The Failure of Ariane 5

Ariane 5 was scheduled for launch June 4th 1996. Everything went smooth until 37 seconds after launch when Ariane 5 suddenly, during its maiden voyage, tilted over and exploded approximately 3700m above sea level.

The event:
In Kourou, French Guiana, the weather was acceptable in the morning, and presented no obstacle for the shuttle transport to the launch pad. All criterias were fine, apart from the visibility, which made the launch committee delay the launch by 59 minutes.

59 seconds past 09:33 (local time) the Vulcain engine and the two solid boosters were ignited, and the shuttle lifted off. Everything went smoothly until +37 seconds when the shuttle veered off its flight path, and turned sideways. 39 seconds after the launch, the angle of attack was more than 20 degrees. The aerodynamical pressure on the hull was too great, so the shuttle broke up and exploded.

The Failure:
  • The tilt of the shuttle was caused by nozzle deflections of the solid boosters and the Vulcain main engine.

  • These nozzle deflections were commanded by the shuttle's On-Board Computer (OBC), on the basis of information supplied by the active Inertial Reference System (SRI2*). This information was not proper flight data, but a diagnostic bit pattern from the SRI 2, which was interpreted by the OBC as flight data.

    *SRI stands for Système de Référence Inertielle

  • SRI2 sent the diagnostic pattern after a failure caused by a software exception.

  • OBC was unable to switch on SRI1 (the backup), because SRI1 had encountered the same error, one data cycle (72 milliseconds) earlier.

  • The software exceptions in both SRI1 and SRI2 were caused by a data conversion from 64-bit floating point to 16-bit signed integer value. The floating point number that was being converted was greater than what could be represented as a 16-bit integer. The result was an operand error.

  • This error occurred in a part of the software that serves no purpose after lift-off (for Ariane 5, this was +3 seconds after ignition) but the software still remains operative for approx. 40 seconds. This time is based on the requirements of Ariane 4, but Ariane 5 does not require this.

  • The error was caused by an unexpected high result of an internal alignment function called 'Horizontal Bias' (BH). This value is calculated as an indicator for alignment precision over time. The reason for the high BH-result was that the trajectory of Ariane 5 differs from Ariane 4 (which the function was programmed for), which results in a much higher horizontal velocity.
(The telemetry data showed many other anomalies, but none of these are considered significant to the accident.)

The Result:
The self-destruct sequence was initiated, which made the shuttle explode in an height of aprox 3700 metres, and aprox. 1km from the launch pad. The debris was scattered over an area of 5 x 2.5 km2. Amongst the debris the two SRIs were found, and used for analysis.

It took the European Space Agency 10 years and $7 billion to produce Ariane 5. In addition to the building costs, the shuttle contained 4 research-satellites (The satellites were not insured).

The shuttle was unmanned, so no lives were lost, and since the launch area wasn't populated, none were injured by debris.

  • CNN
  • The report made by the Ariane 5 Inquiry Board
  • Various documents regarding the accident