In high altitude operation, the low pressure rotor will increase its speed when the compressor load decreases in the lower density air.

• A. Increasing in speed
• B. Decreasing in speed
• C. Remaining at the same speed
• D. Stalling

Answer: (A)

The key idea is that the low-pressure rotor speed is set by the balance of power on the shaft: the turbine provides driving torque, while the compressor presents aerodynamic load. In high altitude, air is thinner, so the compressor requires less work (the load is lower) to spin at a given speed. If that load drops, the same turbine torque will accelerate the shaft, raising the rotor speed until the reduced load is again balanced at a higher speed. In other words, with less resistance on the shaft, the available turbine power pushes the rotor faster instead of keeping it the same or slowing it down. Stalling would imply a loss of airflow and is not the normal response to a reduced compressor load in this context.