Guidance on detection of audio anomalies on CVR recordings
Flight recorders, better known to the general public as "black boxes", are installed on transport aeroplanes and, during the flight, record data destined to be used after any accident or incident.
Transport aeroplanes are equipped with two flight recorders:
- The CVR (Cockpit Voice Recorder) records audio data: exchanges between the pilots and with controllers as well as the acoustic environment in the cockpit (conversations, noises, aural warnings).
- The FDR (Flight Data Recorder) records the values of the aeroplane parameters (speed, altitude, engine power, connection and disconnection of automatic systems, position of flight control surfaces, flight controls, etc). According to the age and the type aircraft the number of parameters recorded varies from a few dozen to several thousand.
The total length of the recording is 25h for the FDR and 2h for the CVR. During the flight the recorded data continuously replaces the older data, thus ensuring that the FDR permanently has the values of the previous 25 hours of flight of the aircraft and, equally, on the CVR, the audio data for the previous 2 hours of flight.
Flight recorders are designed to protect the recordings that they contain in case of an accident. For this they are certified to resist numerous constraints: acceleration of up to 3. 400 g (3400 times the intensity of terrestrial gravity), a temperature of up to 1,100 °C for one hour, a month submerged in water six miles deep, etc. These characteristics allow investigators to successfully access data from the flight recorders after an accident in almost 100% of cases.
In civil aviation, there are two flight recorder technologies today: magnetic tape and solid state memory. However the solid state recorders are progressively replacing those with magnetic technology, which should be completely replaced in the coming years.
A beacon attached to each of the protected cases is intended to allow the recorders to be found after an accident at sea. These beacons transmit a signal that is broadcast under water as an impulsion every second. This signal can be picked up by appropriate detectors. The transmission time, which was 30 days, has now been increased to 90 days.
Readout of Flight Recorder Content
The first step in the download of flight recorder data involves reading out all of the data that they contain and backing up this data. This data may then be decoded and analyzed within the framework of the safety investigation.
In the case of recorders with electronic cards as a recording medium (the most common type today) the memory card(s) are extracted from the protected casing (memory module) and are then subject to a number of operations in the BEA lab that make it possible to check the integrity of the memories containing the recorded data. The following step generally consists of connecting the memory card(s) to a recorder chassis of the same type, whose write function has been inhibited, and to use the latter as a reader.
In the case of a magnetic recorder, the magnetic tape is extracted, cleaned then read out with a dedicated reader. The BEA has developed specific readout tools that make it possible to deal with situations in which the condition of the tape is degraded (immersion in water, exposure to fire, deterioration over time specific to magnetic media).
In the case of an incident, the recorders not being damaged, these can be read out directly by connecting the recorder to an appropriate tool. The readout tools are generally those developed by recorder manufacturers that have been acquired by the BEA.
Once the data is read out and saved, it can be analyzed by investigators.
The data from the FDR must first be converted into aeroplane parameters. In fact the file extracted from the recorder is a binary file known as a « raw data file »; the decoding of this file is performed using a decoding document specific to each aeroplane. The values of the parameters and their evolution over time can then be represented in the form of graphs or table, and can then be used to calculate the behaviour of the aeroplane.
The data from the CVR enables investigators to listen and transcribe the audio recordings. It is also possible, based on these audio recordings, to undertake specific spectral analyses that make it possible to interpret warnings and noises present in the cockpit.
Analysis of the recorders and readout and analysis of the data are carried out in the context of investigations conducted by the BEA or, for events that occur abroad, in the context of its participation in an investigation involving a French-operated or manufactured aircraft. Countries that have no flight recorder readout capacity often seek technical assistance from the BEA to undertake work on flight recorders.