When I worked in aircraft manufacturing, we only used “flight hours”.
During functional system check (prior to first flight of the aircraft) of components and systems, operating time was never factored into the mix. If the component failed during this time it was considered infant mortality and the time was calculated as “zero”. Even though the component itself may have had up to 20 to 30 hours “operational time” prior to flight time, it wasn’t flying when it failed, so it was considered zero.
If the component failed a post flight operational check, the flight hours were used to calculate the time on the component.
