score:6
It is unlawful to depart the US without a valid US passport, but this is almost never enforced, and there is no penalty. I have never tried leaving without my US passport, but I usually don't show it to anyone when I leave, and I've never been prevented from leaving the US or asked to show my US passport or to prove my immigration status. However, I suspect that some US airlines might do so; I usually fly on European carriers.
There is a small chance that you might be spot-checked by CBP before leaving, and they might stop you from leaving. The risk is small, but if you want to eliminate it altogether, you can make an expedited passport application. It takes just a few days.
Upvote:-4
Also (as an addition to phoog's answer), as a German citizen, when returning to Germany, you are required to use your German document for entry.
The United Kingdom will probably prefer you that you use the passport with which you have some form of leave to enter.
An Airline will probably require the passport that allows entry in the country of destination.
Wichtiger Hinweis für Doppelstaater
Bei Doppelstaatern (Erwachsene und Kinder) ist zu beachten, dass nach internationaler Übung
- die Einreise nach und die Ausreise aus Deutschland nur mit deutschem Reisepass oder Passersatz, z.B. Personalausweis
- die Einreise in und die Ausreise aus dem anderen Staat (dessen Staatsangehörigkeit die Betreffenden ebenfalls besitzen) nur mit den nationalen Dokumenten des anderen Staates erfolgen sollte.
Important note for dual nationals
In the case of dual states (adults and children) it should be noted that according to international practice
- Entry to and exit from Germany only with a German passport or passport substitute, e.g. Identity card
- Entry into and exit from the other state (whose citizenship they also have) should only take place with the national documents of the other state.
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