Which of the following is NOT a sign of engine failure in a helicopter?

The correct answer is that an increase in rotor RPM is not a sign of engine failure in a helicopter. When an engine fails, the engine's power output decreases, which in turn can lead to a decrease in rotor RPM, not an increase.

In most helicopter configurations, the rotor system relies on engine power to maintain proper RPM. When there is a failure, typically, the opposite effect occurs, which aligns with the other choices provided in the question. For example, RPM horn activation and oil pressure light illumination indicate critical abnormalities, and a nose left yaw often results from a loss of power on one side, which describes engine performance issues. Therefore, an increase in rotor RPM during an engine failure scenario would be inconsistent and is not a recognized sign of such an event. This understanding is critical for recognizing engine failure symptoms and managing emergency situations effectively.