Turbine Block 13 Practice Exam

In some high-volume two-spool engines, when operated at high altitude, what happens to the low-pressure rotor as compressor load decreases in low-density air?

Decrease in speed

Maintain constant speed

Stall

Increase in speed as the compressor load decreases in the lower density air

In a high-volume two-spool engine, the low-pressure rotor speed is set by the balance between the power the turbine provides to drive the low-pressure compressor and the load the compressor presents. At high altitude, air density is low, so the compressor requires less air to achieve a given pressure rise; the compressor load drops.

When compressor load decreases, less energy is being drawn from the low-pressure turbine to drive the compressor. With that reduced drag on the shaft, and with the turbine still supplying power, the low-pressure rotor tends to accelerate until the engine controls adjust fuel flow or airflow to restore balance. The net effect is that the low-pressure rotor speeds up as compressor load falls in thin air.

Stalling isn’t the typical outcome of just a load decrease at altitude, and a constant speed or a decrease in speed doesn’t align with the reduced mechanical load on the low-pressure spool.