Here's a NASA blog with slightly more detail:<p><a href="https://mars.nasa.gov/technology/helicopter/status/296/mars-helicopters-flight-four-rescheduled/" rel="nofollow">https://mars.nasa.gov/technology/helicopter/status/296/mars-...</a><p>> An issue identified earlier this month showed a 15% chance for each time the helicopter attempts to fly that it would encounter a watchdog timer expiration and not transition to flight mode. Today’s delay is in line with that expectation and does not prevent future flights.
In short :<p>> <i>NASA engineers are assessing the data, since it's not yet clear what caused the failure. One potential cause is a software issue that first showed up during a high-speed spin test ahead of the chopper's first flight. That test failed because Ingenuity's flight computer was unable to transition from "pre-flight" to "flight" mode. Within a few days, though, NASA engineers resolved the issue with a quick software rewrite.</i><p>> <i>But those engineers determined that their fix would only successfully transition the helicopter into flight mode 85% of the time. So Thursday's attempt may have fallen into the 15% of instances in which it doesn't work.</i>