The Ariane 5 Flight 501 incident occurred on June 4, 1996, during its maiden test flight. The rocket veered off its flight path 37 seconds after launch and was destroyed by its automated destruct system at 39 seconds due to high aerodynamic forces. This resulted in the loss of the Ariane 5 launcher and its payload, four Cluster mission spacecraft, valued at over US$370 million.

The malfunction stemmed from a software error in the control system. Code reused from the Ariane 4, specifically for re-alignment, was not necessary for the Ariane 5 but remained active. The Ariane 5’s higher acceleration caused a rapid build-up of horizontal velocity, which led to a data conversion from a 64-bit floating-point value to a 16-bit signed integer. This conversion resulted in an arithmetic overflow because the value exceeded the 16-bit integer’s capacity.

A critical design flaw was the disabling of the software handler for this specific error trap, done for efficiency. While other variable conversions were protected, this one was not. Furthermore, pre-flight tests for the re-alignment code were not performed under simulated Ariane 5 flight conditions, meaning the design error went undiscovered. The review process also failed to fully analyze the implications of this software operating during flight.

The incident highlighted the risks of complex computing systems and spurred research into the reliability of safety-critical systems. Post-flight simulations faithfully reproduced the chain of events. The failure also led to the first large-scale static code analysis by abstract interpretation for the Ariane code.