Customs Pre-Clearance: How Does it Work?

This may be a stupid question, but when you do customs pre-clearance before flying into the USA, what happens next? Do you go through it with all of your luggage before boarding? Does that mean you can't buy duty free on board?

Regarding BA's all CW flight (A318) from LCY-JFK with stop in Shannon, does that mean everyone has to get off the plane with all their stuff to go through customs?

Customs checks your bags and then you go through regular security. I never buy duty-free so I never pay attention to that part. That is my experience for Canada and the Caribbean.

So what is stopping someone from not going through customs (say OLCI), and then still boarding?

Physical barriers.

With Pre-Clearance:

1. Check-in with your airline, either via OLCI or at a kiosk at the airport or in-person. Receive your boarding pass.
2. Fill out US Customs Declaration Form.
3. Proceed with your boarding pass, and all your bags for the journey, to the Pre-Clearance Area. Depending on the airport (for example, YVR), drop your bags off at the baggage carousel before proceeding into pre-clearance area.
4. Proceed through USA CBP formalities - immigration and customs.
5. Proceed through Pre-board screening.
6. Purchase duty-free, depending on airport. YVR/YYZ has duty-free shops AFTER you clear CBP formalities and security.
6. Board your flight to the USA, with duty free goods if any in carry-on.
7. Land in domestic terminal area in USA airport. If transiting within same terminal, no further security checks are usually required.

And CBP officials with weapons.

My experience in SNN is very similar to the previous post, although I think the bags went straight from check in to the plane, and I was shown a picture of the bags by the CBP official to confirm they were mine.

My understanding helped with the video from the DUB thread.
Regarding SNN, for the BA all CW flight, do all pax disembark upon landing in SNN?

Yes. They line up with all of their cabin carryon items.

I think you'll find them to be un/disarmed in Canada.

my experiance in DUB was similar.


Checked the bags and at the combined CBP showed me a picture of my bag and asked a couple of questions. I guess that if they are not happy with your response they can pull the bag and search it. As for duty free, as most things are cheaper in the US I don't think they are too bothered.

The CBP officer was unarmed and friendly.

Why can't they show a picture of one's bag for regular customs clearance in the US? This could make it where people do not need to re-clear security upon connecting.

Farce of habbit, probably.

When would the picture be taken? They're probably not set up in airports to do that, and it would probably be hard to set up at each airport for arriving bags to each be photographed.

As for re-clearing security, that's separate from customs inspecting bags. Even if this was done, I highly doubt the TSA would give up the opportunity to grope more people.

Yes - including any carry on bags, and the plane is checked while passengers are dealing with pre-clearance to make sure nothing has been left on board.
You are shown a photo of your checked bag(s), asked to confirm it is yours, and then the usual immigration formalities - resulting in a nice SNN stamp.

Incidentally, last time I was on the LCY-SNN-JFK flight, when boarding back after pre-clearance I asked the first officer what would happen if someone had immigration 'issues' and was told the plane wouldn't wait for them more than an extra 20 mins or so, leaving the passenger in a potentially awkward situation - especially if on the later flight