Which instrument plays a crucial role in indicating overspeed conditions in a gas turbine engine?

The tachometer is essential for indicating overspeed conditions in a gas turbine engine because it measures the rotational speed of the engine's components, typically in revolutions per minute (RPM). By continuously displaying the engine speed, the tachometer allows operators to monitor if the engine is operating within safe parameters. If the RPM exceeds the manufacturer's specified limits, this indicates an overspeed condition, which can lead to engine damage or failure. Thus, the tachometer serves as a critical tool for ensuring the safe operation of the engine by providing real-time feedback on the engine's performance.

Other instruments like the oil pressure gauge, exhaust gas temperature indicator, and fuel pressure indicator provide important information regarding engine health and performance but do not specifically measure rotational speed or directly indicate overspeed conditions.