A great update on the highly unusual, last second (literally) abort of the Ariane 5 as reported in Spaceflight Now:
The Ariane 5 rocket’s on-board computer detected an "incorrect displacement" of one of the Vulcain 2 engine actuators less than a half-second before the launcher was supposed to lift off from the launch pad, according to an anomaly summary provided to Spaceflight Now.
The flight computer detected the anomaly March 30 about 6.6 seconds into the Vulcain engine’s burn, just a fraction of a second before the solid rocket boosters would have ignited, according to Arianespace, the commercial operator of the Ariane 5.
The solution, as might be expected, was to remove and replace a number of the actuators.
In the old days, this sort of telemetry and state-of-health would be monitored by a human being watching a strip-chart recorder on the count-down net. It’s unlikely human beings would have been able to abort a launch with less than a half second to go.
When the Ariane 5 returns to flight flies (scheduled now for Friday), the payloads will be the Yahsat 1A and Intelsat’s New Dawn.