A murder happened in an airplane flying from country A to country B when crossing above country C.
Who will have jurisdiction?
Bonus question: What if countries A, B and C are members of the European Union?
International aviation law is tricky. One effort to set some standards down was the Tokyo Convention, also known as the Convention on Offences and Certain Other Acts Committed On Board Aircraft. Here are some excerpts:
- The State of registration of the aircraft is competent to exercise jurisdiction over offences and acts committed on board.
. . .
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.
The commander of the aircraft is also given some powers to restrain the perpetrator or any other person that poses a danger to the aircraft and/or its occupants. S/he is given some other options, such as the choice of where to deliver the offender (though this by no means implies that the person will be prosecuted by that State).
The Convention (and related documents) was ratified by numerous nations.
As for the bonus question, there doesn't appear to be any special EU law regarding this. The EASA does not cover these sort of offenses, though it would be the closest to a Europe-wide regulatory body that would be relevant here.
Whether a state has jurisdiction over a crime or not is determined exclusively by the laws of that state, including any treaties the state has signed. It is entirely possible for multiple countries to have jurisdiction over a crime; this is likely to be just such a situation.
As a practical matter, if there is a murder on an airplane the plane will be making an emergency landing at the nearest airport (or nearest friendly airport, if, say, the pilot finds himself over a war zone) and will be met by local police (because they're the police who are actually around), who will take the suspect into custody. From there, the legal procedures kick in -- either the suspect will be locally charged, he will be set free, or he will be extradited.
The Tokyo Convention does not actually preclude country C from bringing charges. It bans interfering with the aircraft in flight; the country can't automatically order the plane to the ground, although they can do so if they think it's a security risk (e.g. terrorist risk). However, article 3 paragraph 3 explicitly says that the convention's grant of jurisdiction to the country of registration is not exclusive; it specifically does not forbid national legislation from establishing jurisdiction over foreign-registered aircraft.
If country C is the US, they have authority to charge the suspect with murder: 49 USC 46501(2)(C) defines the "special aircraft jurisdiction of the United States" to include aircraft in flight in the United States, and 49 USC 46506(1) says that violation of several federal criminal laws (including 18 USC 1111, the federal murder law) in that jurisdiction is punishable as a federal crime.
If country C is the UK, the Crown Prosecution Service states that UK law gives jurisdiction over offenses committed on aircraft in flight over the UK. Blackstone's Criminal Practice 2012 states that this is a common-law principle. No mention is made of any EU rules on the CPS page, although that doesn't necessarily mean there aren't any.
If the aircraft in fact lands in country B, and country B is the US or UK, national law gives that country's courts jurisdiction. If the act endangers the plane or is part of a hijacking, both the country of registration and the country where the plane ultimately lands with the person on board have jurisdiction, regardless of where the incident happened (even if it was over international waters). Both of those treaties do not rule out any other country having jurisdiction either.