N° 33–1996: Ariane 501 - Presentation of Inquiry Board report

Mr Jean-Marie Luton, ESA Director General, and Mr Alain Bensoussan, CNES Chairman, immediately set up an independent Inquiry Board (see ESA-CNES Press Release of 10 June 1996), which has now submitted its report.

The report begins by presenting the causes of the failure, analysis of the flight data having indicated:

• nominal behaviour of the launcher up to H0 + 36 seconds;
• simultaneous failure of the two inertial reference systems;
• swivelling into the extreme position of the nozzles of the two solid boosters and, slightly later, of the Vulcain engine, causing the launcher to veer abruptly;
• self-destruction of the launcher correctly triggered by rupture of the electrical links between the solid boosters and the core stage.

A chain of events, their inter-relations and causes have been established, starting with the destruction of the launcher and tracing back in time towards the primary cause. These provide the technical explanations for the failure of the 501 flight, which lay in the flight control and guidance system. A detailed account is given in the report, which concludes:

" The failure of Ariane 501 was caused by the complete loss of guidance and attitude information 37 seconds after start of the main engine ignition sequence (30 seconds after lift-off). This loss of information was due to specification and design errors in the software of the inertial reference system. The extensive reviews and tests carried out during the Ariane 5 development programme did not include adequate analysis and testing of the inertial reference system or of the complete flight control system, which could have detected the potential failure."

Despite the series of tests and reviews carried out under the programme, in the course of which thousands of corrections were made, shortcomings in the system approach concerning the software resulted in failure to detect the fault. It is stressed that alignement function of the inertial reference system, which served a purpose only before lift-off (but remained operative afterwards), was not taken into account in the simulations and that the equipment and system tests were not sufficiently representative.

Without implicating the system architecture, the report makes a series of recommendations for ensuring that the launcher's software operates correctly. The Ariane 5 programme will be taking action in line with all these recommendations, as follows:

• correction of the problem in the SRI (inertial reference system) that led to the accident;
• reexamination of all software embedded in equipment;
• improvement of the representativeness (vis-a-vis the launcher) of the qualification testing environment;
• introduction of overlaps and deliberate redundancy between successive tests:
  

  -at equipment level,
  

  -at stage level,
  

  -at system level;
  

  • improvement and systematisation of the two-way flow of information:

    -up from equipment to system: nominal and failure-mode behaviour;
    

    -down from system to equipment: use of equipment items in flight.

  More specifically, the following corrective measures will be applied:

  • to the inertial reference system:
    

    -switch-off or inhibition of the alignment function after liftoff,
    

    -analysis/modification of processing, particularly on detection of a fault (no processor shutdown),
    

    -testing to check the coverage of the SRI flight domain;
    

    • to the system qualification environment:

      -general improvement of representativeness through systematic use of real equipment and components wherever possible,
      

      -simulation of real trajectories on SRI electronics.
      

      • In addition, the following general measures will be taken:
        

        -critical reappraisal of all software (flight program and embedded software),
        

        -review of mechanisms for managing double failures,
        

        -improvement of facilities for acquisition and retrieval of telemetry data,
        

        -improvement of overall coordination relating to software.

      The ESA Director General and CNES Chairman will be making a joint presentation of the plan of action put into effect and its programmatic consequences at a press conference in September.

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