Design Management Group

Ariane 5 flight 501

"On 4 June 1996, the maiden flight of the Ariane 5 launcher ended in a failure. Only about 40 seconds after initiation of the flight sequence, at an altitude of about 3700 m, the launcher veered off its flight path, broke up and exploded."

So begins the official report into the failure of Ariane 5 flight 501. The report makes fascinating reading, with a step-by-step explanation of how an obscure software bug ended up crashing the program and with it, the whole mission.

The error arose because a numeric overflow in the Inertial Reference System program, during conversion of a 64-bit to a 16-bit number. The piece of code had been originally written for the Ariane 4 and was being reused in the Ariane 5. The original designers had chosen not to check for this error (to save on computational overhead) as it could not happen in practice - on the Ariane 4 that is. Unfortunately, the Ariane 5 take-off trajectory was different and the bug caused both Inertial Reference Systems to crash, causing them to output a test pattern. This was interpreted by the On Board Computer as real flight data, leading to extreme deflections of the booster engine nozzles.

The report states " The launcher started to disintegrate at about H0 + 39 seconds because of high aerodynamic loads due to an angle of attack of more than 20 degrees that led to separation of the boosters from the main stage, in turn triggering the self-destruct system of the launcher".

Ironically, the errant code was responsible for the initial alignment of the Inertial Reference System, and had absolutely no purpose once the launcher had lifted off. For historic reasons however, the code was kept running for a short while to facilitate restart on the Ariane 4 launch.

Further information

• ESA press release No. 33-1996: Ariane 501 - Presentation of Inquiry Board report
• Jean-Marc Jézéquel and Bertrand Meyer (1997) Design by Contract: The Lessons of Ariane (examines the software engineering issues involved with code reuse)

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