Who legally owns the Flight Data Recorder?
Who legally owns the Flight Data Recorder after an aircrash?


Whoever owned it before the crash!

I'd guess this might be the aircraft owner (e.g. an airline or a leasing company) or an aircraft operator (e.g. an airline).

Ownership is less important than the obligations imposed by the relevant regulatory authorities, and probably by international treaties and agreements, effective in the area where the FDR is located after an accident.

For relevant discussion see NTSB FDR Handbook


  And how is it decided who carries out the crash investigation?


There are a complicated set of rules agreed internationally. From memory I think it goes something like this:


The government appointed body in whose territory(airspace?) the accident occurred. Or
The government appointed body in whose country the aircraft was registered.


EMSA state


  5.1 The State of Occurrence shall institute an investigation into the circumstances of the accident and be 
  responsible for the conduct of the investigation, but it may delegate the whole or any part of the conducting of such 
  investigation to another State by mutual arrangement and consent. In any event the State of Occurrence shall use 
  every means to facilitate the investigation. 
  
  5.3 When the location of the accident or the serious incident cannot definitely be established as being in the 
  territory of any State, the State of Registry shall institute and conduct any necessary investigation of the accident or 
  serious incident. However, it may delegate the whole or any part of the investigation to another State by mutual arrangement and consent.


Example 1


  The UK Air Accidents Investigation Branch (AAIB) is part of the Department for Transport and is responsible for the investigation of civil aircraft accidents and serious incidents within the UK and its overseas territories.


Example 2


  The NTSB is an independent Federal agency charged by Congress with investigating every civil aviation accident in the U.S.




Normally an air accident investigation organisation takes the lead role but co-opts experts from the airline, the manufacturer and other air-accident organisations with an interest in the incident or with special expertise. Involvement of interested parties (airline, ATC, manufacturer) obviously has to be handled very carefully to avoid detracting from the independence of the investigation.

Most investigations focus almost exclusively on determining facts and recommending actions to prevent recurrences. They generally are not mainly concerned with assigning blame, that may be the focus of a separate legal inquiry.