Definition
A propeller system feature on turboprop engines that automatically detects when the propeller is driving the engine — rather than the engine driving the propeller — and responds by moving the propeller blades toward a higher pitch (coarser) angle to reduce the drag the propeller is creating. NTS does not feather the propeller; it limits the amount of negative torque the engine experiences while the system continues to operate.
Plain English
If the engine quits or loses power, the spinning propeller can start dragging the engine around like a windmill, which creates a lot of drag on the airplane. NTS senses this and twists the propeller blades to a flatter-into-the-wind angle so they catch less air and slow down, reducing that drag until the pilot can take further action.
Context Anchor
You will see NTS discussed in fixed-shaft turboprop engine operation, especially during engine failure, very low power, or rapid power changes.
Derivation
Negative torque means torque in the reverse direction — the propeller is turning the engine instead of the engine turning the propeller. Sensing simply means detecting it. The name describes exactly what the system does: it senses negative torque and acts on it.
Why Pilots Care
It prevents a windmilling propeller from creating excessive drag that could impair aircraft control or performance after an engine failure.
Analogy
It is like a bicycle wheel on a trainer. Normally your legs drive the wheel. If the wheel somehow started forcing the pedals around instead, the driving direction has reversed. NTS detects that kind of reversal in the propeller system.
Intuition Check
Negative does not mean bad here. It means the twisting force has reversed direction: the propeller is trying to turn the engine, rather than the engine turning the propeller.
Example Sentence 1
When the left engine lost power in cruise, NTS activated and moved the propeller toward a higher pitch, reducing drag while the crew completed the engine-failure checklist.
Example Sentence 2
The NTS activates during low-power conditions to keep the propeller from driving the engine and creating unwanted resistance.