ECU (high side failure) or operation in ECU lockout renders TGT limiting inoperative, enabling the possibility of exceeding 850o C at T4.5 and thermally stressing the hot sections components. This is only one example of how the TGT could exceed 850o C.

(b)
Hot Section Airflow-P-3 air protects hot section components from "heat destruction" by diluting combustion gasses to reduce heat, and by maintaining a protective boundary layer between the flame and the metal (P-3 air is also directed inside hot section components to conduct heat away). The interruption of P-3 air (as in compressor stall) can stress or damage hot section components. TGT/T4.5 represents the temperature at the Gas Generator output (same as P/T input temp.); this temperature is limited to 850o C by the ECU. 850o C at T-4.5 equates to about 1700-1800o C within the combustion chamber. Malfunction of the ECU (high side failure) or operation in ECU lockout renders TGT limiting inoperative, enabling the possibility of exceeding 850o C at T-4.5 and thermally stressing hot section components.

(3)
Power Turbine Module-The power turbine module converts gas pressure into rotational shaft energy to power the drive train.  Power Turbine Rotor and Drive Shaft Components-The power turbine is

comprised of a two stage power turbine rotor, each prefaced by nozzles, and a power turbine drive shaft. The power turbine shaft passes through the center of the Ng section, but has no mechanical link with the Ng. A splined adapter mates the power turbine shaft to the high speed shaft. The high speed shaft transfers engine power to the input module and it rotates at the same speed as the power turbine rotors. Reductions in rpm occur in the input and main module.
Np Operational Overview-Expanding gases produced by the Ng section attempt to drive the power turbine at 20,900 rpm, which is equal to 258 RPM R, 100% Np/Nr.
Np to Nr Output-Np, the power turbine, is mechanically connected to the Nr, a variable load. Due to this mechanical connection, changes in the main rotor load, the collective pitch, affect the power turbine load. Changes in the power turbine load necessitate changes in the Ng output. The Ng is trimmed by the collective movement through the LDS and the engine governing system, the ECU.  
(a)
Overspeed Protection-The power turbine is protected from damaging overspeed conditions by the Np overspeed system. If the Np speed reaches 106 ± 1%, the Np overspeed system activates and reduces the Ng output to slow down the Np. Two magnetic pickups are housed in the power turbine section. The Np sensors relay magnetic pulses to the ECU for Np governing purposes and for cockpit indications of % RPM 1 or 2. A torque and Np overspeed sensor forwards Np speed and torque signals to the ECU for torque matching and Np overspeed functions. The ECU also conditions the signal for cockpit torque indications.

(b)
Temperature Sensors-Seven thermocouple probes are placed just prior to the power turbine, station 4.5. These sensors measure the temperature of the gasses between the two turbine sections. The thermocouple probes feed temperature data to the ECU for the TGT limiting function. The ECU relays the TGT signal to the CDU through the Signal Data Converter (SDC).
　
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