Ariane 5: the rocket brought down by old code

The story

The Ariane 5 was Europe’s new, larger rocket. Some of its navigation software was carried over from the Ariane 4, which had a long record of successful launches.

One piece of that old code converted a measurement of the rocket’s sideways speed into a smaller number format. On the Ariane 4, the value never got big enough to cause a problem. The Ariane 5 flew differently and moved sideways much faster early in flight. The number overflowed, and the navigation system shut itself down.

The backup navigation unit ran identical software, so it had already failed the same way. The main computer read the error data as real flight data and steered hard. Thirty-seven seconds after launch, the rocket broke apart and destroyed itself, along with four research satellites.

The code that failed served no purpose after liftoff on the Ariane 5. It had been kept because it worked before.

Why it mattered

It is one of the best-known software failures in history. It showed that code that worked perfectly in one system can fail in another, and that a backup running identical software is not much of a backup, because it fails in exactly the same way.

The lesson today

“It worked on the old system” is not the same as “it will work on the new one.” When businesses move to a new server, upgrade software or change equipment, the old settings and assumptions come along too. Testing before the change, and having a backup that does not fail in the same way, is what keeps a small mismatch from becoming an outage.