Ariane 5: The Integer Overflow That Destroyed a Rocket (Ep. 726)
Video from the channel DatabaseHistory about Ariane 5, linked on its Above Ten Thousand entry. Duration: 6:08. Language: English. Video type: Documentary.
What it's about
An explainer episode, with recreations and graphics, about the failure of Ariane 5's first flight on 4 June 1996. The cause was a numeric overflow in the guidance software.
Historical context
The inertial reference system program was carried over directly from the Ariane 4, on the logic that code with a flight history is more reliable than new code. But its assumptions no longer held for a more powerful launcher. Ariane 5 followed a steeper ascent path, and the measured horizontal velocity far exceeded the values of the earlier model.
Key facts
The failure occurred when converting a 64-bit floating-point number into a 16-bit signed integer, which cannot hold values above 32,767. The protection against this overflow had been removed for this variable on the Ariane 4 to save processing cycles. With no error handling, the primary unit shut down. The backup unit, running the same software, failed the same way milliseconds later. With no valid data, the flight computer commanded maximum nozzle deflections to correct a nonexistent rotation, and the aerodynamic loads broke up the vehicle. The episode concludes that any architecture change in a real-time system requires revalidating the software's limits and ranges.