With fixed throttle and increasing altitude, the fan RPM will:

• A. Increase RPM.
• B. Decrease RPM.
• C. Stay the same RPM.
• D. Might vary unpredictably.

Answer: (A)

When the throttle is fixed, the amount of fuel entering the combustion chamber stays the same regardless of altitude. What changes with increasing altitude is air density, so the mass flow through the engine decreases.

The turbine and compressor are linked on the same shaft. With less air coming in, the compressor requires less work to spin and deliver a given pressure rise, so the mechanical load on the shaft drops. Meanwhile, the fixed fuel flow tends to push more energy into the turbine gases (a higher energy release per unit of air isn’t directly offset by the reduced air mass). That combination tends to let the shaft accelerate until the reduced air flow and the available energy balance out. In practice, the result is an increase in the fan (low‑pressure compressor) RPM as altitude rises under a fixed throttle setting.

So, as you climb with the throttle unchanged, the fan RPM tends to rise rather than fall or stay the same. The other possibilities don’t capture this balancing act between reduced air mass flow and the steady energy input from a fixed fuel flow.