As altitude increases with fixed throttle for a high by-pass engine, the fan RPM will:

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Multiple Choice

As altitude increases with fixed throttle for a high by-pass engine, the fan RPM will:

Explanation:
With a fixed throttle, the energy the engine can put into the fan is essentially constant. As altitude increases, the air becomes thinner, so the mass flow through the fan and compressor drops. That means the aerodynamic load on the fan decreases. With the same energy input from the fuel, the reduced load allows the fan (the low‑pressure spool) to spin faster until a new balance is reached between the turbine power and the reduced air load, subject to temperature limits. So the fan RPM rises with increasing altitude at fixed throttle.

With a fixed throttle, the energy the engine can put into the fan is essentially constant. As altitude increases, the air becomes thinner, so the mass flow through the fan and compressor drops. That means the aerodynamic load on the fan decreases. With the same energy input from the fuel, the reduced load allows the fan (the low‑pressure spool) to spin faster until a new balance is reached between the turbine power and the reduced air load, subject to temperature limits. So the fan RPM rises with increasing altitude at fixed throttle.

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