Ariane 5 Flight 501 failure (1996)
The loss of the first Ariane 5 less than forty seconds after liftoff, caused by software carried over from Ariane 4 without being revalidated.
A launcher's maiden flight carries the least understood risk in space, because no reliability record exists for it yet. The Flight 501 failure became the reference case, first for a technical reason: the inertial reference system reused Ariane 4 code, in which one numeric conversion had never been tested against the faster trajectories of the new vehicle. The overflow shut down both redundant inertial units, which ran the same software, and the launcher lost its guidance. Redundancy was therefore useless, since both channels shared the same fault, which insurers recognize in every common-mode risk. The inquiry report also set the rule, now standard, never to reuse a component without revalidating the assumptions of its new environment.
On June 4, 1996, at Kourou, the first Ariane 5 self-destructed about 37 seconds after liftoff, taking with it the European Space Agency's four Cluster science satellites; the inquiry board chaired by Jacques-Louis Lions delivered its report on July 19, 1996.
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