Which engine indicator is expressed as a percentage of engine speed?

The engine indicator that is expressed as a percentage of engine speed is RPM, which stands for Revolutions Per Minute. RPM is a vital measurement used in aviation and other engineering fields to denote how fast an engine is rotating. It provides critical insights regarding the engine's performance and efficiency. When expressed as a percentage, it allows pilots and engineers to understand how close the engine is operating to its maximum rated speed, facilitating better management of power and performance during flight operations.

In the context of aircraft, monitoring RPM is essential for ensuring that the engine operates within safe limits, helping to avoid potential damage from overheating or mechanical failure. This measurement is typically displayed on flight instruments in a way that allows the crew to easily ascertain the current operating conditions of the engine in relation to its maximum capabilities.