What is a sign that an aircraft engine may be overheating?

A decrease in oil pressure is a significant indicator that an aircraft engine may be overheating. Oil in an engine serves multiple critical purposes, including lubrication, cooling, and cleaning. If the engine temperature rises excessively, it can lead to a breakdown of the oil's lubricating qualities, potentially causing it to thin out. This thinning can result in a drop in oil pressure, as there is less viscous liquid to maintain pressure in the system. Monitoring oil pressure is essential for maintaining engine integrity, and a sudden decrease signals that the engine may not be cooling properly, increasing the risk of overheating.

The other options might suggest various conditions but do not directly imply overheating. Unexpected engine noise could relate to mechanical issues, an increase in RPM might indicate the engine is performing well under load, and unusual vibrations can signal imbalance or other concerns, none of which specifically indicate that the engine is overheating in the same direct manner that a drop in oil pressure does.