Negative torque occurs when air loading on a propeller drives the engine instead of the engine driving the propeller. This situation is often seen in turboprop engines with shaft-driven constant speed propellers and some free turbine systems. When the engine power levers are moved to flight idle, the propeller’s blade pitch adjusts to its minimum angle to maintain RPM. If the aircraft is moving fast enough, this adjustment might not be sufficient to keep the propeller at the desired speed. As a result, the NTS system activates.
The activation of negative torque can create a lot of drag, which may even lead to a mechanical separation between the engine and the propeller. To counteract this, the NTS system briefly engages the propeller feathering mechanism. This action moves the blades to a coarser position, which helps reduce the drag caused by the negative torque. The feathering system only operates long enough to eliminate the negative torque signal. Without pilot intervention to increase engine power or a decrease in airspeed, the system will cycle back to its normal state.
In cases of engine failure in a free turbine setup, the engine’s torque decreases rapidly. The NTS system detects this drop and responds by adjusting the blade angle toward the minimum drag or full feather position. This ensures optimal performance and safety during critical situations.
