For UK/USA bound flights, are details entered during online check-in, such as date of birth or passport number ever checked by airlines/immigration?

Is passenger information (as entered during online check-in) other than name and visa ever validated by airlines?

For US/UK flights the information you entered during online check-in seem to be always thrown away. What seem to happen in practice nowadays is that your passport is always scanned/swiped during the “document check” (which happens either during check-in, or at the gate) and the information you have entered during online checkin is overwritten. The gate agent explained it was faster for them to do it this way than cross-check the data and correct mistakes. You can watch it yourself when they do it.

Is that information ever validated by immigration?

It depends on how you define “validated” (I assume we’re mostly concerned about the practical aspect, i.e. “what happens if the airline transmitted the incorrect information I entered, and it doesn’t match my documents?)”. However some of us routinely travel with two passports, and usually the information from only one of them is transmitted. For example, I routinely fly to Istanbul showing only the US passport to the airline, but enter Turkey using a different passport (which the airline knows nothing about) – never had any issues.

Even if they did, you can’t be blamed for this because you did not transmit this information to immigration yourself. More, the law puts this responsibility (and liability) on airlines, not on you. If they let the inaccurate information through, it is their fault – and their fine.

Update: last month I have requested (FOIA) the CBP arrival records for my Mom, and they had inaccuracies. In one case she missed the flight and took a later flight 7 days late, but CBP had record of her arrival in USA on the date of the missed flight – and this arrival was NOT recorded in CBP database. Thus it is pretty evident that this information is not checked at least by US immigration.

Will you be denied on-board because you’ve made a mistake during
check-in?

Every time I get a new passport, when I do online checkin, I typically make mistakes in passport number and the expiration day. It has never been a problem, and I was never denied boarding. This included multiple flights to USA/UK.

Final: since the data you entered during online checkin is always overwritten during the document check, it is impossible to be denied boarding for the mistakes/typos made during online checkin*. Even if for some reason the airline decides to verify it, they would simply enter the correct the data and let you through. As mentioned in previous version of this answer, I have done this in 2016, and I have done it a lot in 2017 (being pissed off that airline requires you to enter a shitload of data which they throw away anyway).

(*) of course you can still be denied boarding for other reasons, like bringing a wrong passport. But this was not the scope of the questions.

To clarity, validate is a very strong word with different meanings.

Also, these answer are limited exclusively to the information enter in the airline’s website at check-in or booking, not at the airport or by an Agent unless otherwise noted.

1. NO. The carrier has no means to validate any information you enter on line. They take you at you word at this stage. They may compare what you enter for a particular itinerary with your existing profile and flag any differences, but some variations are allowed, such as middle initial vs full middle name. But, the airline gets it validated because of #2.

2. YES. For flights to/from and within the US and UK, the airline does transmit what you enter online to the appropriate government agency (CBP, TSA, Home Office) ahead of time, sometimes days ahead of time (3 for CBP). YES, the agencies validate the information such as having a valid ESTA for the Passport. This is done to flag any errors or…mistakes…as far ahead of time as possible. If you make a mistake during booking and don’t notice it at online check-in, you may not be able to complete online check-in.

3. UNLIKELY BUT POSSIBLE. In most cases, it will depend on how much time there is to correct any mistakes and this doesn’t happen often because the vetting process begins well ahead of the flight. Important note: You can be denied boarding because the information is incorrect and cannot be corrected, not merely because you entered it wrong on the website.

For instance, if you mistype the passport number, entering one that matches to someone else or does not match a current ESTA, they may/should reject that forcing you to complete check-in at the airport by scanning your Passport. At this point, the scanned data overrides what you messed up online and the the carrier resends the APIS message. That alone will clear up most issues.

Sources:
APIS: Advance Passenger Information System – The airline must report accurately so they have ‘check’ on their own first.

CBP/TSA Consolidated User Guide – Probably a newer version, but this one’s directly linkable.

International travel document requirements – Specifies denied boarding and UA was the subject of the other thread.

Passports, visas and API

What do US customs see when they swipe passports?

“Do not board” message at the airport check-in? – Both the airline and CPB had to ‘check’ the passengers data for this to happen.