Which of the following is NOT an engine variable detected by the fuel control unit?

The correct answer is that fuel composition is not an engine variable detected by the fuel control unit.

The fuel control unit's primary role is to manage the amount of fuel being delivered to the engine based on operational parameters that directly affect performance. The variables that it typically monitors, like power lever position, compressor intake temperature, and engine r.p.m., are all critical for determining the appropriate fuel flow to ensure optimal combustion and engine efficiency.

Power lever position indicates the desired thrust level, while compressor intake temperature and engine r.p.m. influence the air-fuel mixture's requirements. In contrast, fuel composition, although important for understanding combustion characteristics and engine performance in a broader sense, is not monitored in real-time by the fuel control unit during engine operation. Instead, it is inferred based on the fuel type being used and does not change dynamically like the other variables. Thus, recognizing that fuel composition does not fall under the purview of real-time engine variable detection is essential for understanding how the fuel control unit operates.