in — but only if they actually know that you are a citizen. Otherwise everybody in the world could just say "hey guys, I'm a citizen, let me in" and that would be that.

This is the whole point of identity documents.

There is no magic, secret handshake that reveals you to be a bona fide citizen in the absence of said documents.

So it should not be surprising that when your documents are out of date (effectively, you have no valid evidence of being a citizen), the airline will not take a chance on transporting you.

If you got to the US border with a valid, non-expired passport and CBP did not let you in, that would be a problem.

None of the other answers seem to address why a person with an expired US passport might not be a US citizen.

Here are two reasons:

1. After the passport expired, the individual did something which caused an automatic loss of US citizenship. Here's a list of possibilities. Generally these things don't immediately result in a loss of citizenship; there would be a court hearing.

2. After the passport expired, the individual intentionally lost his/her US citizenship by obtaining another citizenship, and then renouncing his/her US citizenship before a US consular officer.

The airline has no way of knowing whether an expired passport indicates a loss or renunciation of citizenship, or whether the person merely forgot to renew. The airline has to err on the side of caution by not allowing into the United States a person who might not legally be able to be there.

Airlines are not able to track what the actual requirements for crossing a border are. These requirements change, are sometimes really unclear, and sometimes quite ideosyncratic. Airlines have to rely on databases (timatic) that do this job for them. If the database says that your documentation is insufficent, and they have no evidence to the contrary, they will deny boarding (in order to avoid possible fines, as mentioned elsewhere).

I am familiar with a smallish country where it was - over years - possible (and legally possible) to get a visa at the border, even though timatic said otherwise. If you were flying with their national carrier, there was no problem relying on this, because there was always some representative from the airline in the vicinity of the gate who would confirm that these people could board. But if you were relying on getting a visa-on-arrival and were flying some other airline, possibly on a non-direct flight - well, bad luck for you.

This situation is rather similar to the one the OP describes in that entry was legally possible, but this was not reflected in the references available to the airlines.

P.S. as for why the CBP would tell timatic that people can only board with valid passports, I suspect there is some practical reason. E.g. higher incidence of people using someone else's expired passport. Or people misplacing their old passport, reporting it as stolen and then forgetting about it. When travelling in a foreign country, would you rather be told "Your cannot board that plane because your passport is expired. Please visit your embassy." or "You cannot board that plane because you are travelling on a passport that has been reported stolen. Please call a lawyer."?