Ariane 5 Exploded 37 Seconds After Launch. A 16-Bit Variable From 1988 Cost $370 Million
Video from the channel Macro Lens about Ariane 5, linked on its Above Ten Thousand entry. Duration: 6:10. Language: English. Video type: Documentary.
What it's about
An explainer piece reconstructing the loss of the first Ariane 5, on 4 June 1996. The rocket was destroyed about 37 seconds after liftoff with four satellites on board. The video explains how a software fault inherited from the Ariane 4 caused the accident.
Historical context
The new launcher reused the Ariane 4's inertial reference system (SRI). Its gyroscopes and accelerometers feed the flight computer the rocket's position and orientation. An alignment function, designed to let the Ariane 4 recover quickly after a countdown hold, remained active in flight even though it served no purpose on the Ariane 5. After the accident, an inquiry board chaired by Jacques-Louis Lions was set up and delivered its report in July.
Key facts
The horizontal bias variable, a 64-bit floating-point value, was converted into a 16-bit signed integer, whose maximum is 32,767. Ariane 5's faster trajectory made it overflow at around 36.7 seconds. The Ada exception had no handler, and the unit shut down. The backup unit, running the same code, had failed 72 milliseconds earlier. The flight computer interpreted the diagnostic report as flight data, and the nozzles went to maximum deflection. The rocket broke apart, and the self-destruct system completed its destruction.
Of seven variables exposed to the overflow, only four were protected, to save on computing load. The SRI was also never tested against Ariane 5's actual trajectory. The report called for four things: revalidating reused software, handling every exception, testing the complete system under real conditions, and preventing a failed unit from transmitting erroneous data.