[Tim Wang]

Today I did a ground run test on the rear engine. The battery charge level was good and there was no problem on the start up. Run up checks as usual were completed without issues. I waited for the engine and the oil temperature to get warmer running the engine around 1800 rpm's. As I moved the prop lever to cycle the prop a drop in rpm's appeared however returning the lever to "fine" pitch didn't change the rpm's as expected and it stayed at the lower rpm reading. I moved the prop lever again to cycle the prop and the rpm's would drop even more once the prop lever moved further towards the "coarse" position and no return to the high rpm setting. Moving the prop lever only caused a drop in rpm's and no increase in rpm's. GOVERNOR fault.
In discussing the matter with my AME, a governor fault fits with my situation since the oil would be thicker at the start of my flight making the seals in the governer work as normal, but as the flight progressed the hotter oil got thinner and the seals inside the governer failed to move the oil around. Since the front prop is pushing air towards the rear prop this would cause the prop to twist to a coarser pitch. The subsequent drop in rpm's would then cause me to increase power to the front engine which leads to forcing more air towards the rear prop, twisting the blades more causing more rpm drop and the cycle continues until the engine quits or shut down. This wouldn't have happened on a side by side engine configuration without a front engine pushing air causing the rear prop blades to twist when a governor fails. Since the run up checks found no issue with the governor there wouldn't be a reason to suspect a problem with this part.

Cheers.