Which of the following is a common indication of an impending engine failure?

A decrease in power is a significant indication of an impending engine failure because it directly reflects the engine's inability to produce the necessary thrust. Power loss can result from various issues such as fuel supply problems, mechanical failures, or issues with the ignition system. When the pilot observes a decrease in power during flight, it is critical to assess the situation immediately, as it may indicate that the engine is not functioning properly and could lead to complete failure if not addressed.

In contrast, increasing fuel efficiency typically suggests that the engine is operating within normal parameters, stable oil pressure readings indicate that the lubrication system is functioning well, and normal vibration levels suggest that the engine components are balanced and operating harmoniously. None of these conditions indicate an impending engine failure like a decrease in power does, emphasizing the importance of power output in engine monitoring and safety protocols.