What determines legally the "departure" from the US when you leave by plane?

Question

Related to this question: https://travel.stackexchange.com/questions/171475/if-an-international-flight-is-turned-around-mid-air-do-the-passengers-have-to-p

Entry to the US is quite well defined: once you are through the doors of the customs hall, you are in the country. However, this is less clear for departure. Most countries have exit controls, and once you are through it, you have officially left the country. The US doesn't have exit controls, so the international departure area is no different from the domestic departure area.

The question: When you leave the US by plane, what point in time or event determines when you transition from being legally inside the US to being legally outside the US?

Could be

1. Entering the secure area (unlikely since it's shared with domestic passengers)
2. Boarding the plane
3. Plane doors closed
4. Plane pushing off the gate (which is the official departure time)
5. Plane taking off
6. Plane leaving US air space
7. Plane landing in a non US airport
8. Getting off the plane
9. Passing immigration and being admitted to new country
10. Something else ?

Answer 1

The answer would depend what you mean by "legally" in/outside. The U.S., like all other countries, is fully sovereign in executing its laws over its territories (land, air, space, aircrafts, ships etc.) regardless of the legal fiction of passing through the customs and border controls, subject to its laws and international treaties it has agreed to and ratified.

Even in a country with exit control or separate areas for international departure, its law still fully applies after exit control or at the departure area, but certain legal provisions may not apply in a certain area for certain purposes (customs, taxation, VAT, etc.), only because that country's law says so.

For example, with respect to criminal offences, the Tokyo Convention limits (and extends) the exercise of criminal jurisdiction over an aircraft in flight:

Art. 1, para. 3 For the purposes of this Convention, an aircraft is considered to be in flight from the moment when power is applied for the purpose of take-off until the moment when the landing run ends.

Art. 4 A Contracting State which is not the State of registration may not interfere with an aircraft in flight in order to exercise its criminal jurisdiction over an offence committed on board except in the following cases:

• (a) the offence has effect on the territory of such State;
• (b) the offence has been committed by or against a national or permanent resident of such State;
• (c) the offence is against the security of such State;
• (d) the offence consists of a breach of any rules or regulations relating to the flight or manoeuvre of aircraft in force in such State;
• (e) the exercise of jurisdiction is necessary to ensure the observance of any obligation of such State under a multilateral international agreement.

however, Art. 4 is a limitation of the manner of exercising a State's criminal jurisdiction over foreign vessels (you cannot interfere with operation of international flights except in limited circumstances); it does not abrogate the claimed jurisdiction itself, nor does it limit the State's authority of aircrafts registered under its laws:

Art. 3, para. 3. This Convention does not exclude any criminal jurisdiction exercised in accordance with national law.

The United States (the federal government) claims a special aircraft jurisdiction as well as a special maritime and territorial jurisdiction of the United States in regard to various offences (e.g. offences against aircrafts, murder, certain sex offences etc.). For the purpose of the special aircraft jurisdiction which applies to certain foreign aircrafts (that have a scheduled destination or departure point in the U.S.),

An aircraft is "in flight" from the moment when all external doors are closed following embarkation until the moment when one such door is opened for disembarkation, or in the case of a forced landing, until competent authorities take responsibility for the aircraft. 49 U.S.C. § 46501(1).

So in cases where this jurisdiction could apply (or be considered to be as if it would apply), you do not leave the jurisdiction until your aircraft is ready for disembarkment. Of course, this line of argument could be a slippery slope that leads to the conclusion that you never "leave" (even if you never "entered") the U.S. (or any given country) since there are also offences that the U.S. (or another country) claims worldwide jurisdiction.

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For cases related to your linked question

CBP publishes a guide on diverted flights, which states

Passengers and crew on flights departing the United States for foreign destinations that make an unscheduled stop in the United States due to any emergency, including but not limited to fuelrelated, weather-related, mechanical-related, illness on board, or other emergency reason, generally are not be processed by CBP, but treated as a domestic arrival.

Answer 2

There is a definition of "depart from the United States" at 22 CFR 46.1(h) which applies to Part 46:

depart by land, water, or air (1) from the United States for any foreign place, or (2) from one geographical part of the United States for a separate geographical part of the United States: Provided, That a trip or journey upon a public ferry, passenger vessel sailing coastwise on a fixed schedule, excursion vessel, or aircraft, having both termini in the continental United States or in any one of the other geographical parts of the United States and not touching any territory or waters under the jurisdiction or control of a foreign power, shall not be deemed a departure from the United States.

The definition is not framed in terms of "how far you get", so when you depart (leave), you have departed. But this is for the part of the federal regulations on Control of Aliens Departing from the US, which boils down to saying when a departure-control officer can prevent an alien from departing from the United Stated.

There no no law that pertains to a "transition" from being in the US to not being in the US, instead the applicable question might be "when are you in the US?". That's not the same question as "When must you go through Customs and Border Control formalities"?". There is some complexity to the legal question of whether you are "in the United States", but for example when you fly over the US-Canada border, you are then in Canada and not the US. At some point when you fly over the ocean, you are no longer above US territorial waters so you are out of the US.

One may or may not have to go through customs formalities when your plane lands: it is a separate question under what conditions one must "enter" the US.

from being legally inside the US to being legally outside the US