Why do we check the RPM difference during an engine run-up?

The practice of checking RPM difference during an engine run-up primarily focuses on ensuring proper magneto timing. When testing the magnetos, the RPM drop observed when each magneto is turned on individually should fall within a specific range. This drop indicates how well each magneto is operating and whether it is efficiently igniting the fuel-air mixture in the engine cylinders. An excessive RPM drop, or an uneven drop between the two magnetos, could suggest a malfunction or issue requiring further inspection, such as a fouled spark plug or a problem with the ignition system.

This check is crucial for confirming that both magnetos are functioning correctly, as a failure in one can lead to poor engine performance, a rough running engine, or, in severe cases, engine failure. Therefore, monitoring the RPM difference is essential in assessing the ignition system’s effectiveness during the run-up procedure.

Other choices, while relevant to engine checks, do not directly relate to the specific action of checking RPM differences. Fuel flow checks, engine health metrics, and propeller functionality tests are part of broader pre-flight inspections but are not directly tied to the purpose of observing RPM differences during the run-up.