HOLD mode at takeoff completely manual?
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Join Date: Dec 2007
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HOLD mode at takeoff completely manual?
Hi, a question about the 747-400 with GE engines. After the TOGA switch is pressed at takeoff, the thrust goes into THR REF mode and sticks to the green reference N1. At 65 knots the speed mode changes into HOLD and the A/T disconnects until 400 ft. In this interval, if TAT, speed and altitude change, I suppose the green reference N1 will slightly change (say it goes from 98.2 to 98.3). Will there be a consequent increase in the actual thrust from 98.2 to 98.3, commanded by the EEC, or is the thrust completely manual and disconnected from any sort of 'protection' by the EEC, so that if the pilot doesn't touch the thrust levers the thrust does not change at all?
Thanks.
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Join Date: Jul 2012
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Thrust demand to the EEC is solely by thrust lever position. Thus, if the reference N1 changes with the A/T in HOLD, the thrust levers will not move and so the thrust will not vary. It is my understanding that the EEC will only intervene to prevent exceedance of a physical RPM limit, not a variation in a reference N1 limit.
Join Date: Jun 2002
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A/T HOLD mode is where the thrust levers are de-clutched, temporarily rendering them incapable of being driven by the A/T servo motors. The thrust developed will be proportional to power lever position and resultant thrust will change according to ambient conditions. During the HOLD period the engines are still protected by overspeed governors and there should always be a wide enough ITT margin to allow for significant changes in the environment.
