There are two kinds of black boxes in modern planes: the FDR (Flight Data Recorder) which record instructions sent to the electronic systems on an aircraft and the CVR (Cockpit Voice Recorder) which records all audio in the flight deck, i.e. ambient noises and pilot discussions.
The movie focuses the whole time on the CVR.
The name of the protagonist, Mathieu Vasseur, is the same as the name of the protagonist of director Yann Gozlan's previous movie (Un hombre ideal (2015), and both characters are played by Pierre Niney.
Modern planes also use an independent system to send messages to the company hat's Aircraft Communications Addressing and Reporting System (ACARS) is a digital datalink system for transmission of short messages between aircraft and ground stations via airband radio or satellite.