Some turboprop and turbojet engines are equipped with two spool or split compressors. When operated at high altitudes, the

• A. Low pressure rotor will increase in speed as the compressor load decreases in the lower density air
• B. Low pressure rotor will decrease in speed as the compressor load decreases in the lower density air
• C. Throttle must be retarded to prevent overspeeding of the high pressure rotor due to the lower density air
• D. High pressure rotor speed remains constant

Answer: (A)

When a two-spool compressor operates in thin air at high altitude, the air density drops and the mass flow through the compressor falls. That means the compressor doesn’t need as much power to achieve its pressure ratio, so the load on the low-pressure (LP) compressor decreases. With less load on the LP compressor, the LP turbine experiences less drag on the LP shaft, allowing the LP rotor to accelerate until the engine’s condition (fuel control and speed balance) settles to a new operating point. In this situation the LP rotor speed tends to rise as the load diminishes.

The high-pressure (HP) rotor is governed by its own stage and by the engine’s control system, which keeps the HP side within a relatively narrow range, so its speed does not change as noticeably as the LP rotor in this scenario. That’s why the LP rotor increasing in speed is the characteristic response described, rather than a decrease or a need to retard throttle for HP overspeed.