Background

On June 4th 1996, before most of you were born, an Ariane 5 Heavy Lift Vehicle exploded 37 seconds after launch. After launch, the rocket deviated from it's predicted flight trajectory to geostationary orbit due to high aerodynamic load. This was caused by the angle of attack exceeding 20 degrees, so the starting accelerators separated from its main stage, which triggered the missile Autodestruct System. The change of the angle of attack happened because of a malfunction in the nozzle rotation of the solid accelerators, which was caused by a command from an on-board computer based on the information from the active Navigation System (IRS 2). Some of the diagnostic data from the computer was mistaken as flight data. After further investigation, conversion of data from a 64-bit floating point format to a 16-bit signed integer was to blame for the crash. If correct coding was implemented, the rocket would have been fine and the explosion could have been prevented.

Figure 1: Conversion from 64-bit to 16-bit Signed Integer Error