TM 1-1520-238-10 Change 9 2-31 warning light and audio will activate. If the knurled ring is released  after  turning  to  CHOP,  it  will  snap  back  to  the center position, but the engines will remain at idle and the ENGINE CHOP warning light will remain illuminated. Re- tard both PWR levers to IDLE. If the knurled ring is then turned 45° to the left to RESET the engines will perform in accordance with  PWR lever settings. When the knurled ring is released from RESET, it will snap back to the center position, but the circuit will remain in the RESET normal condition. Power for the engine-cut circuit is obtained by way of the ENG CUT circuit breaker on the pilot overhead circuit breaker panel. 2.31.3  Engine Load Demand System. When the en- gine PWR lever is in FLY, the ECU/DECU and HMU re- spond to collective pitch position to automatically control engine speed and provide required power. During emer- gency operations when the PWR lever is moved to LOCK- OUT and then retarded to an intermediate position, be- tween IDLE and FLY, the engine will respond to collective signals, but control of engine speed is no longer automatic and must be managed manually using the PWR lever. WARNING The T700-GE-701 and T700-GE-701C en- gine is designed to shut down when an overspeed  condition  is  sensed.  The OVSP TEST circuit trips at 95 – 97% N_P and should never be performed in flight. A  power  loss  will  result.  ONLY  mainte- nance  personnel  are  authorized  to  per- form this check. a.   Engine Overspeed Protection System (OPS). The engine overspeed protection system prevents power turbine  overspeed.  The  system  receives  power  turbine speed signals from the torque and overspeed sensors lo- cated in the exhaust frame section of the engine. When N_p meets or exceeds 119.6% 1%, two frequency sensing circuits  output  a  signal  to  the  overspeed  system  which causes the ODV to shut off fuel to the engine. Two over- speed test switches are located on the EMER PWR CHK OVSP  TEST  panel  (fig.  2–19).  These  switches  permit testing of the OPS A and B circuits in normal power tur- bine speed range. An advisory light on the panel indicates emergency airframe electrical power. The system normal- ly  uses  engine  alternator  power.  The  overspeed  test switches  are  used  during  maintenance  operational checks. 2.32  ENGINE INSTRUMENTS. The engine instruments are vertical scale type. A common feature of all vertical-scale instruments is the power-on in- dication: when electrical power is supplied to these instru- ments, the blue segment of each vertical-scale is illumi- nated. Another common feature is indicator-light dimming (not dimming of the edge-lighted panels but dimming of the vertical-scale segments and the digital displays). This dimming is accomplished for each crew station by the DIM control on the engine instrument test panels (fig 2-20) lo- cated in each crew station. An additional feature of the pi- lot engine instrument test panel is automatic dimming of the pilot engine instruments by a photoelectric cell located on the test panel. The cell dims or brightens in response to ambient  crew  station  light  but  may  be  overridden  by manual control. A third common feature of all engine in- struments in each crew station is that they share power supplies.  Each  power  supply  energizes  alternate  lamp segments  and  one  digital  readout  for  all  engine  instru- ments in each respective crew station. If one power sup- ply fails, every other lamp segment and all of ENG 1 or all of ENG 2 digits will extinguish. In addition, if one of the pi- lot power supplies fails, the AUX PWR light on the pilot engine  instrument  test  panel  illuminates.  The  CPG  test panel does not have this feature. One of the pilot power supplies  receives  28  vdc  from  the  emergency  dc  bus through the ENG INST circuit breaker on the pilot circuit breaker  panel.  The  other  pilots  power  supply  and  both CPG power supplies receive 28 vdc from the emergency dc bus through the ENG INST circuit breaker on the CPG No. 1 circuit breaker panel. The illuminated segments at the vertical scale instruments are referenced to the adja- cent instrument indices and utilize a technique called opti- mistic scaling. This means, for example, that for proper in- dication of 100% N_p/N_r the segments immediately above the instrument index line for 100% should be at the thresh- old of illumination.