The failed of calculation cause Ariane 501 loss of altitude information in 37 second from the start after engine initialize after the lift-up. This disintegration had begun an instant before, when the spacecraft swerved off course under the pressure of the three powerful nozzles in its boosters and main engine. The rocket was making an abrupt course correction that was not needed, compensating for a wrong turn that had not taken place.

 Steering was controlled by the on-board computer, which mistakenly thought the rocket needed a course change because of numbers coming from the inertial guidance system. That device uses gyroscopes and accelerometers to track motion. The numbers looked like flight data bizarre and impossible flight data but were actually a diagnostic error message. The guidance system had in fact shut down.

When the guidance system's own computer tried to convert one piece of data the sideways velocity of the rocket from a 64-bit format to a 16-bit format. The number was too big, and an overflow error resulted. When the guidance system shut down, it passed control to an identical, redundant unit, which was there to provide backup in case of just such a failure. But the second unit had failed in the identical manner a few milliseconds before. It was running the same software.
 
 

This bug belongs to a species that has existed since the first computer programmers realized they could store numbers as sequences of bits, atoms of data, ones and zeroes: 1001010001101001 in binary number. A bug like this might crash example computer program such as a spreadsheet or word processor on a bad day. Ordinarily, though, when a program converts data from one form to another, the conversions are protected by extra lines of code that watch for errors and recover gracefully. Indeed, many of the data conversions in the guidance system's programming included such protection.
    

But in this case, the programmers had decided that this particular velocity figure would never be large enough to cause trouble. After all, it never had been before. Unluckily, Ariane 5 was a faster rocket than Ariane 4. One extra absurdity the calculation containing the bug, which shut down the guidance system, which confused the on-board computer, which forced the rocket off course, actually served no purpose once the rocket was in the air. Its only function was to align the system before launch suppose should have been turned off. But engineers chose long ago, in an earlier version of the Ariane, to leave this function running for the first 40 seconds of flight meant to make it easy to restart the system in the event of a brief hold in the countdown.