Regression testing lessons from F1's wet-weather throttle failure

The Formula One race in Malaysia faced an unexpected software issue on October 4. During formation laps behind the safety car, Max Verstappen experienced a lack of power, followed by other cars encountering the same problem. Race control stopped proceedings with a red flag, highlighting the severity of the situation.

The FIA later attributed the failure to a wet-weather software rule that didn't account for a very-low-speed condition, which testing had missed. The control software reduced electrical power to 250 kilowatts, causing cars to slow down almost to a standstill when operating in certain track sections with wet-weather restrictions. When this happened, a loop prevented power from being redeployed, resulting in cars being unable to accelerate again.

The failure spread across multiple teams, as they all shared the same control software. An emergency update was released to lift the implicated restrictions, allowing the race to continue. FIA director Nikolas Tombazis stated that the updates were installed without testing under intense time pressure, but he believed safety and fairness were preserved.

The incident underscores the importance of regression testing, particularly for combinations of conditions that are individually ordinary but can create unexpected interactions. In this case, the failure was exposed when cars entered restricted sections at very low speeds, attempted acceleration, and encountered an unexpected control loop. To prevent similar issues in the future, engineers should test the edges of the operating envelope, especially during transitions out of constrained states.