Which of the following is a sign of an abnormal engine behavior noted during flight?

An abnormal engine behavior noted during flight is often indicated by vibration. When an engine operates normally, it typically runs smoothly and without excessive vibration. Vibration can suggest issues such as misalignment, imbalance, or mechanical malfunctions within the engine. It may indicate pilot-induced problems or concerns related to engine operation integrity, making it essential for pilots to monitor this closely.

Steady operation, consistent power output, and normal temperature readings are indicative of a healthy engine status and would not typically suggest any abnormal behavior during flight. These signs show that the engine is functioning as expected, while vibration is a clear indicator that further investigation is needed to assess the underlying cause.