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Since the engine runs at constant RPM, torque is proportional to power and is used as the
primary parameter for engine thrust settings and ratings. Normally, the engine drives the
propeller, resulting in positive torque values.
There are two systems within the gearbox that work to mitigate the drag caused by negative
torque. The primary means of reducing negative torque is the negative torque system (NTS). If
an intermediate value of negative torque is sensed, propeller control is commanded to increase
the blade angle (move propeller blades towards feather) to relieve the negative torque. When the
propeller is driving the engine, the NTS system tends to engage intermittently, each NTS
engagement driving the propeller to a higher blade angle until the negative torque is relieved,
then disengaging. Centrifugal force returns the propeller to a lower blade angle, building up
negative torque until NTS reengages. The NTS is deactivated when the throttles are moved
below flight idle, into the ground (beta) range, to permit reverse thrust
To back up the NTS, a safety coupling is installed that actuates at a much higher negative torque
than the NTS. The safety coupling acts as a mechanical “fuse” that minimizes the drag from a
propeller that fails to feather. If an in flight decoupling occurs, the flight manual directs that the
engine be immediately shutdown. The power plant is no longer producing thrust.
The C-130’s propeller is equipped with a low pitch stop. The purpose is to prevent the propeller
blades from entering the ground (beta) range while the aircraft is in flight. The low pitch stop
physically prevents the blade angle from decreasing below 23 degrees with the throttles in the
flight (alpha) range. When the throttles are retarded below the flight idle detent, the low pitch
stop retracts.
The propeller is also equipped with a pitch lock mechanism. The purpose is to prevent the blade
angle from decreasing if propeller hydraulic pressure is lost or the propeller over speeds
(indicating loss of speed control). The pitch lock engages if RPM > 103%, and consists of
ratchets that lock the propeller at its existing pitch and prevent the blade angle from decreasing.
A pitch locked propeller functions as a fixed pitch propeller and can continue to provide thrust
with a complete loss of propeller governing fluid.
The T56 fuel control is completely mechanical. To permit use of different fuels with potentially
different energy contents, the engines are equipped with a temperature datum (TD) system. The
