Extended "Stopover"

If I understand your question correctly, it sounds like you plan to engage in "hidden city ticketing." Most airlines will cancel all subsequent flights if you miss a leg of an itinerary. So, if you book a round trip PDX - FRA ticket with stopover in AMS on the outgoing itinerary and then skip the AMS-FRA leg, you will likely have your FRA-PDX itinerary cancelled. You will not, however, have any trouble getting through immigration at AMS and exiting the airport.

As Dave stated, it is the airline you need to worry about. They will cancel your remaining flights (e.g., your flight home to PDX) if you miss one of the legs. You’d be violating the contract of carriage.

Thank you for the info! It sounded too good to be true...

Tried a random dates in March PDX-AMS, FRA-PDX (on expedia) and got $845 rt. one stop each way.

Did not try to find prices directly with the airlines. ( not knowing your dates makes it difficult to produce meaningful results)

Seems reasonable to me, but that is a subjective assessment.

Happy hunting!

As Dave stated, it is the airline you need to worry about. They will cancel your remaining flights (e.g., your flight home to PDX) if you miss one of the legs. You’d be violating the contract of carriage.

Indeed. The airlines take a very dim view of this. They do retaliate against those who make a habit of doing it (most commonly, confiscating frequent flyer miles, in extreme cases, banning passengers). If you do this once (or a few times spread over time), the risk of adverse action is limited (they surely would cancel any subsequent legs that you have booked once you are a no-show for a leg). Do this often, and they'll do their best to punish you.

The additional expense of multi-city (open jaw) flights will probably be just a fraction of buying last-minute one-way flights back to Portland. Keep checking -- when you see deals into either of your destinations, take a look at multi-city airfares. Scott's Cheap Flights has a daily free email listing of buy-them-now bargains listing airlines involved.

Hi,
yes if you will not board the connecting flight, the remaining ticket will be cancelled.
I just had a quick look. In this case its the best to book KLM cause AMS is their base. If you check their page and check the prices, select MULTI-CITY. This way you can book a ticket with different departure/arrival.
So the price I saw was around $850. This is going to Frankfurt and going back from Amsterdam. Then you could do the trip frankfurt to ams with the train.
another option is trying to book a KLM ticket with a few days stopover in Amsterdam. I did this once and it worked out perfectly.
Good luck

In case you don't know about it, there is a schedule website called Matrix ITA Software. It is the old travel agent flight software now available to all. It allows you to do a variety of options, like cheapest flight in a set month, or from/ to close by airports. From there, you buy on the airline website. It is owned by Google now and similar in method to Google Flights, but I like Matrix better. Maybe that will build something you didn't think of.

Thanks everyone for chiming in and even attempting itineraries for me! So glad I asked the question. We are now going to do an "open-jaw" trip. Happy traveling!

Or you could try flying RT to AMS, and check domestic flights from Frankfurt to AMS. Flight can be very inexpensive (~$30 OW) in Europe. Perhaps the RT to AMS is cheaper, and then the domestic OW is cheap...

Yes, do not "jump". In 2015, we were going to Zagreb, on Polish Air. Our plane was late. We were taken off the plane in Warsaw, put into a lounge, and then bussed quickly to the connecting flight. We didn't check in. On our return trip, we found our flight had been cancelled. Fortunately, the Lufthansa desk attendant was able to recover it from the bowels of the system, but it took almost 2 hours. And the entire issue was due to a problem caused by Polish Air, not by us.

Yes, search for PDX-AMS and FRA-PDX. What dates? if you're going on spring break, the prices are probably higher. The Delta direct flight to Amsterdam is a good one - they are an Air France/KLM partner. There are no direct return flights from Frankfurt until later in the spring (Condor has direct service late May-Sept). A quick search of random March dates shows a 6:55a.m. departure from Frankfurt that connects in Amsterdam (1.5 hour layover). That's doable. The unfortunate thing about getting back to PDX is you typically have to leave Europe at an unholy hour. Good luck!

The issue isn't with immigration.

We did similar a few years back. Originally, a change in Amsterdam and on to Cologne. It turned out leasing a car in Amsterdam was the better option so left the flight in Amsterdam.

As far as I can tell immigration didn't care at all (that may have changed but I doubt it) which port of entry we used to enter the Schengen zone.

I had booked the flight to Cologne as a second ticket with Lufthanse, rather than all on one KLM ticket. The flight home, from Rome, wasn't affected.

The biggest danger in leaving your ticketed flight is airlines are more picky now. They're likely to cancel everything else on your ticket when you're a no-show for a leg. There was a time when people used this as a technique to find cheaper flights so the airlines countered with a fairly brutal cancellation policy.

Immigration (passport control) will take place when you enter the Schengen Area - that is at Amsterdam.
Provided you are "hand luggage only" there is nothing at the airport to stop you doing this, it is the airline.
1) If you have any bags the will be labelled through to Frankfurt, you will not be able to retrieve them in Amsterdam.
2) The airline will cancel the rest of your ticket for "breach of terms", including any return flight back to Portland.

Note customs and immigration are two different things. By searching for "customs" you were looking in the wrong place.