Which instrument alerts pilots to a potential engine failure?

The engine pressure ratio (EPR) gauge is specifically designed to provide pilots with vital information regarding engine performance, particularly in relation to potential engine failures. The EPR is a measurement that compares the pressure of the exhaust gases exiting the engine to the pressure of the air entering the engine. This ratio is crucial for understanding the engine's operational efficiency and can indicate when an engine is not functioning optimally. If the EPR falls below the normal operating range, it can signify an issue within the engine that may lead to failure, allowing pilots to take appropriate corrective action.

The other instruments serve different purposes. The vertical speed indicator measures the rate of climb or descent of the aircraft but does not provide engine-specific information. The altimeter indicates the altitude of the aircraft above sea level based on atmospheric pressure but does not reflect engine performance. Similarly, the horizontal situation indicator offers information regarding aircraft position and navigation but does not monitor engine status. Therefore, the EPR gauge is the critical instrument for alerting pilots to potential engine failure.