They Deleted One Switch Because the Software Had It Covered — Therac-25

Between 1985 and 1987, a radiation therapy machine gave at least six patients doses roughly a hundred times larger than prescribed. Three of them died. Every part of the machine worked as designed. After the first accidents the manufacturer investigated and reported, in writing, that an overdose was not possible. The trigger was typing speed. If an operator entered the treatment, spotted a mistake and corrected it within about eight seconds, the machine ended up in a state its designers believed could not exist. ▶ Watch the full series:    • The Wreckage Report — Every Episode   🔔 Subscribe:    / @thewreckagereport   In this video we break down the engineering: why a linear accelerator needs two beam settings that differ by a factor of a hundred, what the turntable does and why the target must be in place, and how a race condition between the editing code and the turntable control let the two disagree. But the real subject is the hardware interlock. The previous machines had a microswitch that made the fault physically impossible. It was removed from the Therac-25 because the software already performed the same check — and the software had years of clean operation behind it. Except that record belonged to the switch. Every time the software got it wrong, the microswitch silently stopped it and nobody ever found out. Nine months after that bug was found and fixed, the machine killed again — in a different hospital, through a completely unrelated defect that had been sitting in the same code the whole time. This is the founding case study of software safety engineering, and it is still taught that way. Educational and respectful — no patients, no injuries and no clinical footage of people are shown. The people harmed are not named. Chapters: 0:00 A hundred times the dose, and nothing was broken 1:17 One machine, two completely different beams 3:08 The switch that used to make it impossible 5:01 Eight seconds, and a state nobody planned for 6:58 Malfunction 54, and a number the manual never explained 8:57 One programmer, no review, no log 11:30 The accident that created software safety engineering 13:08 A clean record that belonged to a microswitch ────────────── MORE FROM THE WRECKAGE REPORT ▶ Every episode, in order:    • The Wreckage Report — Every Episode   🔔 New disaster breakdown every week:    / @thewreckagereport   This channel covers engineering failures to understand them, never to sensationalize. No graphic content. Sources listed in each video. #engineering #documentary #softwareengineering #safetycritical #therac25 #medicaldevices