What performance indicator would show a deviation from optimal performance?

The fuel flow gauge serves as a critical performance indicator for assessing an aircraft’s fuel consumption in relation to its operational efficiency. A deviation from optimal performance can be indicated by unusual readings on the fuel flow gauge, such as higher than normal fuel consumption for a given power setting or operational profile. This could suggest potential issues such as incorrect fuel mixtures, engine malfunctions, or excessive drag, all of which can impact overall aircraft performance.

In contrast, the other instruments primarily measure specific flight parameters (airspeed, altitude, vertical speed) but do not directly indicate overall performance inefficiencies. While changes in airspeed, altitude, and vertical speed can certainly relate to performance, they do not provide a direct measure of fuel efficiency or consumption relative to the aircraft's intended operational norms. Thus, the fuel flow gauge is specifically designed to highlight deviations in fuel usage that could signal suboptimal performance.