Flight 501, which took place on June 4, 1996, was the first test flight of the European Ariane 5 expendable launch system. It was not successful. Due to a malfunction in the control software the rocket veered off its flight path 37 seconds after launch. It was torn apart by high aerodynamic forces caused by excess change of attitude commanded by the onboard computer.

The breakup caused the loss of the payload: four Cluster mission spacecraft, resulting in a loss of more than US$370 million^[1].

It is one of the most famous computer bugs in history.

CAUSE OF THE FAILURE

The failure of the 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.