What happens to prop RPM and torque if the primary governor fails?

When the primary governor fails, it typically results in a loss of control over the engine speed and propeller RPM. The primary governor’s function is to regulate the propeller RPM by adjusting the pitch of the propeller blades. If the governor fails, the system may eliminate this regulation, leading to unintended changes.

In this situation, prop RPM will likely decrease because the governor is not able to maintain the necessary speed settings. The propeller may not be able to pitch correctly to demand more power from the engine, which generally causes a reduction in RPM. On the other hand, the torque often increases when the RPM decreases, particularly if the engine is still producing power without an efficient way to translate that into propeller thrust.

This dynamic shows that upon governor failure, the expected behavior is indeed a decrease in prop RPM coupled with an increase in torque.