European Space Agency and CNES

Ariane 5 Flight 501 is often reduced to one unsafe numeric conversion in reused code. The official record supports that technical fact, but not that narrow account of responsibility. A pre-launch alignment function that Ariane 5 did not need remained active in flight; a range assumption inherited from Ariane 4 was not tested against the new trajectory; two identical inertial systems failed from the same software condition; diagnostic data then crossed an interface as if it were valid guidance. The deeper failure was a qualification system that treated proven service in one mission domain as evidence for another without making the inherited assumptions, operating limits and common-mode risks visible enough to challenge.